1
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Lv W, Wang Y, Fu H, Liang Z, Huang B, Jiang R, Wu J, Zhao Y. Recent advances of multifunctional zwitterionic polymers for biomedical application. Acta Biomater 2024; 181:19-45. [PMID: 38729548 DOI: 10.1016/j.actbio.2024.05.006] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2023] [Revised: 04/27/2024] [Accepted: 05/03/2024] [Indexed: 05/12/2024]
Abstract
Zwitterionic polymers possess equal total positive and negative charges in the repeating units, making them electrically neutral overall. This unique property results in superhydrophilicity, which makes the zwitterionic polymers highly effective in resisting protein adsorption, thus endowing the drug carriers with long blood circulation time, inhibiting thrombus formation on biomedical devices in contact with blood, and ensuring the good sensitivity of sensors in biomedical application. Moreover, zwitterionic polymers have tumor-targeting ability and pH-responsiveness, rendering them ideal candidates for antitumor drug delivery. Additionally, the high ionic conductivity of zwitterionic polymers makes them an important raw material for ionic skin. Zwitterionic polymers exhibit remarkable resistance to bacterial adsorption and growth, proving their suitability in a wide range of biomedical applications such as ophthalmic applications, and wound dressings. In this paper, we provide an in-depth analysis of the different structures and characteristics of zwitterionic polymers and highlight their unique qualities and suitability for biomedical applications. Furthermore, we discuss the limitations and challenges that must be overcome to realize the full potential of zwitterionic polymers and present an optimistic perspective for zwitterionic polymers in the biomedical fields. STATEMENT OF SIGNIFICANCE: Zwitterionic polymers have a series of excellent properties such as super hydrophilicity, anti-protein adsorption, antibacterial ability and good ionic conductivity. However, biomedical applications of multifunctional zwitterionic polymers are still a major field to be explored. This review focuses on the design and application of zwitterionic polymers-based nanosystems for targeted and responsive delivery of antitumor drugs and cancer diagnostic agents. Moreover, the use of zwitterionic polymers in various biomedical applications such as biomedical devices in contact with blood, biosensors, ionic skin, ophthalmic applications and wound dressings is comprehensively described. We discuss current results and future challenges for a better understanding of multifunctional zwitterionic polymers for biomedical applications.
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Affiliation(s)
- Wenfeng Lv
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Yanhui Wang
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Huayu Fu
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Ziyang Liang
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Bangqi Huang
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Ruiqin Jiang
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China
| | - Jun Wu
- Bioscience and Biomedical Engineering Thrust, The Hong Kong University of Science and Technology (Guangzhou), Nansha, Guangzhou, 511400, Guangdong, China; Division of Life Science, The Hong Kong University of Science and Technology, Hong Kong SAR, China.
| | - Yi Zhao
- School of Biomedical Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China.
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2
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Papadourakis M, Sinenka H, Matricon P, Hénin J, Brannigan G, Pérez-Benito L, Pande V, van Vlijmen H, de Graaf C, Deflorian F, Tresadern G, Cecchini M, Cournia Z. Alchemical Free Energy Calculations on Membrane-Associated Proteins. J Chem Theory Comput 2023; 19:7437-7458. [PMID: 37902715 PMCID: PMC11017255 DOI: 10.1021/acs.jctc.3c00365] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2023] [Indexed: 10/31/2023]
Abstract
Membrane proteins have diverse functions within cells and are well-established drug targets. The advances in membrane protein structural biology have revealed drug and lipid binding sites on membrane proteins, while computational methods such as molecular simulations can resolve the thermodynamic basis of these interactions. Particularly, alchemical free energy calculations have shown promise in the calculation of reliable and reproducible binding free energies of protein-ligand and protein-lipid complexes in membrane-associated systems. In this review, we present an overview of representative alchemical free energy studies on G-protein-coupled receptors, ion channels, transporters as well as protein-lipid interactions, with emphasis on best practices and critical aspects of running these simulations. Additionally, we analyze challenges and successes when running alchemical free energy calculations on membrane-associated proteins. Finally, we highlight the value of alchemical free energy calculations calculations in drug discovery and their applicability in the pharmaceutical industry.
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Affiliation(s)
- Michail Papadourakis
- Biomedical
Research Foundation, Academy of Athens, 4 Soranou Ephessiou, 11527 Athens, Greece
| | - Hryhory Sinenka
- Institut
de Chimie de Strasbourg, UMR7177, CNRS, Université de Strasbourg, F-67083 Strasbourg Cedex, France
| | - Pierre Matricon
- Sosei
Heptares, Steinmetz Building,
Granta Park, Great Abington, Cambridge CB21 6DG, United
Kingdom
| | - Jérôme Hénin
- Laboratoire
de Biochimie Théorique UPR 9080, CNRS and Université Paris Cité, 75005 Paris, France
| | - Grace Brannigan
- Center
for Computational and Integrative Biology, Rutgers University−Camden, Camden, New Jersey 08103, United States of America
- Department
of Physics, Rutgers University−Camden, Camden, New Jersey 08102, United States
of America
| | - Laura Pérez-Benito
- CADD,
In Silico Discovery, Janssen Research &
Development, Turnhoutseweg 30, 2340 Beerse, Belgium
| | - Vineet Pande
- CADD,
In Silico Discovery, Janssen Research &
Development, Turnhoutseweg 30, 2340 Beerse, Belgium
| | - Herman van Vlijmen
- CADD,
In Silico Discovery, Janssen Research &
Development, Turnhoutseweg 30, 2340 Beerse, Belgium
| | - Chris de Graaf
- Sosei
Heptares, Steinmetz Building,
Granta Park, Great Abington, Cambridge CB21 6DG, United
Kingdom
| | - Francesca Deflorian
- Sosei
Heptares, Steinmetz Building,
Granta Park, Great Abington, Cambridge CB21 6DG, United
Kingdom
| | - Gary Tresadern
- CADD,
In Silico Discovery, Janssen Research &
Development, Turnhoutseweg 30, 2340 Beerse, Belgium
| | - Marco Cecchini
- Institut
de Chimie de Strasbourg, UMR7177, CNRS, Université de Strasbourg, F-67083 Strasbourg Cedex, France
| | - Zoe Cournia
- Biomedical
Research Foundation, Academy of Athens, 4 Soranou Ephessiou, 11527 Athens, Greece
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3
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Frangos ZJ, Cantwell Chater RP, Vandenberg RJ. Glycine Transporter 2: Mechanism and Allosteric Modulation. Front Mol Biosci 2021; 8:734427. [PMID: 34805268 PMCID: PMC8602798 DOI: 10.3389/fmolb.2021.734427] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2021] [Accepted: 10/25/2021] [Indexed: 01/19/2023] Open
Abstract
Neurotransmitter sodium symporters (NSS) are a subfamily of SLC6 transporters responsible for regulating neurotransmitter signalling. They are a major target for psychoactive substances including antidepressants and drugs of abuse, prompting substantial research into their modulation and structure-function dynamics. Recently, a series of allosteric transport inhibitors have been identified, which may reduce side effect profiles, compared to orthosteric inhibitors. Allosteric inhibitors are also likely to provide different clearance kinetics compared to competitive inhibitors and potentially better clinical outcomes. Crystal structures and homology models have identified several allosteric modulatory sites on NSS including the vestibule allosteric site (VAS), lipid allosteric site (LAS) and cholesterol binding site (CHOL1). Whilst the architecture of eukaryotic NSS is generally well conserved there are differences in regions that form the VAS, LAS, and CHOL1. Here, we describe ligand-protein interactions that stabilize binding in each allosteric site and explore how differences between transporters could be exploited to generate NSS specific compounds with an emphasis on GlyT2 modulation.
