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Frotscher E, Krainer G, Schlierf M, Keller S. Dissecting Nanosecond Dynamics in Membrane Proteins with Dipolar Relaxation upon Tryptophan Photoexcitation. J Phys Chem Lett 2018; 9:2241-2245. [PMID: 29652505 DOI: 10.1021/acs.jpclett.8b00834] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/07/2023]
Abstract
The structural dynamics of proteins on the nanosecond time scale can be probed with dipolar relaxation in response to photoexcitation of intrinsic tryptophan (Trp) residues. For membrane proteins, however, the complexity due to overlapping contributions from the protein itself, the membrane mimic, and the aqueous solvent impairs detailed analysis and interpretation. To disentangle these contributions, we measured time-resolved emission spectra of Trp in the protein Mistic in detergent micelles of various polarities. By comparison with Trp analogues in water and micelles, we could dissect the contributions from hydration, micelle, and protein matrix to dipolar relaxation on the nanosecond time scale. Our results demonstrate that ultrafast, subnanosecond relaxation reports on the extent of Trp shielding from water, with micelle and protein moieties making additive contributions. By contrast, relaxation in the low nanosecond regime is due to dipolar rearrangement of micelle and protein moieties upon photoexcitation, thereby probing conformational dynamics around the intrinsic fluorophore.
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Affiliation(s)
- Erik Frotscher
- Molecular Biophysics , Technische Universität Kaiserslautern (TUK) , Erwin-Schrödinger-Str. 13 , 67663 Kaiserslautern , Germany
| | - Georg Krainer
- Molecular Biophysics , Technische Universität Kaiserslautern (TUK) , Erwin-Schrödinger-Str. 13 , 67663 Kaiserslautern , Germany
- B CUBE - Center for Molecular Bioengineering , Technische Universität Dresden , Arnoldstr. 18 , 01307 Dresden , Germany
| | - Michael Schlierf
- B CUBE - Center for Molecular Bioengineering , Technische Universität Dresden , Arnoldstr. 18 , 01307 Dresden , Germany
| | - Sandro Keller
- Molecular Biophysics , Technische Universität Kaiserslautern (TUK) , Erwin-Schrödinger-Str. 13 , 67663 Kaiserslautern , Germany
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Role of Cationic Side Chains in the Antimicrobial Activity of C18G. Molecules 2018; 23:molecules23020329. [PMID: 29401708 PMCID: PMC6017431 DOI: 10.3390/molecules23020329] [Citation(s) in RCA: 37] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2018] [Revised: 01/23/2018] [Accepted: 02/01/2018] [Indexed: 01/08/2023] Open
Abstract
Antimicrobial peptides (AMPs) have been an area of great interest, due to the high selectivity of these molecules toward bacterial targets over host cells and the limited development of bacterial resistance to these molecules throughout evolution. The peptide C18G has been shown to be a selective, broad spectrum AMP with a net +8 cationic charge from seven lysine residues in the sequence. In this work, the cationic Lys residues were replaced with other natural or non-proteinogenic cationic amino acids: arginine, histidine, ornithine, or diaminopropionic acid. These changes vary in the structure of the amino acid side chain, the identity of the cationic moiety, and the pKa of the cationic group. Using a combination of spectroscopic and microbiological methods, the influence of these cationic groups on membrane binding, secondary structure, and antibacterial activity was investigated. The replacement of Lys with most other cationic residues had, at most, 2-fold effects on minimal inhibitory concentration against a variety of Gram-positive and Gram-negative bacteria. However, the peptide containing His as the cationic group showed dramatically reduced activity. All peptide variants retained the ability to bind lipid vesicles and showed clear preference for binding vesicles that contained anionic lipids. Similarly, all peptides adopted a helical conformation when bound to lipids or membrane mimetics, although the peptide containing diaminopropionic acid exhibited a decreased helicity. The peptides exhibited a wider variety of activity in the permeabilization of bacterial membranes, with peptides containing Lys, Arg, or Orn being the most broadly active. In all, the antibacterial activity of the C18G peptide is generally tolerant to changes in the structure and identity of the cationic amino acids, yielding new possibilities for design and development of AMPs that may be less susceptible to immune and bacterial recognition or in vivo degradation.
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Jeong BS, Dyer RB. Proton Transport Mechanism of M2 Proton Channel Studied by Laser-Induced pH Jump. J Am Chem Soc 2017; 139:6621-6628. [PMID: 28467842 DOI: 10.1021/jacs.7b00617] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The M2 proton transport channel of the influenza virus A is an important model system because it conducts protons with high selectivity and unidirectionally when activated at low pH, despite the relative simplicity of its structure. Although it has been studied extensively, the molecular details of the pH-dependent gating and proton conductance mechanisms are incompletely understood. We report direct observation of the M2 proton channel activation process using a laser-induced pH jump coupled with tryptophan fluorescence as a probe. Biphasic kinetics is observed, with the fast phase corresponding to the His37 protonation, and the slow phase associated with the subsequent conformation change. Unusually fast His37 protonation was observed (2.0 × 1010 M-1 s-1), implying the existence of proton collecting antennae for expedited proton transport. The conformation change (4 × 103 s-1) was about 2 orders of magnitude slower than protonation at endosomal pH, suggesting that a transporter model is likely not feasible.
