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Pinto M, Saliminasab M, Harris A, Lazaratos M, Bondar AN, Ladizhansky V, Brown LS. The retinal chromophore environment in an inward light-driven proton pump studied by solid-state NMR and hydrogen-bond network analysis. Phys Chem Chem Phys 2024; 26:24090-24108. [PMID: 39248601 DOI: 10.1039/d4cp02611j] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 09/10/2024]
Abstract
Inward proton pumping is a relatively new function for microbial rhodopsins, retinal-binding light-driven membrane proteins. So far, it has been demonstrated for two unrelated subgroups of microbial rhodopsins, xenorhodopsins and schizorhodopsins. A number of recent studies suggest unique retinal-protein interactions as being responsible for the reversed direction of proton transport in the latter group. Here, we use solid-state NMR to analyze the retinal chromophore environment and configuration in an inward proton-pumping Antarctic schizorhodopsin. Using fully 13C-labeled retinal, we have assigned chemical shifts for every carbon atom and, assisted by structure modelling and molecular dynamics simulations, made a comparison with well-studied outward proton pumps, identifying locations of the unique protein-chromophore interactions for this functional subclass of microbial rhodopsins. Both the NMR results and molecular dynamics simulations point to the distinctive polar environment in the proximal part of the retinal, which may result in a hydration pattern dramatically different from that of the outward proton pumps, causing the reversed proton transport.
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Affiliation(s)
- Marie Pinto
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
| | - Maryam Saliminasab
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
| | - Andrew Harris
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
| | - Michalis Lazaratos
- Freie Universität Berlin, Physics Department, Theoretical Molecular Biophysics Group, D-14195 Berlin, Germany
| | - Ana-Nicoleta Bondar
- University of Bucharest, Faculty of Physics, Măgurele 077125, Romania
- Forschungszentrum Jülich, Institute for Computational Biomedicine (IAS-5/INM-9), 52428 Jülich, Germany
| | - Vladimir Ladizhansky
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
| | - Leonid S Brown
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
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2
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de Grip WJ, Ganapathy S. Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering. Front Chem 2022; 10:879609. [PMID: 35815212 PMCID: PMC9257189 DOI: 10.3389/fchem.2022.879609] [Citation(s) in RCA: 14] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2022] [Accepted: 05/16/2022] [Indexed: 01/17/2023] Open
Abstract
The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.
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Affiliation(s)
- Willem J. de Grip
- Leiden Institute of Chemistry, Department of Biophysical Organic Chemistry, Leiden University, Leiden, Netherlands
- Radboud Institute for Molecular Life Sciences, Radboud University Medical Center, Nijmegen, Netherlands
| | - Srividya Ganapathy
- Department of Imaging Physics, Delft University of Technology, Netherlands
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3
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Abstract
In the last two decades, solid-state nuclear magnetic resonance (ssNMR) spectroscopy has transformed from a spectroscopic technique investigating small molecules and industrial polymers to a potent tool decrypting structure and underlying dynamics of complex biological systems, such as membrane proteins, fibrils, and assemblies, in near-physiological environments and temperatures. This transformation can be ascribed to improvements in hardware design, sample preparation, pulsed methods, isotope labeling strategies, resolution, and sensitivity. The fundamental engagement between nuclear spins and radio-frequency pulses in the presence of a strong static magnetic field is identical between solution and ssNMR, but the experimental procedures vastly differ because of the absence of molecular tumbling in solids. This review discusses routinely employed state-of-the-art static and MAS pulsed NMR methods relevant for biological samples with rotational correlation times exceeding 100's of nanoseconds. Recent developments in signal filtering approaches, proton methodologies, and multiple acquisition techniques to boost sensitivity and speed up data acquisition at fast MAS are also discussed. Several examples of protein structures (globular, membrane, fibrils, and assemblies) solved with ssNMR spectroscopy have been considered. We also discuss integrated approaches to structurally characterize challenging biological systems and some newly emanating subdisciplines in ssNMR spectroscopy.
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Affiliation(s)
- Sahil Ahlawat
- Tata Institute of Fundamental Research Hyderabad, Survey No. 36/P Gopanpally, Serilingampally, Ranga Reddy District, Hyderabad 500046, Telangana, India
| | - Kaustubh R Mote
- Tata Institute of Fundamental Research Hyderabad, Survey No. 36/P Gopanpally, Serilingampally, Ranga Reddy District, Hyderabad 500046, Telangana, India
| | - Nils-Alexander Lakomek
- University of Düsseldorf, Institute for Physical Biology, Universitätsstraße 1, 40225 Düsseldorf, Germany
| | - Vipin Agarwal
- Tata Institute of Fundamental Research Hyderabad, Survey No. 36/P Gopanpally, Serilingampally, Ranga Reddy District, Hyderabad 500046, Telangana, India
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4
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Abstract
Microbial rhodopsins represent the most abundant phototrophic systems known today. A similar molecular architecture with seven transmembrane helices and a retinal cofactor linked to a lysine in helix 7 enables a wide range of functions including ion pumping, light-controlled ion channel gating, or sensing. Deciphering their molecular mechanisms therefore requires a combined consideration of structural, functional, and spectroscopic data in order to identify key factors determining their function. Important insight can be gained by solid-state NMR spectroscopy by which the large homo-oligomeric rhodopsin complexes can be studied directly within lipid bilayers. This chapter describes the methodological background and the necessary sample preparation requirements for the study of photointermediates, for the analysis of protonation states, H-bonding and chromophore conformations, for 3D structure determination, and for probing oligomer interfaces of microbial rhodopsins. The use of data extracted from these NMR experiments is discussed in the context of complementary biophysical methods.
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Affiliation(s)
- Clara Nassrin Kriebel
- Institute of Biophysical Chemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
- Centre for Biomolecular Magnetic Resonance, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Johanna Becker-Baldus
- Institute of Biophysical Chemistry, Goethe University Frankfurt, Frankfurt am Main, Germany
- Centre for Biomolecular Magnetic Resonance, Goethe University Frankfurt, Frankfurt am Main, Germany
| | - Clemens Glaubitz
- Institute of Biophysical Chemistry, Goethe University Frankfurt, Frankfurt am Main, Germany.
- Centre for Biomolecular Magnetic Resonance, Goethe University Frankfurt, Frankfurt am Main, Germany.
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5
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Bada Juarez JF, Judge PJ, Adam S, Axford D, Vinals J, Birch J, Kwan TOC, Hoi KK, Yen HY, Vial A, Milhiet PE, Robinson CV, Schapiro I, Moraes I, Watts A. Structures of the archaerhodopsin-3 transporter reveal that disordering of internal water networks underpins receptor sensitization. Nat Commun 2021; 12:629. [PMID: 33504778 PMCID: PMC7840839 DOI: 10.1038/s41467-020-20596-0] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2020] [Accepted: 12/10/2020] [Indexed: 12/11/2022] Open
Abstract
Many transmembrane receptors have a desensitized state, in which they are unable to respond to external stimuli. The family of microbial rhodopsin proteins includes one such group of receptors, whose inactive or dark-adapted (DA) state is established in the prolonged absence of light. Here, we present high-resolution crystal structures of the ground (light-adapted) and DA states of Archaerhodopsin-3 (AR3), solved to 1.1 Å and 1.3 Å resolution respectively. We observe significant differences between the two states in the dynamics of water molecules that are coupled via H-bonds to the retinal Schiff Base. Supporting QM/MM calculations reveal how the DA state permits a thermodynamic equilibrium between retinal isomers to be established, and how this same change is prevented in the ground state in the absence of light. We suggest that the different arrangement of internal water networks in AR3 is responsible for the faster photocycle kinetics compared to homologs.
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Affiliation(s)
- Juan F Bada Juarez
- Biochemistry Department, Oxford University, South Parks Road, Oxford, OX1 3QU, UK
| | - Peter J Judge
- Biochemistry Department, Oxford University, South Parks Road, Oxford, OX1 3QU, UK
| | - Suliman Adam
- Fritz Haber Center for Molecular Dynamics Research, Institute of Chemistry, Hebrew University of Jerusalem, Jerusalem, 9190401, Israel
| | - Danny Axford
- Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK
| | - Javier Vinals
- Biochemistry Department, Oxford University, South Parks Road, Oxford, OX1 3QU, UK
| | - James Birch
- Diamond Light Source, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK
- Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0FA, UK
| | - Tristan O C Kwan
- Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0FA, UK
- National Physical Laboratory, Hampton Road, Teddington, London, TW11 0LW, UK
| | - Kin Kuan Hoi
- Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA, UK
| | - Hsin-Yung Yen
- OMass Therapeutics, The Schrodinger Building, Oxford Science Park, Oxford, OX4 4GE, UK
| | - Anthony Vial
- Centre de Biochimie Structurale (CBS), INSERM, CNRS, University of Montpellier, Montpellier, France
| | - Pierre-Emmanuel Milhiet
- Centre de Biochimie Structurale (CBS), INSERM, CNRS, University of Montpellier, Montpellier, France
| | - Carol V Robinson
- Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA, UK
| | - Igor Schapiro
- Fritz Haber Center for Molecular Dynamics Research, Institute of Chemistry, Hebrew University of Jerusalem, Jerusalem, 9190401, Israel
| | - Isabel Moraes
- Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, OX11 0FA, UK.
- National Physical Laboratory, Hampton Road, Teddington, London, TW11 0LW, UK.
| | - Anthony Watts
- Biochemistry Department, Oxford University, South Parks Road, Oxford, OX1 3QU, UK.