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Affiliation(s)
- Zachary J Frangos
- Transporter Biology Group, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia
| | - Ryan P Cantwell Chater
- Transporter Biology Group, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia
| | - Robert J Vandenberg
- Transporter Biology Group, School of Medical Sciences, Faculty of Medicine and Health, University of Sydney, Sydney, NSW, Australia
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4
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Scopelliti AJ, Font J, Vandenberg RJ, Boudker O, Ryan RM. Structural characterisation reveals insights into substrate recognition by the glutamine transporter ASCT2/SLC1A5. Nat Commun 2018; 9:38. [PMID: 29295993 PMCID: PMC5750217 DOI: 10.1038/s41467-017-02444-w] [Citation(s) in RCA: 47] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2017] [Accepted: 11/30/2017] [Indexed: 12/21/2022] Open
Abstract
Cancer cells undergo a shift in metabolism where they become reliant on nutrients such as the amino-acid glutamine. Glutamine enters the cell via the alanine/serine/cysteine transporter 2 (ASCT2) that is upregulated in several cancers to maintain an increased supply of this nutrient and are therefore an attractive target in cancer therapeutic development. ASCT2 belongs to the glutamate transporter (SLC1A) family but is the only transporter in this family able to transport glutamine. The structural basis for glutamine selectivity of ASCT2 is unknown. Here, we identify two amino-acid residues in the substrate-binding site that are responsible for conferring glutamine selectivity. We introduce corresponding mutations into a prokaryotic homologue of ASCT2 and solve four crystal structures, which reveal the structural basis for neutral amino acid and inhibitor binding in this family. This structural model of ASCT2 may provide a basis for future development of selective ASCT2 inhibitors to treat glutamine-dependent cancers. Cancer cells are reliant on nutrients such as glutamine, which enter the cell via the alanine/serine/cysteine transporter 2 (ASCT2). Here, authors use crystallography to show which amino-acid residues in the substrate-binding site are responsible for conferring glutamine selectivity to ASCT2.
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Affiliation(s)
- Amanda J Scopelliti
- Transporter Biology Group, Discipline of Pharmacology, Sydney Medical School, University of Sydney, Sydney, NSW, 2006, Australia.,Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY, 10065, USA
| | - Josep Font
- Transporter Biology Group, Discipline of Pharmacology, Sydney Medical School, University of Sydney, Sydney, NSW, 2006, Australia
| | - Robert J Vandenberg
- Transporter Biology Group, Discipline of Pharmacology, Sydney Medical School, University of Sydney, Sydney, NSW, 2006, Australia
| | - Olga Boudker
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY, 10065, USA. .,Howard Hughes Medical Institute, Weill Cornell Medicine, New York, NY, 10065, USA.
| | - Renae M Ryan
- Transporter Biology Group, Discipline of Pharmacology, Sydney Medical School, University of Sydney, Sydney, NSW, 2006, Australia.
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5
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Grouleff J, Koldsø H, Miao Y, Schiøtt B. Ligand Binding in the Extracellular Vestibule of the Neurotransmitter Transporter Homologue LeuT. ACS Chem Neurosci 2017; 8:619-628. [PMID: 27966884 DOI: 10.1021/acschemneuro.6b00359] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The human monoamine transporters (MATs) facilitate the reuptake of monoamine neurotransmitters from the synaptic cleft. MATs are linked to a number of neurological diseases and are the targets of both therapeutic and illicit drugs. Until recently, no high-resolution structures of the human MATs existed, and therefore, studies of this transporter family have relied on investigations of the homologues bacterial transporters such as the leucine transporter LeuT, which has been crystallized in several conformational states. A two-substrate transport mechanism has been suggested for this transporter family, which entails that high-affinity binding of a second substrate in an extracellular site is necessary for the substrate in the central binding site to be transported. Compelling evidence for this mechanism has been presented, however, a number of equally compelling accounts suggest that the transporters function through a mechanism involving only a single substrate and a single high-affinity site. To shed light on this apparent contradiction, we have performed extensive molecular dynamics simulations of LeuT in the outward-occluded conformation with either one or two substrates bound to the transporter. We have also calculated the substrate binding affinity in each of the two proposed binding sites through rigorous free energy simulations. Results show that substrate binding is unstable in the extracellular vestibule and the substrate binding affinity within the suggested extracellular site is very low (0.2 and 3.3 M for the two dominant binding modes) compared to the central substrate binding site (14 nM). This suggests that for LeuT in the outward-occluded conformation only a single high-affinity substrate binding site exists.