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Affiliation(s)
- Ban-Seok Jeong
- Department of Chemistry, Emory University , Atlanta, Georgia 30322, United States
| | - R Brian Dyer
- Department of Chemistry, Emory University , Atlanta, Georgia 30322, United States
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Markiewicz BN, Lemmin T, Zhang W, Ahmed IA, Jo H, Fiorin G, Troxler T, DeGrado WF, Gai F. Infrared and fluorescence assessment of the hydration status of the tryptophan gate in the influenza A M2 proton channel. Phys Chem Chem Phys 2016; 18:28939-28950. [PMID: 27725984 PMCID: PMC5157935 DOI: 10.1039/c6cp03426h] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The M2 proton channel of the influenza A virus has been the subject of extensive studies because of its critical role in viral replication. As such, we now know a great deal about its mechanism of action, especially how it selects and conducts protons in an asymmetric fashion. The conductance of this channel is tuned to conduct protons at a relatively low biologically useful rate, which allows acidification of the viral interior of a virus entrapped within an endosome, but not so great as to cause toxicity to the infected host cell prior to packaging of the virus. The dynamic, structural and chemical features that give rise to this tuning are not fully understood. Herein, we use a tryptophan (Trp) analog, 5-cyanotryptophan, and various methods, including linear and nonlinear infrared spectroscopies, static and time-resolved fluorescence techniques, and molecular dynamics simulations, to site-specifically interrogate the structure and hydration dynamics of the Trp41 gate in the transmembrane domain of the M2 proton channel. Our results suggest that the Trp41 sidechain adopts the t90 rotamer, the χ2 dihedral angle of which undergoes an increase of approximately 35° upon changing the pH from 7.4 to 5.0. Furthermore, we find that Trp41 is situated in an environment lacking bulk-like water, and somewhat surprisingly, the water density and dynamics do not show a measurable difference between the high (7.4) and low (5.0) pH states. Since previous studies have shown that upon channel opening water flows into the cavity above the histidine tetrad (His37), the present finding thus provides evidence indicating that the lack of sufficient water molecules near Trp41 needed to establish a continuous hydrogen bonding network poses an additional energetic bottleneck for proton conduction.
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Affiliation(s)
- Beatrice N Markiewicz
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
| | - Thomas Lemmin
- Department of Pharmaceutical Chemistry, University of California San Francisco, San Francisco, California 94143, USA.
| | - Wenkai Zhang
- Ultrafast Optical Processes Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Ismail A Ahmed
- Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Hyunil Jo
- Department of Pharmaceutical Chemistry, University of California San Francisco, San Francisco, California 94143, USA.
| | - Giacomo Fiorin
- Institute for Computational Molecular Science, Temple University, Philadelphia, Pennsylvania 19122, USA
| | - Thomas Troxler
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. and Ultrafast Optical Processes Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - William F DeGrado
- Department of Pharmaceutical Chemistry, University of California San Francisco, San Francisco, California 94143, USA.
| | - Feng Gai
- Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. and Ultrafast Optical Processes Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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Ridgway Z, Picciano AL, Gosavi PM, Moroz YS, Angevine CE, Chavis AE, Reiner JE, Korendovych IV, Caputo GA. Functional characterization of a melittin analog containing a non-natural tryptophan analog. Biopolymers 2016; 104:384-394. [PMID: 25670241 DOI: 10.1002/bip.22624] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2014] [Revised: 01/13/2015] [Accepted: 01/18/2015] [Indexed: 02/06/2023]
Abstract
Tryptophan (Trp) is a naturally occurring amino acid, which exhibits fluorescence emission properties that are dependent on the polarity of the local environment around the Trp side chain. However, this sensitivity also complicates interpretation of fluorescence emission data. A non-natural analogue of tryptophan, β-(1-azulenyl)-L-alanine, exhibits fluorescence insensitive to local solvent polarity and does not impact the structure or characteristics of several peptides examined. In this study, we investigated the effect of replacing Trp with β-(1-azulenyl)-L-alanine in the well-known bee-venom peptide melittin. This peptide provides a model framework for investigating the impact of replacing Trp with β-(1-azulenyl)-L-alanine in a functional peptide system that undergoes significant shifts in Trp fluorescence emission upon binding to lipid bilayers. Microbiological methods including assessment of the antimicrobial activity by minimal inhibitory concentration (MIC) assays and bacterial membrane permeability assays indicated little difference between the Trp and the β-(1-azulenyl)-L-alanine-substituted versions of melittin. Circular dichroism spectroscopy showed both that peptides adopted the expected α-helical structures when bound to phospholipid bilayers and electrophysiological analysis indicated that both created membrane disruptions leading to significant conductance increases across model membranes. Both peptides exhibited a marked protection of the respective fluorophores when bound to bilayers indicating a similar membrane-bound topology. As expected, while fluorescence quenching and CD indicate the peptides are stably bound to lipid vesicles, the peptide containing β-(1-azulenyl)-L-alanine exhibited no fluorescence emission shift upon binding while the natural Trp exhibited >10 nm shift in emission spectrum barycenter. Taken together, the β-(1-azulenyl)-L-alanine can serve as a solvent insensitive alternative to Trp that does not have significant impacts on structure or function of membrane interacting peptides.
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Affiliation(s)
- Zachary Ridgway
- Department of Chemistry and Biochemistry, Rowan University, Glassboro NJ 08028
| | - Angela L Picciano
- Department of Chemistry and Biochemistry, Rowan University, Glassboro NJ 08028
| | | | - Yurii S Moroz
- Department of Chemistry, Syracuse University, Syracuse NY 13244.,Present affiliation: ChemBioCenter, Kyiv National Taras Shevchenko University, 61 Chervonotkatska Street, Kyiv 02094, Ukraine
| | | | - Amy E Chavis
- Department of Physics, Virginia Commonwealth University, Richmond VA 23284
| | - Joseph E Reiner
- Department of Physics, Virginia Commonwealth University, Richmond VA 23284
| | | | - Gregory A Caputo
- Department of Chemistry and Biochemistry, Rowan University, Glassboro NJ 08028.,School of Biomedical Sciences, Rowan University, Glassboro NJ, 08028
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