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Jirasko V, Lakomek N, Penzel S, Fogeron M, Bartenschlager R, Meier BH, Böckmann A. Proton-Detected Solid-State NMR of the Cell-Free Synthesized α-Helical Transmembrane Protein NS4B from Hepatitis C Virus. Chembiochem 2020; 21:1453-1460. [PMID: 31850615 PMCID: PMC7318649 DOI: 10.1002/cbic.201900765] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Indexed: 01/01/2023]
Abstract
Proton-detected 100 kHz magic-angle-spinning (MAS) solid-state NMR is an emerging analysis method for proteins with only hundreds of microgram quantities, and thus allows structural investigation of eukaryotic membrane proteins. This is the case for the cell-free synthesized hepatitis C virus (HCV) nonstructural membrane protein 4B (NS4B). We demonstrate NS4B sample optimization using fast reconstitution schemes that enable lipid-environment screening directly by NMR. 2D spectra and relaxation properties guide the choice of the best sample preparation to record 2D 1 H-detected 1 H,15 N and 3D 1 H,13 C,15 N correlation experiments with linewidths and sensitivity suitable to initiate sequential assignments. Amino-acid-selectively labeled NS4B can be readily obtained using cell-free synthesis, opening the door to combinatorial labeling approaches which should enable structural studies.
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Affiliation(s)
- Vlastimil Jirasko
- ETH ZürichPhysical ChemistryVladimir-Prelog Weg 28093ZürichSwitzerland
| | | | - Susanne Penzel
- ETH ZürichPhysical ChemistryVladimir-Prelog Weg 28093ZürichSwitzerland
| | - Marie‐Laure Fogeron
- Institut de Biologie et Chimie des ProteinesMMSBLabex EcofectUMR 5086 CNRSUniversité de Lyon7 passage du Vercors69367LyonFrance
| | - Ralf Bartenschlager
- Department of Infectious DiseasesMolecular VirologyHeidelberg UniversityIm Neuenheimer Feld 34569120HeidelbergGermany
- Division of Virus-Associated Carcinogenesis (Germany)Cancer Research Center (DKFZ)Im Neuenheimer Feld 24269120HeidelbergGermany
| | - Beat H. Meier
- ETH ZürichPhysical ChemistryVladimir-Prelog Weg 28093ZürichSwitzerland
| | - Anja Böckmann
- Institut de Biologie et Chimie des ProteinesMMSBLabex EcofectUMR 5086 CNRSUniversité de Lyon7 passage du Vercors69367LyonFrance
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7
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Munro RA, de Vlugt J, Ward ME, Kim SY, Lee KA, Jung KH, Ladizhansky V, Brown LS. Biosynthetic production of fully carbon-13 labeled retinal in E. coli for structural and functional studies of rhodopsins. JOURNAL OF BIOMOLECULAR NMR 2019; 73:49-58. [PMID: 30719609 DOI: 10.1007/s10858-019-00225-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/26/2018] [Accepted: 01/16/2019] [Indexed: 06/09/2023]
Abstract
The isomerization of a covalently bound retinal is an integral part of both microbial and animal rhodopsin function. As such, detailed structure and conformational changes in the retinal binding pocket are of significant interest and are studied in various NMR, FTIR, and Raman spectroscopy experiments, which commonly require isotopic labeling of retinal. Unfortunately, the de novo organic synthesis of an isotopically-labeled retinal is complex and often cost-prohibitive, especially for large scale expression required for solid-state NMR. We present the novel protocol for biosynthetic production of an isotopically labeled retinal ligand concurrently with an apoprotein in E. coli as a cost-effective alternative to the de novo organic synthesis. Previously, the biosynthesis of a retinal precursor, β-carotene, has been introduced into many different organisms. We extended this system to the prototrophic E. coli expression strain BL21 in conjunction with the inducible expression of a β-dioxygenase and proteo-opsin. To demonstrate the applicability of this system, we were able to assign several new carbon resonances for proteorhodopsin-bound retinal by using fully 13C-labeled glucose as the sole carbon source. Furthermore, we demonstrated that this biosynthetically produced retinal can be extracted from E. coli cells by applying a hydrophobic solvent layer to the growth medium and reconstituted into an externally produced opsin of any desired labeling pattern.
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Affiliation(s)
- Rachel A Munro
- Departments of Physics, and Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada
| | - Jeffrey de Vlugt
- Departments of Physics, and Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada
| | - Meaghan E Ward
- Departments of Physics, and Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada
| | - So Young Kim
- Deptartment of Life Science, Institute of Biological Interfaces, Sogang University, Shinsu-Dong 1, Mapo-Gu, Seoul, 121-742, Republic of Korea
- Division of Biotechnology, College of Environmental & Bioresource Sciences, Chonbuk National University, Jeonju, Republic of Korea
| | - Keon Ah Lee
- Deptartment of Life Science, Institute of Biological Interfaces, Sogang University, Shinsu-Dong 1, Mapo-Gu, Seoul, 121-742, Republic of Korea
| | - Kwang-Hwan Jung
- Deptartment of Life Science, Institute of Biological Interfaces, Sogang University, Shinsu-Dong 1, Mapo-Gu, Seoul, 121-742, Republic of Korea
| | - Vladimir Ladizhansky
- Departments of Physics, and Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada
| | - Leonid S Brown
- Departments of Physics, and Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, ON, N1G 2W1, Canada.
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8
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Brinkmann A, Sternberg U, Bovee-Geurts PHM, Fernández Fernández I, Lugtenburg J, Kentgens APM, DeGrip WJ. Insight into the chromophore of rhodopsin and its Meta-II photointermediate by 19F solid-state NMR and chemical shift tensor calculations. Phys Chem Chem Phys 2018; 20:30174-30188. [PMID: 30484791 DOI: 10.1039/c8cp05886e] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
19F nuclei are useful labels in solid-state NMR studies, since their chemical shift and tensor elements are very sensitive to the electrostatic and space-filling properties of their local environment. In this study we have exploited a fluorine substituent, strategically placed at the C-12-position of 11-cis retinal, the chromophore of visual rhodopsins. This label was used to explore the local environment of the chromophore in the ground state of bovine rhodopsin and its active photo-intermediate Meta II. In addition, the chemical shift and tensor elements of the chromophore in the free state in a membrane environment and the bound state in the protein were determined. Upon binding of the chromophore into rhodopsin and Meta II, the isotropic chemical shift changes in the opposite direction by +9.7 and -8.4 ppm, respectively. An unusually large isotropic shift difference of 35.9 ppm was observed between rhodopsin and Meta II. This partly originates in the light-triggered 11-cis to all-trans isomerization of the chromophore. The other part reflects the local conformational rearrangements in the chromophore and the binding pocket. These NMR data were correlated with the available X-ray structures of rhodopsin and Meta II using bond polarization theory. For this purpose hydrogen atoms have to be inserted and hereto a family of structures were derived that best correlated with the well-established 13C chemical shifts. Based upon these structures, a 12-F derivative was obtained that best corresponded with the experimentally determined 19F chemical shifts and tensor elements. The combined data indicate strong changes in the local environment of the C-12 position and a substantially different interaction pattern with the protein in Meta II as compared to rhodopsin.
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Affiliation(s)
- Andreas Brinkmann
- Metrology, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada.
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9
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Lends A, Ravotti F, Zandomeneghi G, Böckmann A, Ernst M, Meier BH. Direct amide 15N to 13C transfers for solid-state assignment experiments in deuterated proteins. JOURNAL OF BIOMOLECULAR NMR 2018; 72:69-78. [PMID: 30206780 DOI: 10.1007/s10858-018-0207-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/29/2018] [Accepted: 09/01/2018] [Indexed: 06/08/2023]
Abstract
The assignment of protein backbone and side-chain NMR chemical shifts is the first step towards the characterization of protein structure. The recent introduction of proton detection in combination with fast MAS has opened up novel opportunities for assignment experiments. However, typical 3D sequential-assignment experiments using proton detection under fast MAS lead to signal intensities much smaller than the theoretically expected ones due to the low transfer efficiency of some of the steps. Here, we present a selective 3D experiment for deuterated and (amide) proton back-exchanged proteins where polarization is directly transferred from backbone nitrogen to selected backbone or sidechain carbons. The proposed pulse sequence uses only 1H-15N cross-polarization (CP) transfers, which are, for deuterated proteins, about 30% more efficient than 1H-13C CP transfers, and employs a dipolar version of the INEPT experiment for N-C transfer. By avoiding HN-C (HN stands for amide protons) and C-C CP transfers, we could achieve higher selectivity and increased signal intensities compared to other pulse sequences containing long-range CP transfers. The REDOR transfer is designed with an additional selective π pulse, which enables the selective transfer of the polarization to the desired 13C spins.
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Affiliation(s)
- Alons Lends
- Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zurich, Switzerland
| | - Francesco Ravotti
- Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zurich, Switzerland
| | - Giorgia Zandomeneghi
- Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zurich, Switzerland
| | - Anja Böckmann
- Molecular Microbiology and Structural Biochemistry, Labex Ecofect, UMR 5086 CNRS/Université de Lyon, 69367, Lyon, France
| | - Matthias Ernst
- Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zurich, Switzerland.
| | - Beat H Meier
- Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zurich, Switzerland.