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Affiliation(s)
- Julie Grouleff
- Center
for Insoluble Protein Structures (inSPIN) and Interdisciplinary Nanoscience
Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade
140, 8000 Aarhus
C, Denmark
| | - Heidi Koldsø
- Center
for Insoluble Protein Structures (inSPIN) and Interdisciplinary Nanoscience
Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade
140, 8000 Aarhus
C, Denmark
| | - Yinglong Miao
- Howard
Hughes Medical Institute and Department of Pharmacology, University of California at San Diego, La Jolla, California 92093, United States
| | - Birgit Schiøtt
- Center
for Insoluble Protein Structures (inSPIN) and Interdisciplinary Nanoscience
Center (iNANO), Department of Chemistry, Aarhus University, Langelandsgade
140, 8000 Aarhus
C, Denmark
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6
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Sohail A, Jayaraman K, Venkatesan S, Gotfryd K, Daerr M, Gether U, Loland CJ, Wanner KT, Freissmuth M, Sitte HH, Sandtner W, Stockner T. The Environment Shapes the Inner Vestibule of LeuT. PLoS Comput Biol 2016; 12:e1005197. [PMID: 27835643 PMCID: PMC5105988 DOI: 10.1371/journal.pcbi.1005197] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2016] [Accepted: 10/12/2016] [Indexed: 12/27/2022] Open
Abstract
Human neurotransmitter transporters are found in the nervous system terminating synaptic signals by rapid removal of neurotransmitter molecules from the synaptic cleft. The homologous transporter LeuT, found in Aquifex aeolicus, was crystallized in different conformations. Here, we investigated the inward-open state of LeuT. We compared LeuT in membranes and micelles using molecular dynamics simulations and lanthanide-based resonance energy transfer (LRET). Simulations of micelle-solubilized LeuT revealed a stable and widely open inward-facing conformation. However, this conformation was unstable in a membrane environment. The helix dipole and the charged amino acid of the first transmembrane helix (TM1A) partitioned out of the hydrophobic membrane core. Free energy calculations showed that movement of TM1A by 0.30 nm was driven by a free energy difference of ~15 kJ/mol. Distance measurements by LRET showed TM1A movements, consistent with the simulations, confirming a substantially different inward-open conformation in lipid bilayer from that inferred from the crystal structure. Crystal structures of the bacterial small amino acid transporter LeuT provided structural evidence for the alternating access model. Thereby, these structures shaped our understanding of the mechanisms underlying substrate translocation by neurotransmitter transporters. However, it has been questioned, if the crystallized inward-open conformation of LeuT can exist in the membrane environment. Here we show that, while stable in detergent micelles, the inward-open conformation of LeuT is of high energy and undergoes structural readjustments. We use a multi-faceted approach including molecular dynamics simulations, scintillation proximity assays, free energy calculations and apply for the first time lanthanide resonance energy transfer measurements to verify the in silico predictions. In silico and in vitro approaches using the same conditions allowed us to combine the macroscopic experimental data with microscopic all atom results from simulations to identify the underlying driving forces: partitioning of charged and polar groups from the hydrophobic membrane interior to the hydrophilic environment. We propose that the inward-facing state shows a much smaller movement of TM1A, but large enough to create an access path to the S1 substrate binding site from the vestibule.
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Affiliation(s)
- Azmat Sohail
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
| | - Kumaresan Jayaraman
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
| | - Santhoshkannan Venkatesan
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
| | - Kamil Gotfryd
- University of Copenhagen, Faculty of Health and Medical Sciences Denmark, Department of Neuroscience and Pharmacology, Copenhagen, Denmark
- University of Copenhagen, Faculty of Health Sciences Denmark, Department of Biomedical Sciences, Copenhagen, Denmark
| | - Markus Daerr
- Ludwig Maximilians University Munich, Department of Pharmacy, Center of Drug Research, Munich, Germany
| | - Ulrik Gether
- University of Copenhagen, Faculty of Health and Medical Sciences Denmark, Department of Neuroscience and Pharmacology, Copenhagen, Denmark
| | - Claus J. Loland
- University of Copenhagen, Faculty of Health and Medical Sciences Denmark, Department of Neuroscience and Pharmacology, Copenhagen, Denmark
| | - Klaus T. Wanner
- Ludwig Maximilians University Munich, Department of Pharmacy, Center of Drug Research, Munich, Germany
| | - Michael Freissmuth
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
| | - Harald H. Sitte
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
- * E-mail:
| | - Walter Sandtner
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
| | - Thomas Stockner
- Medical University of Vienna, Center for Physiology and Pharmacology, Institute of Pharmacology, Vienna, Austria
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7
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Zhang JL, Zheng QC, Yu LY, Li ZQ, Zhang HX. Effect of External Electric Field on Substrate Transport of a Secondary Active Transporter. J Chem Inf Model 2016; 56:1539-46. [PMID: 27472561 DOI: 10.1021/acs.jcim.6b00212] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Substrate transport across a membrane accomplished by a secondary active transporter (SAT) is essential to the normal physiological function of living cells. In the present research, a series of all-atom molecular dynamics (MD) simulations under different electric field (EF) strengths was performed to investigate the effect of an external EF on the substrate transport of an SAT. The results show that EF both affects the interaction between substrate and related protein's residues by changing their conformations and tunes the timeline of the transport event, which collectively reduces the height of energy barrier for substrate transport and results in the appearance of two intermediate conformations under the existence of an external EF. Our work spotlights the crucial influence of external EFs on the substrate transport of SATs and could provide a more penetrating understanding of the substrate transport mechanism of SATs.
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Affiliation(s)
- Ji-Long Zhang
- International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University , Changchun 130023, Jilin, People's Republic of China.,Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education, Jilin University , Changchun 130021, Jilin, People's Republic of China.,Department of Chemistry and Supercomputing Institute, University of Minnesota , Minneapolis, Minnesota 55455, United States
| | - Qing-Chuan Zheng
- International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University , Changchun 130023, Jilin, People's Republic of China
| | - Li-Ying Yu
- International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University , Changchun 130023, Jilin, People's Republic of China
| | - Zheng-Qiang Li
- Key Laboratory for Molecular Enzymology and Engineering of the Ministry of Education, Jilin University , Changchun 130021, Jilin, People's Republic of China
| | - Hong-Xing Zhang
- International Joint Research Laboratory of Nano-Micro Architecture Chemistry, Institute of Theoretical Chemistry, Jilin University , Changchun 130023, Jilin, People's Republic of China
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8
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Kumar S, Bradley CL, Mukashyaka P, Doerrler WT. Identification of essential arginine residues ofEscherichia coliDedA/Tvp38 family membrane proteins YqjA and YghB. FEMS Microbiol Lett 2016; 363:fnw133. [DOI: 10.1093/femsle/fnw133] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/14/2016] [Indexed: 12/15/2022] Open
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9
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Zhao C, Caplan DA, Noskov SY. Evaluations of the Absolute and Relative Free Energies for Antidepressant Binding to the Amino Acid Membrane Transporter LeuT with Free Energy Simulations. J Chem Theory Comput 2015; 6:1900-14. [PMID: 26615849 DOI: 10.1021/ct9006597] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The binding of ligands to protein receptors with high affinity and specificity is central to many cellular processes. The quest for the development of computational models capable of accurately evaluating binding affinity remains one of the main goals of modern computational biophysics. In this work, free energy perturbation/molecular dynamics simulations were used to evaluate absolute and relative binding affinity for three different antidepressants to a sodium-dependent membrane transporter, LeuT, a bacterial homologue of human serotonin and dopamine transporters. Dysfunction of these membrane transporters in mammals has been implicated in multiple diseases of the nervous system, including bipolar disorder and depression. Furthermore, these proteins are key targets for antidepressants including fluoxetine (aka Prozac) and tricyclic antidepressants known to block transport activity. In addition to being clinically relevant, this system, where multiple crystal structures are readily available, represents an ideal testing ground for methods used to study the molecular mechanisms of ligand binding to membrane proteins. We discuss possible pitfalls and different levels of approximation required to evaluate binding affinity, such as the dependence of the computed affinities on the strength of constraints and the sensitivity of the computed affinities to the particular partial charges derived from restrained electrostatic potential fitting of quantum mechanics electrostatic potential. Finally, we compare the effects of different constraint schemes on the absolute and relative binding affinities obtained from free energy simulations.
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Affiliation(s)
- Chunfeng Zhao
- Institute for Biocomplexity and Informatics and Department of Biological Sciences, University of Calgary, 2500 University Drive, BI558, Calgary, AB, Canada T2N 1N4 and Molecular Structure and Function, Hospital for Sick Children and Department of Biochemistry, University of Toronto, Ontario, Canada
| | - David A Caplan
- Institute for Biocomplexity and Informatics and Department of Biological Sciences, University of Calgary, 2500 University Drive, BI558, Calgary, AB, Canada T2N 1N4 and Molecular Structure and Function, Hospital for Sick Children and Department of Biochemistry, University of Toronto, Ontario, Canada
| | - Sergei Yu Noskov
- Institute for Biocomplexity and Informatics and Department of Biological Sciences, University of Calgary, 2500 University Drive, BI558, Calgary, AB, Canada T2N 1N4 and Molecular Structure and Function, Hospital for Sick Children and Department of Biochemistry, University of Toronto, Ontario, Canada
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10
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Song HD, Zhu F. Conformational Changes in Two Inter-Helical Loops of Mhp1 Membrane Transporter. PLoS One 2015; 10:e0133388. [PMID: 26186341 PMCID: PMC4505851 DOI: 10.1371/journal.pone.0133388] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2015] [Accepted: 06/26/2015] [Indexed: 12/17/2022] Open
Abstract
Mhp1 is a bacterial secondary transporter with high-resolution crystal structures available for both the outward- and inward-facing conformations. Through molecular dynamics simulations of the ligand-free Mhp1 as well as analysis of its crystal structures, here we show that two inter-helical loops, respectively located at the extra- and intracellular ends of the "hash motif" in the protein, play important roles in the conformational transition. In the outward- and inward-facing states of the protein, the loops adopt different secondary structures, either wrapped to the end of an alpha-helix, or unwrapped to extended conformations. In equilibrium simulations of 100 ns with Mhp1 in explicit lipids and water, the loop conformations remain largely stable. In targeted molecular dynamics simulations with the protein structure driven from one state to the other, the loops exhibit resistance and only undergo abrupt changes when other parts of the protein already approach the target conformation. Free energy calculations on the isolated loops further confirm that the wrapping/unwrapping transitions are associated with substantial energetic barriers, and consist of multiple sequential steps involving the rotation of certain backbone torsion angles. Furthermore, in simulations with the loops driven from one state to the other, a large part of the protein follows the loops to the target conformation. Taken together, our simulations suggest that changes of the loop secondary structures would be among the slow degrees of freedom in the conformational transition of the entire protein. Incorporation of detailed loop structures into the reaction coordinate, therefore, should improve the convergence and relevance of the resulting conformational free energy.
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Affiliation(s)
- Hyun Deok Song
- Department of Physics, Indiana University—Purdue University Indianapolis, Indianapolis, Indiana, United States of America
| | - Fangqiang Zhu
- Department of Physics, Indiana University—Purdue University Indianapolis, Indianapolis, Indiana, United States of America
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11
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Islam SM, Roux B. Simulating the distance distribution between spin-labels attached to proteins. J Phys Chem B 2015; 119:3901-11. [PMID: 25645890 PMCID: PMC4509421 DOI: 10.1021/jp510745d] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
EPR/DEER spectroscopy is playing
an increasingly important role
in the characterization of the conformational states of proteins.
In this study, force field parameters for the bifunctional spin-label
(RX) used in EPR/DEER are parametrized and tested with molecular dynamics
(MD) simulations. The dihedral angles connecting the Cα atom of the backbone to the nitroxide ring moiety of the RX spin-label
attached to i and i + 4 positions
in a polyalanine α-helix agree very well with those observed
in the X-ray crystallography. Both RXi,i+4 and RXi,i+3 are more rigid than the monofunctional spin-label (R1) commonly
used in EPR/DEER, while RXi,i+4 is more rigid and causes less distortion in a protein backbone
than RXi,i+3. Simplified
dummy spin-label models with a single effective particle representing
the RXi,i+3 and RXi,i+4 are also developed
and parametrized from the all-atom simulations. MD simulations with
dummy spin-labels (MDDS) provide distance distributions that can be
directly compared to distance distributions obtained from EPR/DEER
to rapidly assess if a hypothetical three-dimensional (3D) structural
model is consistent with experiment. The dummy spin-labels can also
be used in the restrained-ensemble MD (re-MD) simulations to carry
out structural refinement of 3D models. Applications of this methodology
to T4 lysozyme, KCNE1, and LeuT are shown to provide important insights
about their conformational dynamics.