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Coupled HOOP signature correlates with quantum yield of isorhodopsin and analog pigments. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2016; 1858:118-125. [PMID: 27836700 DOI: 10.1016/j.bbabio.2016.11.003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/02/2016] [Revised: 09/29/2016] [Accepted: 11/04/2016] [Indexed: 11/21/2022]
Abstract
With a quantum yield of 0.66±0.03 the photoisomerization efficiency of the visual pigment rhodopsin (11-cis⇒all-trans chromophore) is exceptionally high. This is currently explained by coherent coupling of the excited state electronic wavepacket with local vibrational nuclear modes, facilitating efficient cross-over at a conical intersection onto the photoproduct energy surface. The 9-cis counterpart of rhodopsin, dubbed isorhodopsin, has a much lower quantum yield (0.26±0.03), which, however, can be markedly enhanced by modification of the retinal chromophore (7,8-dihydro and 9-cyclopropyl derivatives). The coherent coupling in the excited state is promoted by torsional skeletal and coupled HOOP vibrational modes, in combination with a twisted conformation around the isomerization region. Since such torsion will strongly enhance the infrared intensity of coupled HOOP modes, we investigated FTIR difference spectra of rhodopsin, isorhodopsin and several analog pigments in the spectral range of isolated and coupled HCCH wags. As a result we propose that the coupled HOOP signature in these retinal pigments correlates with the distribution of torsion over counteracting segments in the retinylidene polyene chain. As such the HOOP signature can act as an indicator for the photoisomerization efficiency, and can explain the higher quantum yield of the 7,8-dihydro and 9-cyclopropyl-isorhodopsin analogs.
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11
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Feldman TB, Smitienko OA, Shelaev IV, Gostev FE, Nekrasova OV, Dolgikh DA, Nadtochenko VA, Kirpichnikov MP, Ostrovsky MA. Femtosecond spectroscopic study of photochromic reactions of bacteriorhodopsin and visual rhodopsin. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY. B, BIOLOGY 2016; 164:296-305. [PMID: 27723489 DOI: 10.1016/j.jphotobiol.2016.09.041] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/07/2016] [Revised: 09/29/2016] [Accepted: 09/30/2016] [Indexed: 11/16/2022]
Abstract
Photochromic ultrafast reactions of bacteriorhodopsin (H. salinarum) and bovine rhodopsin were conducted with a femtosecond two-pump probe pulse setup with the time resolution of 20-25fs. The dynamics of the forward and reverse photochemical reactions for both retinal-containing proteins was compared. It is demonstrated that when retinal-containing proteins are excited by femtosecond pulses, dynamics pattern of the vibrational coherent wave packets in the course of the reaction is different for bacteriorhodopsin and visual rhodopsin. As shown in these studies, the low-frequencies that form a wave packets experimentally observed in the dynamics of primary products formation as a result of retinal photoisomerization have different intensities and are clearer for bovine rhodopsin. Photo-reversible reactions for both retinal proteins were performed from the stage of the relatively stable photointermediates that appear within 3-5ps after the light pulse impact. It is demonstrated that the efficiency of the reverse phototransition K-form→bacteriorhodopsin is almost five-fold higher than that of the Batho-intermediate→visual rhodopsin phototransition. The results obtained indicate that in the course of evolution the intramolecular mechanism of the chromophore-protein interaction in visual rhodopsin becomes more perfect and specific. The decrease in the probability of the reverse chromophore photoisomerization (all-trans→11-cis retinal) in primary photo-induced rhodopsin products causes an increase in the efficiency of the photoreception process.
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Affiliation(s)
- Tatiana B Feldman
- Biological Faculty, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 119991, Russia; Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119334, Russia.
| | - Olga A Smitienko
- Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119334, Russia
| | - Ivan V Shelaev
- Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119991, Russia
| | - Fedor E Gostev
- Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119991, Russia
| | - Oksana V Nekrasova
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya st. 16/10, Moscow 117997, Russia
| | - Dmitriy A Dolgikh
- Biological Faculty, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 119991, Russia; Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya st. 16/10, Moscow 117997, Russia
| | - Victor A Nadtochenko
- Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119334, Russia; Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119991, Russia; Institute of Problems of Chemical Physics, Russian Academy of Sciences, Academician Semenov avenue 1, Chernogolovka, Moscow region 142432, Russia
| | - Mikhail P Kirpichnikov
- Biological Faculty, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 119991, Russia; Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya st. 16/10, Moscow 117997, Russia
| | - Mikhail A Ostrovsky
- Biological Faculty, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 119991, Russia; Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin st.4, Moscow 119334, Russia
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12
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Brown LS, Ladizhansky V. Membrane proteins in their native habitat as seen by solid-state NMR spectroscopy. Protein Sci 2015; 24:1333-46. [PMID: 25973959 DOI: 10.1002/pro.2700] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2015] [Revised: 04/07/2015] [Accepted: 04/09/2015] [Indexed: 12/21/2022]
Abstract
Membrane proteins play many critical roles in cells, mediating flow of material and information across cell membranes. They have evolved to perform these functions in the environment of a cell membrane, whose physicochemical properties are often different from those of common cell membrane mimetics used for structure determination. As a result, membrane proteins are difficult to study by traditional methods of structural biology, and they are significantly underrepresented in the protein structure databank. Solid-state Nuclear Magnetic Resonance (SSNMR) has long been considered as an attractive alternative because it allows for studies of membrane proteins in both native-like membranes composed of synthetic lipids and in cell membranes. Over the past decade, SSNMR has been rapidly developing into a major structural method, and a growing number of membrane protein structures obtained by this technique highlights its potential. Here we discuss membrane protein sample requirements, review recent progress in SSNMR methodologies, and describe recent advances in characterizing membrane proteins in the environment of a cellular membrane.
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Affiliation(s)
- Leonid S Brown
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario, Canada, N1G 2W1
| | - Vladimir Ladizhansky
- Department of Physics and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario, Canada, N1G 2W1
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Sproviero EM. Opsin Effect on the Electronic Structure of the Retinylidene Chromophore in Rhodopsin. J Chem Theory Comput 2015; 11:1206-19. [PMID: 26579769 DOI: 10.1021/ct500612n] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Direct examination of experimental NMR parameters combined with electronic structure analysis was used to provide a first-principle interpretation of NMR experiments and give a precise evaluation of how the electronic perturbation of the protein environment affects the electronic properties of the retinylidene chromophere in rhodopsin. To this end, we pursued a theoretical analysis using a combination of tools including quantum mechanics/molecular mechanics (QM/MM) at the Density Functional Theory (DFT) level, in conjunction with gauge independent atomic orbital (GIAO) calculations of (13)C NMR chemical shieldings and (1)J(CC) spin-spin coupling constants obtained with the Coupled Perturbed DFT (CPDFT) method. The opsin effect on the retinylidene chromophere is interpreted as an inductive effect of Glu-113 which readjusts the weighting factors of resonance substructures of the conjugated chain of the chromophere. These changes give a rationalization to the alternating effect of the (13)C chemical shifts magnitudes when comparing the retinylidene chromophere in the presence and absence of the protein environment. Conversely, perturbation of π orbitals has little to no effect over (1)J (13)C-(13)C spin-spin coupling constants, as they are mainly dominated by the Fermi contact term, and hence the counteraion effect is restricted to the vicinity of the perturbation. Thus, the apparent contradiction between experimental findings based on chemical shifts (deep penetration) and one-bond J-couplings (localized effects of the protonated Schiff base at the chain terminus) is in fact a consequence of different properties responding differently to the same external perturbation.
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Affiliation(s)
- Eduardo M Sproviero
- Department of Chemistry & Biochemistry, University of the Sciences in Philadelphia , 600 South 43rd Street, Philadelphia, Pennsylvania 19104-4495, United States
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Wang S, Ladizhansky V. Recent advances in magic angle spinning solid state NMR of membrane proteins. PROGRESS IN NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 2014; 82:1-26. [PMID: 25444696 DOI: 10.1016/j.pnmrs.2014.07.001] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/03/2014] [Revised: 07/16/2014] [Accepted: 07/20/2014] [Indexed: 05/14/2023]
Abstract
Membrane proteins mediate many critical functions in cells. Determining their three-dimensional structures in the native lipid environment has been one of the main objectives in structural biology. There are two major NMR methodologies that allow this objective to be accomplished. Oriented sample NMR, which can be applied to membrane proteins that are uniformly aligned in the magnetic field, has been successful in determining the backbone structures of a handful of membrane proteins. Owing to methodological and technological developments, Magic Angle Spinning (MAS) solid-state NMR (ssNMR) spectroscopy has emerged as another major technique for the complete characterization of the structure and dynamics of membrane proteins. First developed on peptides and small microcrystalline proteins, MAS ssNMR has recently been successfully applied to large membrane proteins. In this review we describe recent progress in MAS ssNMR methodologies, which are now available for studies of membrane protein structure determination, and outline a few examples, which highlight the broad capability of ssNMR spectroscopy.
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Affiliation(s)
- Shenlin Wang
- Beijing Nuclear Magnetic Resonance Center, Peking University, Beijing 100871, China; College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
| | - Vladimir Ladizhansky
- Department of Physics, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada; Biophysics Interdepartmental Group, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada.
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15
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Pope A, Eilers M, Reeves PJ, Smith SO. Amino acid conservation and interactions in rhodopsin: probing receptor activation by NMR spectroscopy. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2013; 1837:683-93. [PMID: 24183693 DOI: 10.1016/j.bbabio.2013.10.007] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/16/2013] [Revised: 10/15/2013] [Accepted: 10/18/2013] [Indexed: 11/30/2022]
Abstract
Rhodopsin is a classical two-state G protein-coupled receptor (GPCR). In the dark, its 11-cis retinal chromophore serves as an inverse agonist to lock the receptor in an inactive state. Retinal-protein and protein-protein interactions have evolved to reduce the basal activity of the receptor in order to achieve low dark noise in the visual system. In contrast, absorption of light triggers rapid isomerization of the retinal, which drives the conversion of the receptor to a fully active conformation. Several specific protein-protein interactions have evolved that maintain the lifetime of the active state in order to increase the sensitivity of this receptor for dim-light vision in vertebrates. In this article, we review the molecular interactions that stabilize rhodopsin in the dark-state and describe the use of solid-state NMR spectroscopy for probing the structural changes that occur upon light-activation. Amino acid conservation provides a guide for those interactions that are common in the class A GPCRs as well as those that are unique to the visual system. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.