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Affiliation(s)
- Shahidul M Islam
- Department of Biochemistry and Molecular Biology and ‡Department of Chemistry, University of Chicago , Chicago, Illinois 60637, United States
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12
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Horn AHC. A consistent force field parameter set for zwitterionic amino acid residues. J Mol Model 2014; 20:2478. [PMID: 25338816 DOI: 10.1007/s00894-014-2478-z] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2014] [Accepted: 09/21/2014] [Indexed: 12/20/2022]
Abstract
Isolated amino acids play an important role in biochemistry and are therefore an interesting object of study. Atomistic molecular dynamics (MD) simulations can provide a high-resolution picture of the dynamic features of these species, especially in their biological environment. Unfortunately, most standard force field packages lack libraries for isolated amino acids in their zwitterionic form. Although several studies have used ad-hoc parameterizations for single amino acids, a consistent force-field parameter set for these molecules is still missing. Here, we present such a parameter library derived from the widely used parm99SB set from the AMBER program package. The parameter derivation for all 20 proteinogenic amino acids transparently followed established procedures with histidine treated in three different protonation states. All amino acids were subjected to MD simulations in four different forms for comparison: zwitterionic, N-teminally capped with acetyl, C-terminally capped with N-methyl, and capped at both termini. Simulation results show similarities between the different forms. Five zwitterionic amino acids-arginine, glutamate, glycine, phenylalanine, leucine-were simulated in a protein environment. Proteins and ligands generally retained their initial structure. The new parameter set will thus facilitate future atomistic simulations of these species.
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Affiliation(s)
- Anselm H C Horn
- Bioinformatik, Institut für Biochemie, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Fahrstr. 17, 91054, Erlangen, Germany,
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13
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Loland CJ. The use of LeuT as a model in elucidating binding sites for substrates and inhibitors in neurotransmitter transporters. Biochim Biophys Acta Gen Subj 2014; 1850:500-10. [PMID: 24769398 DOI: 10.1016/j.bbagen.2014.04.011] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2014] [Revised: 04/09/2014] [Accepted: 04/11/2014] [Indexed: 01/24/2023]
Abstract
BACKGROUND The mammalian neurotransmitter transporters are complex proteins playing a central role in synaptic transmission between neurons by rapid reuptake of neurotransmitters. The proteins which transport dopamine, noradrenaline and serotonin belong to the Neurotransmitter:Sodium Symporters (NSS). Due to their important role, dysfunctions are associated with several psychiatric and neurological diseases and they also serve as targets for a wide range of therapeutic and illicit drugs. Despite the central physiological and pharmacological importance, direct evidence on structure-function relationships on mammalian NSS proteins has so far been unsuccessful. The crystal structure of the bacterial NSS protein, LeuT, has been a turning point in structural investigations. SCOPE OF REVIEW To provide an update on what is known about the binding sites for substrates and inhibitors in the LeuT. The different binding modes and binding sites will be discussed with special emphasis on the possible existence of a second substrate binding site. It is the goal to give an insight into how investigations on ligand binding in LeuT have provided basic knowledge about transporter conformations and translocation mechanism which can pave the road for a deeper understanding of drug binding and function of the mammalian transporters. MAJOR CONCLUSIONS The LeuT is a suitable model for the structural investigation of NSS proteins including the possible location of drug binding sites. It is still debated whether the LeuT is a suitable model for the molecular mechanisms behind substrate translocation. GENERAL SIGNIFICANCE Structure and functional aspects of NSS proteins are central for understanding synaptic transmission. With the purification and crystallization of LeuT as well as the dopamine transporter from Drosophila melanogaster, the application of biophysical methods such as fluorescence spectroscopy, neutron- or x-ray scattering and NMR for understanding its function becomes increasingly available. This article is part of a Special Issue entitled Structural biochemistry and biophysics of membrane proteins.
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Affiliation(s)
- Claus J Loland
- Molecular Neuropharmacology Laboratory, Department of Neuroscience and Pharmacology, The Faculty of Health and Medical Sciences, University of Copenhagen, DK-2200 Copenhagen N, Denmark.
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14
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Cheng MH, Bahar I. Coupled global and local changes direct substrate translocation by neurotransmitter-sodium symporter ortholog LeuT. Biophys J 2014; 105:630-9. [PMID: 23931311 DOI: 10.1016/j.bpj.2013.06.032] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2013] [Revised: 06/12/2013] [Accepted: 06/14/2013] [Indexed: 01/01/2023] Open
Abstract
Significant advances have been made in recent years in characterizing neurotransmitter:sodium symporter (NSS) family structure and function. Yet, many time-resolved events and intermediates that control the various stages of transport cycle remain to be elucidated. Whether NSSs harbor one or two sites for binding their substrates (neurotransmitters or amino acids), and what the role of the secondary site S2 is, if any, are still unresolved. Using molecular modeling and simulations for LeuT, a bacterial NSS, we present a comprehensive account of substrate-binding and -stabilization events, and subsequently triggered interactions leading to substrate (alanine) release. LeuT instantaneous conformation as it reconfigures from substrate-receiving (outward-facing) to -releasing (inward-facing) state appears to be a determinant of its affinity to bind substrate at site S2. In the outward-facing state, S1 robustly binds alanine and regulates subsequent redistribution of interactions to trigger extracellular gate closure; whereas S2 is only a transient binding site. The substrate-binding affinity at S2 increases in an intermediate close to inward-facing state. LeuT harbors the two substrate-binding sites, and small displacements of second substrate near S2 are observed to induce concerted small translocations in the substrate bound to primary site S1, although complete release requires collective structural rearrangements that fully expose the intracellular vestibule to the cytoplasm.