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Affiliation(s)
- Andreyah Pope
- Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794-5215, USA
| | - Markus Eilers
- Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794-5215, USA
| | - Philip J Reeves
- School of Biological Sciences, University of Essex, Wivenhoe Park, Essex CO4 3SQ, UK
| | - Steven O Smith
- Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794-5215, USA.
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16
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Colherinhas G, Fonseca TL, Castro MA, Coutinho K, Canuto S. Isotropic magnetic shielding constants of retinal derivatives in aprotic and protic solvents. J Chem Phys 2013; 139:094502. [PMID: 24028122 DOI: 10.1063/1.4819694] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We investigate the nuclear isotropic shielding constants σ((13)C) and σ((17)O) of isomers of retinoic acid and retinal in gas-phase and in chloroform, acetonitrile, methanol, and water solutions via Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP∕6-311++G(2d,2p) approach. Electronic solute polarization effects due to protic and aprotic solvents are included iteratively and play an important role in the quantitative determination of oxygen shielding constants. Our MP2∕6-31G+(d) results show substantial increases of the dipole moment of both retinal derivatives in solution as compared with the gas-phase results (between 22% and 26% in chloroform and between 55% and 99% in water). For the oxygen atoms the influence of the solute polarization is mild for σ((17)O) of hydroxyl group, even in protic solvents, but it is particularly important for σ((17)O) of carbonyl group. For the latter, there is a sizable increase in the magnitude with increasing solvent polarity. For the carbon atoms, the solvent effects on the σ((13)C) values are in general small, being more appreciable in carbon atoms of the polyene chain than in the carbon atoms of the β-ionone ring and methyl groups. The results also show that isomeric changes on the backbones of the polyene chains have marked influence on the (13)C chemical shifts of carbon atoms near to the structural distortions, in good agreement with the experimental results measured in solution.
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Affiliation(s)
- G Colherinhas
- Instituto de Física, Universidade Federal de Goiás, CP 131, 74001-970 Goia^nia, GO, Brazil
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Kuemmel CM, Sandberg MN, Birge RR, Knox BE. A Conserved Aromatic Residue Regulating Photosensitivity in Short-Wavelength Sensitive Cone Visual Pigments. Biochemistry 2013; 52:5084-91. [DOI: 10.1021/bi400490g] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Colleen M. Kuemmel
- Departments of Neuroscience
and Physiology, Biochemistry and Molecular Biology, and Ophthalmology, SUNY Upstate Medical University, Syracuse, New York
13210, United States
| | - Megan N. Sandberg
- Departments
of Chemistry and
Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut
06269, United States
| | - Robert R. Birge
- Departments
of Chemistry and
Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut
06269, United States
| | - Barry E. Knox
- Departments of Neuroscience
and Physiology, Biochemistry and Molecular Biology, and Ophthalmology, SUNY Upstate Medical University, Syracuse, New York
13210, United States
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18
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Alia A, Buda F, de Groot HJ, Matysik J. Solid-State NMR of Nanomachines Involved in Photosynthetic Energy Conversion. Annu Rev Biophys 2013; 42:675-99. [DOI: 10.1146/annurev-biophys-083012-130415] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Magic-angle spinning NMR, often in combination with photo-CIDNP, is applied to determine how photosynthetic antennae and reaction centers are activated in the ground state to perform their biological function upon excitation by light. Molecular modeling resolves molecular mechanisms by way of computational integration of NMR data with other structure-function analyses. By taking evolutionary historical contingency into account, a better biophysical understanding is achieved. Chlorophyll cofactors and proteins go through self-assembly trajectories that are engineered during evolution and lead to highly homogeneous protein complexes optimized for exciton or charge transfer. Histidine-cofactor interactions allow biological nanomachines to lower energy barriers for light harvesting and charge separation in photosynthetic energy conversion. In contrast, in primordial chlorophyll antenna aggregates, excessive heterogeneity is paired with much less specific characteristics, and both exciton and charge-transfer character are encoded in the ground state.
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Affiliation(s)
- A. Alia
- Solid State NMR, Leiden Institute of Chemistry, Leiden University, Leiden 2300 RB, The Netherlands;, , ,
| | - Francesco Buda
- Solid State NMR, Leiden Institute of Chemistry, Leiden University, Leiden 2300 RB, The Netherlands;, , ,
| | - Huub J.M. de Groot
- Solid State NMR, Leiden Institute of Chemistry, Leiden University, Leiden 2300 RB, The Netherlands;, , ,
| | - Jörg Matysik
- Solid State NMR, Leiden Institute of Chemistry, Leiden University, Leiden 2300 RB, The Netherlands;, , ,
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19
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G-protein-coupled receptor structure, ligand binding and activation as studied by solid-state NMR spectroscopy. Biochem J 2013; 450:443-57. [DOI: 10.1042/bj20121644] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
GPCRs (G-protein-coupled receptors) are versatile signalling molecules at the cell surface and make up the largest and most diverse family of membrane receptors in the human genome. They convert a large variety of extracellular stimuli into intracellular responses through the activation of heterotrimeric G-proteins, which make them key regulatory elements in a broad range of normal and pathological processes, and are therefore one of the most important targets for pharmaceutical drug discovery. Knowledge of a GPCR structure enables us to gain a mechanistic insight into its function and dynamics, and further aid rational drug design. Despite intensive research carried out over the last three decades, resolving the structural basis of GPCR function is still a major activity. The crystal structures obtained in the last 5 years provide the first opportunity to understand how protein structure dictates the unique functional properties of these complex signalling molecules. However, owing to the intrinsic hydrophobicity, flexibility and instability of membrane proteins, it is still a challenge to crystallize GPCRs, and, when this is possible, it is no longer in its native membrane environment and no longer without modification. Furthermore, the conformational change of the transmembrane α-helices associated with the structure activation increases the difficulty of capturing the activation state of a GPCR to a higher resolution by X-ray crystallography. On the other hand, solid-state NMR may offer a unique opportunity to study membrane protein structure, ligand binding and activation at atomic resolution in the native membrane environment, as well as described functionally significant dynamics. In the present review, we discuss some recent achievements of solid-state NMR for understanding GPCRs, the largest mammalian proteome at ~1% of the total expressed proteins. Structural information, details of determination, details of ligand conformations and the consequences of ligand binding to initiate activation can all be explored with solid-state NMR.
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20
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Kaczor A, Reva I, Fausto R. Influence of cage confinement on the photochemistry of matrix-isolated E-β-ionone: FT-IR and DFT study. J Phys Chem A 2013; 117:888-97. [PMID: 23305459 DOI: 10.1021/jp310764u] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
β-ionone, a model compound of carotenoids ring structure, was investigated by FT-IR spectroscopy in a low-temperature argon matrix as well as using B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) quantum-chemical calculations. The spectrum of matrix-isolated E-β-ionone was analyzed and attributed to six conformers of the compound. Then, matrix-isolated E-β-ionone was submitted to UV irradiation using either a broadband source (with different cutoff filters) or a narrowband laser/MOPO system (at various wavelengths). Upon 240 nm narrowband irradiation, the formation of both Z-retro-γ-ionone and Z-β-ionone was observed, the reactant and the photoproducts being in a photostationary equilibrium. Under these conditions, the matrix environment was found to hamper subsequent reactions of Z-retro-γ-ionone and Z-β-ionone, so that this last species could be observed directly for the first time. Furthermore, the formation of Z-retro-γ-ionone was shown to occur directly via an intramolecular [1,5] H-atom shift and thereby, under the constraints imposed by the matrix confinement, the conformations assumed by this photoproduct were found to be strictly determined by those initially assumed by the reactant molecules. Broadband irradiation resulted in the completion of the reaction (disappearance of the reactant) and the sole observation of Z-retro-γ-ionone. These results imply that under these conditions the Z-β-ionone is unstable, very likely decaying to additional conformers of Z-retro-γ-ionone, as reflected in the broader bands due to this photoproduct observed in the infrared spectra of the broadband irradiated matrix.
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Affiliation(s)
- A Kaczor
- Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland.
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21
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Concistrè M, Johannessen OG, McLean N, Bovee-Geurts PHM, Brown RCD, Degrip WJ, Levitt MH. A large geometric distortion in the first photointermediate of rhodopsin, determined by double-quantum solid-state NMR. JOURNAL OF BIOMOLECULAR NMR 2012; 53:247-256. [PMID: 22639195 DOI: 10.1007/s10858-012-9635-4] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2012] [Accepted: 05/01/2012] [Indexed: 06/01/2023]
Abstract
Double-quantum magic-angle-spinning NMR experiments were performed on 11,12-(13)C(2)-retinylidene-rhodopsin under illumination at low temperature, in order to characterize torsional angle changes at the C11-C12 photoisomerization site. The sample was illuminated in the NMR rotor at low temperature (~120 K) in order to trap the primary photointermediate, bathorhodopsin. The NMR data are consistent with a strong torsional twist of the HCCH moiety at the isomerization site. Although the HCCH torsional twist was determined to be at least 40°, it was not possible to quantify it more closely. The presence of a strong twist is in agreement with previous Raman observations. The energetic implications of this geometric distortion are discussed.