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Affiliation(s)
- Mary Hongying Cheng
- Department of Computational and Systems Biology, School of Medicine, University of Pittsburgh, Pennsylvania, USA
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15
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Abstract
Neurotransmitter:sodium symporters (NSSs) play a critical role in signaling by reuptake of neurotransmitters. Eukaryotic NSSs are chloride-dependent, whereas prokaryotic NSS homologs like LeuT are chloride-independent but contain an acidic residue (Glu290 in LeuT) at a site where eukaryotic NSSs have a serine. The LeuT-E290S mutant displays chloride-dependent activity. We show that, in LeuT-E290S cocrystallized with bromide or chloride, the anion is coordinated by side chain hydroxyls from Tyr47, Ser290, and Thr254 and the side chain amide of Gln250. The bound anion and the nearby sodium ion in the Na1 site organize a connection between their coordinating residues and the extracellular gate of LeuT through a continuous H-bond network. The specific insights from the structures, combined with results from substrate binding studies and molecular dynamics simulations, reveal an anion-dependent occlusion mechanism for NSS and shed light on the functional role of chloride binding.
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16
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Shaikh S, Li J, Enkavi G, Wen PC, Huang Z, Tajkhorshid E. Visualizing functional motions of membrane transporters with molecular dynamics simulations. Biochemistry 2013; 52:569-87. [PMID: 23298176 PMCID: PMC3560430 DOI: 10.1021/bi301086x] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2012] [Revised: 12/21/2012] [Indexed: 01/08/2023]
Abstract
Computational modeling and molecular simulation techniques have become an integral part of modern molecular research. Various areas of molecular sciences continue to benefit from, indeed rely on, the unparalleled spatial and temporal resolutions offered by these technologies, to provide a more complete picture of the molecular problems at hand. Because of the continuous development of more efficient algorithms harvesting ever-expanding computational resources, and the emergence of more advanced and novel theories and methodologies, the scope of computational studies has expanded significantly over the past decade, now including much larger molecular systems and far more complex molecular phenomena. Among the various computer modeling techniques, the application of molecular dynamics (MD) simulation and related techniques has particularly drawn attention in biomolecular research, because of the ability of the method to describe the dynamical nature of the molecular systems and thereby to provide a more realistic representation, which is often needed for understanding fundamental molecular properties. The method has proven to be remarkably successful in capturing molecular events and structural transitions highly relevant to the function and/or physicochemical properties of biomolecular systems. Herein, after a brief introduction to the method of MD, we use a number of membrane transport proteins studied in our laboratory as examples to showcase the scope and applicability of the method and its power in characterizing molecular motions of various magnitudes and time scales that are involved in the function of this important class of membrane proteins.
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Affiliation(s)
- Saher
A. Shaikh
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
| | - Jing Li
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
| | - Giray Enkavi
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
| | - Po-Chao Wen
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
| | - Zhijian Huang
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
| | - Emad Tajkhorshid
- Department of Biochemistry, Beckman Institute for Advanced
Science and Technology, and Center for Biophysics and Computational
Biology, University of Illinois at Urbana-Champaign, 405 North Mathews Avenue, Urbana, Illinois 61801, United States
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17
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Kosti V, Lambrinidis G, Myrianthopoulos V, Diallinas G, Mikros E. Identification of the substrate recognition and transport pathway in a eukaryotic member of the nucleobase-ascorbate transporter (NAT) family. PLoS One 2012; 7:e41939. [PMID: 22848666 PMCID: PMC3405029 DOI: 10.1371/journal.pone.0041939] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2012] [Accepted: 06/28/2012] [Indexed: 01/08/2023] Open
Abstract
Using the crystal structure of the uracil transporter UraA of Escherichia coli, we constructed a 3D model of the Aspergillus nidulans uric acid-xanthine/H(+) symporter UapA, which is a prototype member of the Nucleobase-Ascorbate Transporter (NAT) family. The model consists of 14 transmembrane segments (TMSs) divided into a core and a gate domain, the later being distinctly different from that of UraA. By implementing Molecular Mechanics (MM) simulations and quantitative structure-activity relationship (SAR) approaches, we propose a model for the xanthine-UapA complex where the substrate binding site is formed by the polar side chains of residues E356 (TMS8) and Q408 (TMS10) and the backbones of A407 (TMS10) and F155 (TMS3). In addition, our model shows several polar interactions between TMS1-TMS10, TMS1-TMS3, TMS8-TMS10, which seem critical for UapA transport activity. Using extensive docking calculations we identify a cytoplasm-facing substrate trajectory (D360, A363, G411, T416, R417, V463 and A469) connecting the proposed substrate binding site with the cytoplasm, as well as, a possible outward-facing gate leading towards the substrate major binding site. Most importantly, re-evaluation of the plethora of available and analysis of a number of herein constructed UapA mutations strongly supports the UapA structural model. Furthermore, modeling and docking approaches with mammalian NAT homologues provided a molecular rationale on how specificity in this family of carriers might be determined, and further support the importance of selectivity gates acting independently from the major central substrate binding site.
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Affiliation(s)
- Vasiliki Kosti
- Faculty of Biology, University of Athens, Panepistimiopolis, Athens, Greece
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18
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Merchant BA, Madura JD. Insights from molecular dynamics: the binding site of cocaine in the dopamine transporter and permeation pathways of substrates in the leucine and dopamine transporters. J Mol Graph Model 2012; 38:1-12. [PMID: 23079638 DOI: 10.1016/j.jmgm.2012.05.007] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2012] [Revised: 04/30/2012] [Accepted: 05/04/2012] [Indexed: 11/18/2022]
Abstract
The dopamine transporter (DAT) facilitates the regulation of synaptic neurotransmitter levels. As a target for therapeutic and illicit psycho-stimulant drugs like antidepressants and cocaine, DAT has been studied intensively. Despite a wealth of mutational and physiological data regarding DAT, the structure remains unsolved and details of the transport mechanism, binding sites and conformational changes remain debated. A bacterial homolog of DAT, the leucine transporter (LeuT(Aa)) has been used as a template and framework for modeling and understanding DAT. Free energy profiles obtained from Multi-Configuration Thermodynamic Integration simulations allowed us to correctly identify the primary and secondary binding pockets of LeuT(Aa). A comparison of free energy profiles for dopamine and cocaine in DAT suggests that the binding site of cocaine is located in a secondary pocket, not the primary substrate site. Two recurring primary pathways for intracellular substrate release from the primary pocket are identified in both transporters using the Random Acceleration Molecular Dynamics method. One pathway appears to follow transmembranes (TMs) 1a and 6b while the other pathway follows along TMs 6b and 8. Interestingly, we observe that a single sodium ion is co-transported with leucine during both simulation types.