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Affiliation(s)
- Maria Concistrè
- School of Chemistry, University of Southampton, SO17 1BJ, Southampton, UK.
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22
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Carlomagno T. NMR in natural products: understanding conformation, configuration and receptor interactions. Nat Prod Rep 2012; 29:536-54. [PMID: 22456471 DOI: 10.1039/c2np00098a] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Covering: up to 2011. Natural products are of tremendous importance in both traditional and modern medicine. For medicinal chemistry natural products represent a challenge, as their chemical synthesis and modification are complex processes, which require many, often stereo-selective, synthetic steps. A prerequisite for the design of analogs of natural products, with more accessible synthetic routes, is the availability of their bioactive conformation. Nuclear Magnetic Resonance (NMR) spectroscopy and X-ray crystallography are the two techniques of choice to investigate the structure of natural products. In this review, I describe the most recent advances in NMR to study the conformation of natural products either free in solution or bound to their cellular receptors. In chapter 2, I focus on the use of residual dipolar couplings (RDC). On the basis of a few examples, I discuss the benefit of complementing classical NMR parameters, such as NOEs and scalar couplings, with dipolar couplings to simultaneously determine both the conformation and the relative configuration of natural products in solution. Chapter 3 is dedicated to the study of the structure of natural products in complex with their cellular receptors and is further divided in two sections. In the first section, I describe two solution-state NMR methodologies to investigate the binding mode of low-affinity ligands to macromolecular receptors. The first approach, INPHARMA (Interligand Noes for PHArmacophore Mapping), is based on the observation of interligand NOEs between two small molecules binding competitively to a common receptor. INPHARMA reveals the relative binding mode of the two ligands, thus allowing ligand superimposition. The second approach is based on paramagnetic relaxation enhancement (PRE) of ligand resonances in the presence of a receptor containing a paramagnetic center. In the second section, I focus on solid-state NMR spectroscopy as a tool to access the bioactive conformation of natural products in complex with macromolecular receptors.
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Affiliation(s)
- Teresa Carlomagno
- European Molecular Biology Laboratory, Structural and Computational Biology Unit, Meyerhofstrasse 1, D-69117 Heidelberg
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23
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Mertz B, Struts AV, Feller SE, Brown MF. Molecular simulations and solid-state NMR investigate dynamical structure in rhodopsin activation. BIOCHIMICA ET BIOPHYSICA ACTA 2012; 1818:241-51. [PMID: 21851809 PMCID: PMC5270601 DOI: 10.1016/j.bbamem.2011.08.003] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/26/2011] [Revised: 08/01/2011] [Accepted: 08/01/2011] [Indexed: 10/17/2022]
Abstract
Rhodopsin has served as the primary model for studying G protein-coupled receptors (GPCRs)-the largest group in the human genome, and consequently a primary target for pharmaceutical development. Understanding the functions and activation mechanisms of GPCRs has proven to be extraordinarily difficult, as they are part of a complex signaling cascade and reside within the cell membrane. Although X-ray crystallography has recently solved several GPCR structures that may resemble the activated conformation, the dynamics and mechanism of rhodopsin activation continue to remain elusive. Notably solid-state ((2))H NMR spectroscopy provides key information pertinent to how local dynamics of the retinal ligand change during rhodopsin activation. When combined with molecular mechanics simulations of proteolipid membranes, a new paradigm for the rhodopsin activation process emerges. Experiment and simulation both suggest that retinal isomerization initiates the rhodopsin photocascade to yield not a single activated structure, but rather an ensemble of activated conformational states. This article is part of a Special Issue entitled: Membrane protein structure and function.
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Affiliation(s)
- Blake Mertz
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, USA
| | - Andrey V. Struts
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, USA
- Department of Medical Physics, St. Petersburg State Medical University, St. Petersburg 194100, Russia
| | - Scott E. Feller
- Department of Chemistry, Wabash College, Crawfordsville, IN 47933, USA
| | - Michael F. Brown
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, USA
- Department of Physics, University of Arizona, Tucson, AZ 85721, USA
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Solid-state 2H NMR relaxation illuminates functional dynamics of retinal cofactor in membrane activation of rhodopsin. Proc Natl Acad Sci U S A 2011; 108:8263-8. [PMID: 21527723 DOI: 10.1073/pnas.1014692108] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Rhodopsin is a canonical member of the family of G protein-coupled receptors, which transmit signals across cellular membranes and are linked to many drug interventions in humans. Here we show that solid-state (2)H NMR relaxation allows investigation of light-induced changes in local ps-ns time scale motions of retinal bound to rhodopsin. Site-specific (2)H labels were introduced into methyl groups of the retinal ligand that are essential to the activation process. We conducted solid-state (2)H NMR relaxation (spin-lattice, T(1Z), and quadrupolar-order, T(1Q)) experiments in the dark, Meta I, and Meta II states of the photoreceptor. Surprisingly, we find the retinylidene methyl groups exhibit site-specific differences in dynamics that change upon light excitation--even more striking, the C9-methyl group is a dynamical hotspot that corresponds to a crucial functional hotspot of rhodopsin. Following 11-cis to trans isomerization, the (2)H NMR data suggest the β-ionone ring remains in its hydrophobic binding pocket in all three states of the protein. We propose a multiscale activation mechanism with a complex energy landscape, whereby the photonic energy is directed against the E2 loop by the C13-methyl group, and toward helices H3 and H5 by the C5-methyl of the β-ionone ring. Changes in retinal structure and dynamics initiate activating fluctuations of transmembrane helices H5 and H6 in the Meta I-Meta II equilibrium of rhodopsin. Our proposals challenge the Standard Model whereby a single light-activated receptor conformation yields the visual response--rather an ensemble of substates is present, due to the entropy gain produced by photolysis of the inhibitory retinal lock.
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25
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deGrip WJ, Bovee-Geurts PHM, Wang Y, Verhoeven MA, Lugtenburg J. Cyclopropyl and isopropyl derivatives of 11-cis and 9-cis retinals at C-9 and C-13: subtle steric differences with major effects on ligand efficacy in rhodopsin. JOURNAL OF NATURAL PRODUCTS 2011; 74:383-390. [PMID: 21309593 DOI: 10.1021/np100744v] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Retinal is the natural ligand (chromophore) of the vertebrate rod visual pigment. It occurs in either the 11-cis (rhodopsin) or the 9-cis (isorhodopsin) configuration. In its evolution to a G protein coupled photoreceptor, rhodopsin has acquired exceptional photochemical properties. Illumination isomerizes the chromophore to the all-trans isomer, which acts as a full agonist. This process is extremely efficient, and there is abundant evidence that the C-9 and C-13 methyl groups of retinal play a pivotal role in this process. To examine the steric limits of the C-9 and C-13 methyl binding pocket of the binding site, we have prepared C-9 and C-13 cyclopropyl and isopropyl derivatives of its native ligands and of α-retinal at C-9. Most isopropyl analogues show very poor binding, except for 9-cis-13-isopropylretinal. Most cyclopropyl derivatives exhibit intermediate binding activity, except for 9-cis-13-cyclopropylretinal, which presents good binding activity. In general, the binding site shows preference for the 9-cis analogues over the 11-cis analogues. In fact, 13-isopropyl-9-cis-retinal acts as a superagonist after illumination. Another surprising finding was that 9-cyclopropylisorhodopsin is more like native rhodopsin with respect to spectral and photochemical properties, whereas 9-cyclopropylrhodopsin behaves more like native isorhodopsin in these aspects.
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Affiliation(s)
- Willem J deGrip
- Department of Biochemistry, UMCN 286, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands.
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27
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Akinaga Y, Jung J, Ten-no S. QM/MM calculation of protein magnetic shielding tensors with generalized hybrid-orbital method: A GIAO approach. Phys Chem Chem Phys 2011; 13:14490-9. [DOI: 10.1039/c1cp21001g] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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Frähmcke JS, Wanko M, Phatak P, Mroginski MA, Elstner M. The protonation state of Glu181 in rhodopsin revisited: interpretation of experimental data on the basis of QM/MM calculations. J Phys Chem B 2010; 114:11338-52. [PMID: 20698519 DOI: 10.1021/jp104537w] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
The structure and spectroscopy of rhodopsin have been intensely studied in the past decade both experimentally and theoretically; however, important issues still remain unresolved. Of central interest is the protonation state of Glu181, where controversial and contradictory experimental evidence has appeared. While FTIR measurements indicate this residue to be unprotonated, preresonance Raman and UV-vis spectra have been interpreted in favor of a protonated Glu181. Previous computational approaches were not able to resolve this issue, providing contradicting data as well. Here, we perform hybrid QM/MM calculations using DFT methods for the electronic ground state, MRCI methods for the electronically excited states, and a polarization model for the MM part in order to investigate this issue systematically. We constructed various active-site models for protonated as well as unprotonated Glu181, which were evaluated by computing NMR, IR, Raman, and UV-vis spectroscopic data. The resulting differences in the UV-vis and Raman spectra between protonated and unprotonated models are very subtle, which has two major consequences. First, the common interpretation of prior Raman and UV-vis experiments in favor of a neutral Glu181 appears questionable, as it is based on the assumption that a charge at the Glu181 location would have a sizable impact. Second, also theoretical results should be interpreted with care. Spectroscopic differences between the structural models must be related to modeling uncertainties and intrinsic methodological errors. Despite a detailed comparison of various rhodopsins and mutants and consistently favorite results with charged Glu181 models, we find merely weak evidence from UV-vis and Raman calculations. On the contrary, difference FTIR and NMR chemical shift measurements on Rh mutants are indicative of the protonation state of Glu181. Supported by our results, they provide strong and independent evidence for a charged Glu181.
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Affiliation(s)
- Jan S Frähmcke
- Institute for Physical and Theoretical Chemistry, TU Braunschweig, Hans-Sommer-Str. 10, D-38106 Braunschweig, Germany
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Dawadi PBS, Lugtenburg J. Synthesis and use of stable isotope enriched retinals in the field of vitamin A. Molecules 2010; 15:1825-72. [PMID: 20336016 PMCID: PMC6257204 DOI: 10.3390/molecules15031825] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2010] [Revised: 02/18/2010] [Accepted: 03/02/2010] [Indexed: 01/05/2023] Open
Abstract
The role of vitamin A and its metabolites in the life processes starting with the historical background and its up to date information is discussed in the introduction. Also the role of 11Z-retinal in vision and retinoic acid in the biological processes is elucidated. The essential role of isotopically enriched systems in the progress of vision research, nutrition research etc. is discussed. In part B industrial commercial syntheses of vitamin A by the two leading companies Hoffmann-La Roche (now DSM) and BASF are discussed. The knowledge obtained via these pioneering syntheses has been essential for the further synthetic efforts in vitamin A field by other scientific groups. The rest of the paper is devoted to the synthetic efforts of the Leiden group that gives an access to the preparation of site directed high level isotope enrichment in retinals. First the synthesis of the retinals with deuterium incorporation in the conjugated side chain is reviewed. Then, 13C-labeled retinals are discussed. This is followed by the discussion of a convergent synthetic scheme that allows a rational access to prepare any isotopomer of retinals. The schemes that provide access to prepare any possible isotope enriched chemically modified systems are discussed. Finally, nor-retinals and bridged retinals that give access to a whole (as yet incomplete) library of possible isotopomers are reviewed.
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Affiliation(s)
- Prativa B S Dawadi
- Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA, Leiden, The Netherlands.
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30
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Tikhonova IG, Costanzi S. Unraveling the structure and function of G protein-coupled receptors through NMR spectroscopy. Curr Pharm Des 2010; 15:4003-16. [PMID: 20028318 DOI: 10.2174/138161209789824803] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
G protein-coupled receptors (GPCRs) are a large superfamily of signaling proteins expressed on the plasma membrane. They are involved in a wide range of physiological processes and, therefore, are exploited as drug targets in a multitude of therapeutic areas. In this extent, knowledge of structural and functional properties of GPCRs may greatly facilitate rational design of modulator compounds. Solution and solid-state nuclear magnetic resonance (NMR) spectroscopy represents a powerful method to gather atomistic insights into protein structure and dynamics. In spite of the difficulties inherent the solution of the structure of membrane proteins through NMR, these methods have been successfully applied, sometimes in combination with molecular modeling, to the determination of the structure of GPCR fragments, the mapping of receptor-ligand interactions, and the study of the conformational changes associated with the activation of the receptors. In this review, we provide a summary of the NMR contributions to the study of the structure and function of GPCRs, also in light of the published crystal structures.
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Affiliation(s)
- Irina G Tikhonova
- Laboratory of Biological Modeling, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, DHHS, Bethesda, MD 20892, USA
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31
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Ahuja S, Eilers M, Hirshfeld A, Yan ECY, Ziliox M, Sakmar TP, Sheves M, Smith SO. 6-s-cis Conformation and polar binding pocket of the retinal chromophore in the photoactivated state of rhodopsin. J Am Chem Soc 2010; 131:15160-9. [PMID: 19795853 DOI: 10.1021/ja9034768] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The visual pigment rhodopsin is unique among the G protein-coupled receptors in having an 11-cis retinal chromophore covalently bound to the protein through a protonated Schiff base linkage. The chromophore locks the visual receptor in an inactive conformation through specific steric and electrostatic interactions. This efficient inverse agonist is rapidly converted to an agonist, the unprotonated Schiff base of all-trans retinal, upon light activation. Here, we use magic angle spinning NMR spectroscopy to obtain the (13)C chemical shifts (C5-C20) of the all-trans retinylidene chromophore and the (15)N chemical shift of the Schiff base nitrogen in the active metarhodopsin II intermediate. The retinal chemical shifts are sensitive to the conformation of the chromophore and its molecular interactions within the protein-binding site. Comparison of the retinal chemical shifts in metarhodopsin II with those of retinal model compounds reveals that the Schiff base environment is polar. In particular, the (13)C15 and (15)Nepsilon chemical shifts indicate that the C horizontal lineN bond is highly polarized in a manner that would facilitate Schiff base hydrolysis. We show that a strong perturbation of the retinal (13)C12 chemical shift observed in rhodopsin is reduced in wild-type metarhodopsin II and in the E181Q mutant of rhodopsin. On the basis of the T(1) relaxation time of the retinal (13)C18 methyl group and the conjugated retinal (13)C5 and (13)C8 chemical shifts, we have determined that the conformation of the retinal C6-C7 single bond connecting the beta-ionone ring and the retinylidene chain is 6-s-cis in both the inactive and the active states of rhodopsin. These results are discussed within the general framework of ligand-activated G protein-coupled receptors.
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Affiliation(s)
- Shivani Ahuja
- Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-5215, USA
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Bovee-Geurts PHM, Fernández Fernández I, Liu RSH, Mathies RA, Lugtenburg J, DeGrip WJ. Fluoro Derivatives of Retinal Illuminate the Decisive Role of the C12-H Element in Photoisomerization and Rhodopsin Activation. J Am Chem Soc 2009; 131:17933-42. [DOI: 10.1021/ja907577p] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Petra H. M. Bovee-Geurts
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
| | - Isabelle Fernández Fernández
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
| | - Robert S. H. Liu
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
| | - Richard A. Mathies
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
| | - Johan Lugtenburg
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
| | - Willem J. DeGrip
- Department of Biochemistry, UMCN 286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, Department of BioOrganic Photochemistry, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Chemistry, University of Hawaii at Manao, 2545 The Mall, Honolulu, Hawaii 96822, and Department of Chemistry, University of California, Berkeley, California 94720
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Landrum JT, Chatfield DC, Mebel AM, Alvarez-Calderon F, Fernandez MV. The conformation of end-groups is one determinant of carotenoid topology suitable for high fidelity molecular recognition: a study of beta- and epsilon-end-groups. Arch Biochem Biophys 2009; 493:169-74. [PMID: 19850003 DOI: 10.1016/j.abb.2009.10.007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2009] [Revised: 10/13/2009] [Accepted: 10/15/2009] [Indexed: 11/18/2022]
Abstract
Conformation affects a carotenoid's ability to bind selectively to proteins. We calculated adiabatic energy profiles for rotating the ring end-groups around the C6C7 bond and for flexing of the ring with respect to the polyene chain. The choice of computational methods is important. A low, 4.2 kcal/mol barrier to rotation exists for a beta-ring. An 8.3 kcal/mol barrier exists for rotation of an epsilon-ring. Rotation of the epsilon-ring is sensitive to substitution at C3. In the absence of external forces neither beta- nor epsilon-rings are rotationally constrained. The nearly parallel alignment of the beta-ring to the C6C7 bond axis contrasts to the more perpendicular orientation of the epsilon-ring. Flexion of a beta-ring to the minimized epsilon-ring conformation requires approximately 23 kcal/mol; extension of the epsilon-ring to the minimized beta-ring conformation requires approximately 8 kcal/mol. Selectivity associated with beta- versus epsilon-rings is dominated by the inability of the beta-ring to flex to minimize protein/ring steric interactions and maximize van der Waal's attractions with the binding site.
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Affiliation(s)
- John T Landrum
- Florida International University, Department of Chemistry and Biochemistry, Miami, FL 33199, USA.
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34
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Brown MF, Salgado GFJ, Struts AV. Retinal dynamics during light activation of rhodopsin revealed by solid-state NMR spectroscopy. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2009; 1798:177-93. [PMID: 19716801 DOI: 10.1016/j.bbamem.2009.08.013] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2009] [Revised: 07/25/2009] [Accepted: 08/12/2009] [Indexed: 11/28/2022]
Abstract
Rhodopsin is a canonical member of class A of the G protein-coupled receptors (GPCRs) that are implicated in many of the drug interventions in humans and are of great pharmaceutical interest. The molecular mechanism of rhodopsin activation remains unknown as atomistic structural information for the active metarhodopsin II state is currently lacking. Solid-state (2)H NMR constitutes a powerful approach to study atomic-level dynamics of membrane proteins. In the present application, we describe how information is obtained about interactions of the retinal cofactor with rhodopsin that change with light activation of the photoreceptor. The retinal methyl groups play an important role in rhodopsin function by directing conformational changes upon transition into the active state. Site-specific (2)H labels have been introduced into the methyl groups of retinal and solid-state (2)H NMR methods applied to obtain order parameters and correlation times that quantify the mobility of the cofactor in the inactive dark state, as well as the cryotrapped metarhodopsin I and metarhodopsin II states. Analysis of the angular-dependent (2)H NMR line shapes for selectively deuterated methyl groups of rhodopsin in aligned membranes enables determination of the average ligand conformation within the binding pocket. The relaxation data suggest that the beta-ionone ring is not expelled from its hydrophobic pocket in the transition from the pre-activated metarhodopsin I to the active metarhodopsin II state. Rather, the major structural changes of the retinal cofactor occur already at the metarhodopsin I state in the activation process. The metarhodopsin I to metarhodopsin II transition involves mainly conformational changes of the protein within the membrane lipid bilayer rather than the ligand. The dynamics of the retinylidene methyl groups upon isomerization are explained by an activation mechanism involving cooperative rearrangements of extracellular loop E2 together with transmembrane helices H5 and H6. These activating movements are triggered by steric clashes of the isomerized all-trans retinal with the beta4 strand of the E2 loop and the side chains of Glu(122) and Trp(265) within the binding pocket. The solid-state (2)H NMR data are discussed with regard to the pathway of the energy flow in the receptor activation mechanism.