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Affiliation(s)
- Bonnie A Merchant
- Department of Chemistry and Biochemistry and Center for Computational Sciences, Duquesne University, 600 Forbes Avenue, Pittsburgh, PA 15282, USA
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19
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The structural pathway for water permeation through sodium-glucose cotransporters. Biophys J 2012; 101:1887-95. [PMID: 22004742 DOI: 10.1016/j.bpj.2011.09.019] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2011] [Revised: 09/08/2011] [Accepted: 09/16/2011] [Indexed: 11/21/2022] Open
Abstract
Although water permeation across cell membranes occurs through several types of membrane proteins, the only permeation mechanism resolved at atomic scale is that through aquaporins. Crystallization of the Vibrio parahaemolyticus sodium-galactose transporter (vSGLT) allows investigation of putative water permeation pathways through both vSGLT and the homologous human Na-glucose cotransporter (hSGLT1) using computational methods. Grand canonical Monte Carlo and molecular dynamics simulations were used to stably insert water molecules in both proteins, showing the presence of a water-filled pathway composed of ∼100 water molecules. This provides a structural basis for passive water permeation that is difficult to reconcile with the water cotransport hypothesis. Potential-of-mean-force calculations of water going through the crystal structure of vSGLT shows a single barrier of 7.7 kCal mol(-1), in agreement with previously published experimental data for cotransporters of the SGLT family. Electrophysiological and volumetric experiments performed on hSGLT1-expressing Xenopus oocytes showed that the passive permeation pathway exists in different conformational states. In particular, experimental conditions that aim to mimic the conformation of the crystal structure displayed passive water permeability. These results provide groundwork for understanding the structural basis of cotransporter water permeability.
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20
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Zdravkovic I, Zhao C, Lev B, Cuervo JE, Noskov SY. Atomistic models of ion and solute transport by the sodium-dependent secondary active transporters. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1818:337-47. [PMID: 22138368 DOI: 10.1016/j.bbamem.2011.10.031] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/22/2011] [Revised: 10/24/2011] [Accepted: 10/25/2011] [Indexed: 12/19/2022]
Abstract
The recent determination of high-resolution crystal structures of several transporters offers unprecedented insights into the structural mechanisms behind secondary transport. These proteins utilize the facilitated diffusion of the ions down their electrochemical gradients to transport the substrate against its concentration gradient. The structural studies revealed striking similarities in the structural organization of ion and solute binding sites and a well-conserved inverted-repeat topology between proteins from several gene families. In this paper we will overview recent atomistic simulations applied to study the mechanisms of selective binding of ion and substrate in LeuT, Glt, vSGLT and hSERT as well as its consequences for the transporter conformational dynamics. This article is part of a Special Issue entitled: Membrane protein structure and function.
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Affiliation(s)
- Igor Zdravkovic
- Institute for Biocomplexity and Informatics (IBI), University of Calgary, 2500 University Drive, Calgary, Canada, T2N 1N4
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21
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Kristensen AS, Andersen J, Jørgensen TN, Sørensen L, Eriksen J, Loland CJ, Strømgaard K, Gether U. SLC6 neurotransmitter transporters: structure, function, and regulation. Pharmacol Rev 2011; 63:585-640. [PMID: 21752877 DOI: 10.1124/pr.108.000869] [Citation(s) in RCA: 601] [Impact Index Per Article: 46.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.
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Affiliation(s)
- Anders S Kristensen
- Department of Medicinal Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Copenhagen, Denmark.
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22
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Henry LK, Iwamoto H, Field JR, Kaufmann K, Dawson ES, Jacobs MT, Adams C, Felts B, Zdravkovic I, Armstrong V, Combs S, Solis E, Rudnick G, Noskov SY, DeFelice LJ, Meiler J, Blakely RD. A conserved asparagine residue in transmembrane segment 1 (TM1) of serotonin transporter dictates chloride-coupled neurotransmitter transport. J Biol Chem 2011; 286:30823-30836. [PMID: 21730057 DOI: 10.1074/jbc.m111.250308] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Na(+)- and Cl(-)-dependent uptake of neurotransmitters via transporters of the SLC6 family, including the human serotonin transporter (SLC6A4), is critical for efficient synaptic transmission. Although residues in the human serotonin transporter involved in direct Cl(-) coordination of human serotonin transport have been identified, the role of Cl(-) in the transport mechanism remains unclear. Through a combination of mutagenesis, chemical modification, substrate and charge flux measurements, and molecular modeling studies, we reveal an unexpected role for the highly conserved transmembrane segment 1 residue Asn-101 in coupling Cl(-) binding to concentrative neurotransmitter uptake.
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Affiliation(s)
- L Keith Henry
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Center for Structural Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Department of Pharmacology, Physiology, and Therapeutics, University of North Dakota, Grand Forks, North Dakota 58203.
| | - Hideki Iwamoto
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Julie R Field
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Kristian Kaufmann
- Chemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Eric S Dawson
- Center for Structural Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Vanderbilt Center for Neuroscience Drug Discovery, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Miriam T Jacobs
- Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06520-8066
| | - Chelsea Adams
- Department of Pharmacology, Physiology, and Therapeutics, University of North Dakota, Grand Forks, North Dakota 58203
| | - Bruce Felts
- Department of Pharmacology, Physiology, and Therapeutics, University of North Dakota, Grand Forks, North Dakota 58203
| | - Igor Zdravkovic
- Institute for Biocomplexity and Informatics, Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada
| | - Vanessa Armstrong
- Department of Pharmacology, Physiology, and Therapeutics, University of North Dakota, Grand Forks, North Dakota 58203
| | - Steven Combs
- Chemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Ernesto Solis
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Gary Rudnick
- Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06520-8066
| | - Sergei Y Noskov
- Institute for Biocomplexity and Informatics, Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada
| | - Louis J DeFelice
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Center for Molecular Neuroscience, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Jens Meiler
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Center for Structural Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Chemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548
| | - Randy D Blakely
- Departments of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Center for Molecular Neuroscience, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548; Psychiatry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-8548.