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Affiliation(s)
- Michael F Brown
- Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA; Department of Physics, University of Arizona, Tucson, AZ 85721, USA.
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35
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Concistrè M, Gansmüller A, McLean N, Johannessen OG, Marín Montesinos I, Bovee-Geurts PHM, Brown RCD, DeGrip WJ, Levitt MH. Light penetration and photoisomerization in rhodopsin studied by numerical simulations and double-quantum solid-state NMR spectroscopy. J Am Chem Soc 2009; 131:6133-40. [PMID: 19354207 DOI: 10.1021/ja809878c] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The penetration of light into optically thick samples containing the G-protein-coupled receptor rhodopsin is studied by numerical finite-element simulations and double-quantum solid-state NMR experiments. Illumination with white light leads to the generation of the active bathorhodopsin photostate in the outer layer of the sample but generates a large amount of the side product, isorhodopsin, in the sample interior. The overall yield of bathorhodopsin is improved by using monochromatic 420 nm illumination and by mixing the sample with transparent glass beads. The implications of these findings on the interpretation of previously published rhodopsin NMR data are discussed.
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36
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Gansmüller A, Concistrè M, McLean N, Johannessen OG, Marín-Montesinos I, Bovee-Geurts PHM, Verdegem P, Lugtenburg J, Brown RCD, Degrip WJ, Levitt MH. Towards an interpretation of 13C chemical shifts in bathorhodopsin, a functional intermediate of a G-protein coupled receptor. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2009; 1788:1350-7. [PMID: 19265671 DOI: 10.1016/j.bbamem.2009.02.018] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/16/2008] [Revised: 02/13/2009] [Accepted: 02/18/2009] [Indexed: 10/21/2022]
Abstract
Photoisomerization of the membrane-bound light receptor protein rhodopsin leads to an energy-rich photostate called bathorhodopsin, which may be trapped at temperatures of 120 K or lower. We recently studied bathorhodopsin by low-temperature solid-state NMR, using in situ illumination of the sample in a purpose-built NMR probe. In this way we acquired (13)C chemical shifts along the retinylidene chain of the chromophore. Here we compare these results with the chemical shifts of the dark state chromophore in rhodopsin, as well as with the chemical shifts of retinylidene model compounds in solution. An earlier solid-state NMR study of bathorhodopsin found only small changes in the (13)C chemical shifts upon isomerization, suggesting only minor perturbations of the electronic structure in the isomerized retinylidene chain. This is at variance with our recent measurements which show much larger perturbations of the (13)C chemical shifts. Here we present a tentative interpretation of our NMR results involving an increased charge delocalization inside the polyene chain of the bathorhodopsin chromophore. Our results suggest that the bathochromic shift of bathorhodopsin is due to modified electrostatic interactions between the chromophore and the binding pocket, whereas both electrostatic interactions and torsional strain are involved in the energy storage mechanism of bathorhodopsin.
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Affiliation(s)
- Axel Gansmüller
- School of Chemistry, University of Southampton, SO17 1BJ Southampton, England, UK
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37
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Ahuja S, Crocker E, Eilers M, Hornak V, Hirshfeld A, Ziliox M, Syrett N, Reeves PJ, Khorana HG, Sheves M, Smith SO. Location of the retinal chromophore in the activated state of rhodopsin*. J Biol Chem 2009; 284:10190-201. [PMID: 19176531 DOI: 10.1074/jbc.m805725200] [Citation(s) in RCA: 83] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Rhodopsin is a highly specialized G protein-coupled receptor (GPCR) that is activated by the rapid photochemical isomerization of its covalently bound 11-cis-retinal chromophore. Using two-dimensional solid-state NMR spectroscopy, we defined the position of the retinal in the active metarhodopsin II intermediate. Distance constraints were obtained between amino acids in the retinal binding site and specific (13)C-labeled sites located on the beta-ionone ring, polyene chain, and Schiff base end of the retinal. We show that the retinal C20 methyl group rotates toward the second extracellular loop (EL2), which forms a cap on the retinal binding site in the inactive receptor. Despite the trajectory of the methyl group, we observed an increase in the C20-Gly(188) (EL2) distance consistent with an increase in separation between the retinal and EL2 upon activation. NMR distance constraints showed that the beta-ionone ring moves to a position between Met(207) and Phe(208) on transmembrane helix H5. Movement of the ring toward H5 was also reflected in increased separation between the Cepsilon carbons of Lys(296) (H7) and Met(44) (H1) and between Gly(121) (H3) and the retinal C18 methyl group. Helix-helix interactions involving the H3-H5 and H4-H5 interfaces were also found to change in the formation of metarhodopsin II reflecting increased retinal-protein interactions in the region of Glu(122) (H3) and His(211) (H5). We discuss the location of the retinal in metarhodopsin II and its interaction with sequence motifs, which are highly conserved across the pharmaceutically important class A GPCR family, with respect to the mechanism of receptor activation.
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Affiliation(s)
- Shivani Ahuja
- Departments of Physics & Astronomy and Biochemistry & Cell Biology, Stony Brook University, Stony Brook, New York 11794-5215
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Renault M, Saurel O, Czaplicki J, Demange P, Gervais V, Löhr F, Réat V, Piotto M, Milon A. Solution State NMR Structure and Dynamics of KpOmpA, a 210 Residue Transmembrane Domain Possessing a High Potential for Immunological Applications. J Mol Biol 2009; 385:117-30. [DOI: 10.1016/j.jmb.2008.10.021] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2008] [Revised: 09/16/2008] [Accepted: 10/04/2008] [Indexed: 10/21/2022]
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39
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Three-Layer ONIOM Studies of the Dark State of Rhodopsin: The Protonation State of Glu181. J Mol Biol 2008; 383:106-21. [DOI: 10.1016/j.jmb.2008.08.007] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2008] [Revised: 07/29/2008] [Accepted: 08/01/2008] [Indexed: 11/18/2022]
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40
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Coelho C, Rocha J, Madhu PK, Mafra L. Practical aspects of Lee-Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: implementation and applications. JOURNAL OF MAGNETIC RESONANCE (SAN DIEGO, CALIF. : 1997) 2008; 194:264-282. [PMID: 18703365 DOI: 10.1016/j.jmr.2008.07.019] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2008] [Revised: 07/15/2008] [Accepted: 07/18/2008] [Indexed: 05/26/2023]
Abstract
Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because (1)H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution (1)H MAS techniques. (1)H Lee-Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic-organic hybrid materials, rich in (1)H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the (1)H NMR spectra of the different parameters at play, for example LG (Lee-Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H(4)pmida] [I], and inorganic-organic hybrid materials (C(4)H(12)N(2))[Ge(2)(pmida)(2)OH(2)] x 4H(2)O [II] and (C(2)H(5)NH(3))[Ti(H(1.5)PO(4))(PO(4))](2) x H(2)O [III].
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Affiliation(s)
- Cristina Coelho
- Department of Chemistry, CICECO, University of Aveiro, Aveiro, Portugal
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41
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Wang Y, Bovee-Geurts PHM, Lugtenburg J, DeGrip WJ. Alpha-retinals as Rhodopsin ChromophoresPreference for the 9-ZConfiguration and Partial Agonist Activity. Photochem Photobiol 2008; 84:889-94. [DOI: 10.1111/j.1751-1097.2008.00321.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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42
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Lopez JJ, Kaiser C, Shastri S, Glaubitz C. Double quantum filtering homonuclear MAS NMR correlation spectra: a tool for membrane protein studies. JOURNAL OF BIOMOLECULAR NMR 2008; 41:97-104. [PMID: 18506579 DOI: 10.1007/s10858-008-9245-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/21/2008] [Revised: 04/24/2008] [Accepted: 04/27/2008] [Indexed: 05/26/2023]
Abstract
13C homonuclear correlation spectra based on proton driven spin diffusion (PDSD) are becoming increasingly important for obtaining distance constraints from multiply labeled biomolecules by MAS NMR. One particular challenging situation arises when such constraints are to be obtained from spectra with a large natural abundance signal background which causes detrimental diagonal peak intensities. They obscure cross peaks, and furthermore impede the calculation of a buildup rates matrix which may be used to derive distance constraints, as carried out in "NMR crystallography". Here, we combine double quantum (DQ) filtering with 13C-13C dipolar assisted rotational resonance (DARR) experiments to yield correlation spectra free of natural abundance contributions. Two experimental schemes, using DQ filtering prior to evolution (DOPE), and after mixing (DOAM), have been evaluated. Diagonal peak intensities along the spectrum diagonal are removed completely, and crosspeaks close to the diagonal are easily identifiable. For DOAM spectra with negligible mixing times, it is possible to carry out 'assignment walks' which simplify peak identification substantially. The method is demonstrated on 13C-cys labeled proteorhodopsin, a 27 kDa membrane protein. The magnetization transfer characteristics were studied using buildup curves obtained on uniformly 13C labelled crystalline tripeptide MLF. Our data show that DQ filtered DARR experiments pave the way for obtaining through space constraints for structural studies on ligands, bound to membrane receptors, or on small fragments within large proteins.
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Affiliation(s)
- Jakob J Lopez
- Institute for Biophysical Chemistry, Centre for Biomolecular Magnetic Resonance, J. W. Goethe University Frankfurt, Max-von-Laue-Str. 9, 60438, Frankfurt, Germany
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43
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Schneider R, Ader C, Lange A, Giller K, Hornig S, Pongs O, Becker S, Baldus M. Solid-State NMR Spectroscopy Applied to a Chimeric Potassium Channel in Lipid Bilayers. J Am Chem Soc 2008; 130:7427-35. [DOI: 10.1021/ja800190c] [Citation(s) in RCA: 68] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Affiliation(s)
- Robert Schneider
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Christian Ader
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Adam Lange
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Karin Giller
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Sönke Hornig
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Olaf Pongs
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Stefan Becker
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
| | - Marc Baldus
- Department of NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, and Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie, Universität Hamburg, Falkenried 94, 20251 Hamburg, Germany
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Altun A, Yokoyama S, Morokuma K. Spectral tuning in visual pigments: an ONIOM(QM:MM) study on bovine rhodopsin and its mutants. J Phys Chem B 2008; 112:6814-27. [PMID: 18473437 DOI: 10.1021/jp709730b] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We have investigated geometries and excitation energies of bovine rhodopsin and some of its mutants by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations in ONIOM scheme, employing B3LYP and BLYP density functionals as well as DFTB method for the QM part and AMBER force field for the MM part. QM/MM geometries of the protonated Schiff-base 11- cis-retinal with B3LYP and DFTB are very similar to each other. TD-B3LYP/MM excitation energy calculations reproduce the experimental absorption maximum of 500 nm in the presence of native rhodopsin environment and predict spectral shifts due to mutations within 10 nm, whereas TD-BLYP/MM excitation energies have red-shift error of at least 50 nm. In the wild-type rhodopsin, Glu113 shifts the first excitation energy to blue and accounts for most of the shift found. Other amino acids individually contribute to the first excitation energy but their net effect is small. The electronic polarization effect is essential for reproducing experimental bond length alternation along the polyene chain in protonated Schiff-base retinal, which correlates with the computed first excitation energy. It also corrects the excitation energies and spectral shifts in mutants, more effectively for deprotonated Schiff-base retinal than for the protonated form. The protonation state and conformation of mutated residues affect electronic spectrum significantly. The present QM/MM calculations estimate not only the experimental excitation energies but also the source of spectral shifts in mutants.
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Affiliation(s)
- Ahmet Altun
- Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA
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Altun A, Yokoyama S, Morokuma K. Quantum mechanical/molecular mechanical studies on spectral tuning mechanisms of visual pigments and other photoactive proteins. Photochem Photobiol 2008; 84:845-54. [PMID: 18331400 DOI: 10.1111/j.1751-1097.2008.00308.x] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
The protein environments surrounding the retinal tune electronic absorption maximum from 350 to 630 nm. Hybrid quantum mechanical/molecular mechanical (QM/MM) methods can be used in calculating excitation energies of retinal in its native protein environments and in studying the molecular basis of spectral tuning. We hereby review recent QM/MM results on the phototransduction of bovine rhodopsin, bacteriorhodopsin, sensory rhodopsin II, nonretinal photoactive yellow protein and their mutants.
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Affiliation(s)
- Ahmet Altun
- Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, GA, USA
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Röhrig UF, Sebastiani D. NMR Chemical Shifts of the Rhodopsin Chromophore in the Dark State and in Bathorhodopsin: A Hybrid QM/MM Molecular Dynamics Study. J Phys Chem B 2008; 112:1267-74. [DOI: 10.1021/jp075662q] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Ute F. Röhrig
- Ludwig Institute for Cancer Research and Swiss Institute of Bioinformatics, Molecular Modeling Group, Genopode Building CH-1015 Lausanne, Switzerland, and Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
| | - Daniel Sebastiani
- Ludwig Institute for Cancer Research and Swiss Institute of Bioinformatics, Molecular Modeling Group, Genopode Building CH-1015 Lausanne, Switzerland, and Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
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Bravaya K, Bochenkova A, Granovsky A, Nemukhin A. An opsin shift in rhodopsin: retinal S0-S1 excitation in protein, in solution, and in the gas phase. J Am Chem Soc 2007; 129:13035-42. [PMID: 17924622 DOI: 10.1021/ja0732126] [Citation(s) in RCA: 88] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We considered a series of model systems for treating the photoabsorption of the 11-cis retinal chromophore in the protonated Schiff-base form in vacuum, solutions, and the protein environment. A high computational level, including the quantum mechanical-molecular mechanical (QM/MM) approach for solution and protein was utilized in simulations. The S0-S1 excitation energies in quantum subsystems were evaluated by means of an augmented version of the multiconfigurational quasidegenerate perturbation theory (aug-MCQDPT2) with the ground-state geometry parameters optimized in the density functional theory PBE0/cc-pVDZ approximation. The computed positions of absorption bands lambdamax, 599(g), 448(s), and 515(p) nm for the gas phase, solution, and protein, respectively, are in excellent agreement with the corresponding experimental data, 610(g), 445(s), and 500(p) nm. Such consistency provides a support for the formulated qualitative conclusions on the role of the chromophore geometry, environmental electrostatic field, and the counterion in different media. An essentially nonplanar geometry conformation of the chromophore group in the region of the C14-C15 bond was obtained for the protein, in particular, owing to the presence of the neighboring charged amino acid residue Glu181. Nonplanarity of the C14-C15 bond region along with the influence of the negatively charged counterions Glu181 and Glu113 are found to be important to reproduce the spectroscopic features of retinal chromophore inside the Rh cavity. Furthermore, the protein field is responsible for the largest bond-order decrease at the C11-C12 double bond upon excitation, which may be the reason for the 11-cis photoisomerization specificity.
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Affiliation(s)
- Ksenia Bravaya
- Department of Chemistry, M.V. Lomonosov Moscow State University, 1/3, Leninskie Gory, Moscow 119992, Russian Federation
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Baldus M. ICMRBS founder's medal 2006: biological solid-state NMR, methods and applications. JOURNAL OF BIOMOLECULAR NMR 2007; 39:73-86. [PMID: 17657566 DOI: 10.1007/s10858-007-9177-3] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/23/2007] [Accepted: 06/26/2007] [Indexed: 05/16/2023]
Abstract
Solid-state NMR (ssNMR) provides increasing possibilities to study structure and dynamics of biomolecular systems. Our group has been interested in developing ssNMR-based approaches that are applicable to biomolecules of increasing molecular size and complexity without the need of specific isotope-labelling. Methodological aspects ranging from spectral assignments to the indirect detection of proton-proton contacts in multi-dimensional ssNMR are discussed and applied to (membrane) protein complexes.
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Affiliation(s)
- Marc Baldus
- Research Group Solid-state NMR, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Gottingen, Germany.
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Lau PW, Grossfield A, Feller SE, Pitman MC, Brown MF. Dynamic structure of retinylidene ligand of rhodopsin probed by molecular simulations. J Mol Biol 2007; 372:906-917. [PMID: 17719606 PMCID: PMC5233727 DOI: 10.1016/j.jmb.2007.06.047] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2007] [Revised: 06/13/2007] [Accepted: 06/18/2007] [Indexed: 11/22/2022]
Abstract
Rhodopsin is currently the only available atomic-resolution template for understanding biological functions of the G protein-coupled receptor (GPCR) family. The structural basis for the phenomenal dark state stability of 11-cis-retinal bound to rhodopsin and its ultrafast photoreaction are active topics of research. In particular, the beta-ionone ring of the retinylidene inverse agonist is crucial for the activation mechanism. We analyzed a total of 23 independent, 100 ns all-atom molecular dynamics simulations of rhodopsin embedded in a lipid bilayer in the microcanonical (N,V,E) ensemble. Analysis of intramolecular fluctuations predicts hydrogen-out-of-plane (HOOP) wagging modes of retinal consistent with those found in Raman vibrational spectroscopy. We show that sampling and ergodicity of the ensemble of simulations are crucial for determining the distribution of conformers of retinal bound to rhodopsin. The polyene chain is rigidly locked into a single, twisted conformation, consistent with the function of retinal as an inverse agonist in the dark state. Most surprisingly, the beta-ionone ring is mobile within its binding pocket; interactions are non-specific and the cavity is sufficiently large to enable structural heterogeneity. We find that retinal occupies two distinct conformations in the dark state, contrary to most previous assumptions. The beta-ionone ring can rotate relative to the polyene chain, thereby populating both positively and negatively twisted 6-s-cis enantiomers. This result, while unexpected, strongly agrees with experimental solid-state (2)H NMR spectra. Correlation analysis identifies the residues most critical to controlling mobility of retinal; we find that Trp265 moves away from the ionone ring prior to any conformational transition. Our findings reinforce how molecular dynamics simulations can challenge conventional assumptions for interpreting experimental data, especially where existing models neglect conformational fluctuations.
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Affiliation(s)
- Pick-Wei Lau
- Department of Biochemistry & Molecular Biophysics, University of Arizona, Tucson, Arizona 85721, USA
| | - Alan Grossfield
- IBM TJ Watson Research Center, Yorktown Heights, New York 10598, USA
| | - Scott E. Feller
- Department of Chemistry, Wabash College, Crawfordsville, Indiana 47933, USA
| | - Michael C. Pitman
- IBM TJ Watson Research Center, Yorktown Heights, New York 10598, USA
| | - Michael F. Brown
- Department of Biochemistry & Molecular Biophysics, University of Arizona, Tucson, Arizona 85721, USA
- Department of Chemistry, University of Arizona, Tucson, Arizona 85721, USA
- Department of Physics, University of Arizona, Tucson, Arizona 85721, USA
- Corresponding author. Present address: Pick-Wei Lau, Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA.
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