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Substrate-modulated gating dynamics in a Na+-coupled neurotransmitter transporter homologue. Nature 2011; 474:109-13. [PMID: 21516104 PMCID: PMC3178346 DOI: 10.1038/nature09971] [Citation(s) in RCA: 212] [Impact Index Per Article: 16.3] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2010] [Accepted: 02/25/2011] [Indexed: 12/19/2022]
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24
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Zhao C, Noskov SY. The Role of Local Hydration and Hydrogen-Bonding Dynamics in Ion and Solute Release from Ion-Coupled Secondary Transporters. Biochemistry 2011; 50:1848-56. [DOI: 10.1021/bi101454f] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- Chunfeng Zhao
- Institute for Biocomplexity and Informatics and Department of Biological Sciences, University of Calgary, 2500 University Drive, BI558, Calgary, Alberta, Canada T2N 1N4
| | - Sergei Yu. Noskov
- Institute for Biocomplexity and Informatics and Department of Biological Sciences, University of Calgary, 2500 University Drive, BI558, Calgary, Alberta, Canada T2N 1N4
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25
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The substrate-driven transition to an inward-facing conformation in the functional mechanism of the dopamine transporter. PLoS One 2011; 6:e16350. [PMID: 21298009 PMCID: PMC3029329 DOI: 10.1371/journal.pone.0016350] [Citation(s) in RCA: 100] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2010] [Accepted: 12/13/2010] [Indexed: 11/30/2022] Open
Abstract
Background The dopamine transporter (DAT), a member of the neurotransmitter:Na+ symporter (NSS) family, terminates dopaminergic neurotransmission and is a major molecular target for psychostimulants such as cocaine and amphetamine, and for the treatment of attention deficit disorder and depression. The crystal structures of the prokaryotic NSS homolog of DAT, the leucine transporter LeuT, have provided critical structural insights about the occluded and outward-facing conformations visited during the substrate transport, but only limited clues regarding mechanism. To understand the transport mechanism in DAT we have used a homology model based on the LeuT structure in a computational protocol validated previously for LeuT, in which steered molecular dynamics (SMD) simulations guide the substrate along a pathway leading from the extracellular end to the intracellular (cytoplasmic) end. Methodology/Principal Findings Key findings are (1) a second substrate binding site in the extracellular vestibule, and (2) models of the conformational states identified as occluded, doubly occupied, and inward-facing. The transition between these states involve a spatially ordered sequence of interactions between the two substrate-binding sites, followed by rearrangements in structural elements located between the primary binding site and the cytoplasmic end. These rearrangements are facilitated by identified conserved hinge regions and a reorganization of interaction networks that had been identified as gates. Conclusions/Significance Computational simulations supported by information available from experiments in DAT and other NSS transporters have produced a detailed mechanistic proposal for the dynamic changes associated with substrate transport in DAT. This allosteric mechanism is triggered by the binding of substrate in the S2 site in the presence of the substrate in the S1 site. Specific structural elements involved in this mechanism, and their roles in the conformational transitions illuminated here describe, a specific substrate-driven allosteric mechanism that is directly amenable to experiment as shown previously for LeuT.
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26
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Subbotina J, Yarov-Yarovoy V, Lees-Miller J, Durdagi S, Guo J, Duff HJ, Noskov SY. Structural refinement of the hERG1 pore and voltage-sensing domains with ROSETTA-membrane and molecular dynamics simulations. Proteins 2010; 78:2922-34. [PMID: 20740484 PMCID: PMC2939218 DOI: 10.1002/prot.22815] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
The hERG1 gene (Kv11.1) encodes a voltage-gated potassium channel. Mutations in this gene lead to one form of the Long QT Syndrome (LQTS) in humans. Promiscuous binding of drugs to hERG1 is known to alter the structure/function of the channel leading to an acquired form of the LQTS. Expectably, creation and validation of reliable 3D model of the channel have been a key target in molecular cardiology and pharmacology for the last decade. Although many models were built, they all were limited to pore domain. In this work, a full model of the hERG1 channel is developed which includes all transmembrane segments. We tested a template-driven de-novo design with ROSETTA-membrane modeling using side-chain placements optimized by subsequent molecular dynamics (MD) simulations. Although backbone templates for the homology modeled parts of the pore and voltage sensors were based on the available structures of KvAP, Kv1.2 and Kv1.2-Kv2.1 chimera channels, the missing parts are modeled de-novo. The impact of several alignments on the structure of the S4 helix in the voltage-sensing domain was also tested. Herein, final models are evaluated for consistency to the reported structural elements discovered mainly on the basis of mutagenesis and electrophysiology. These structural elements include salt bridges and close contacts in the voltage-sensor domain; and the topology of the extracellular S5-pore linker compared with that established by toxin foot-printing and nuclear magnetic resonance studies. Implications of the refined hERG1 model to binding of blockers and channels activators (potent new ligands for channel activations) are discussed.
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Affiliation(s)
- Julia Subbotina
- Institute for Biocomplexity and Informatics, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
| | | | - James Lees-Miller
- Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Serdar Durdagi
- Institute for Biocomplexity and Informatics, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
| | - Jiqing Guo
- Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Henry J. Duff
- Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Sergei Yu. Noskov
- Institute for Biocomplexity and Informatics, Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada
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27
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Shaikh SA, Tajkhorshid E. Modeling and dynamics of the inward-facing state of a Na+/Cl- dependent neurotransmitter transporter homologue. PLoS Comput Biol 2010; 6. [PMID: 20865057 PMCID: PMC2928745 DOI: 10.1371/journal.pcbi.1000905] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2010] [Accepted: 07/26/2010] [Indexed: 10/30/2022] Open
Abstract
The leucine transporter (LeuT) has recently commanded exceptional attention due mainly to two distinctions; it provides the only crystal structures available for a protein homologous to the pharmacologically relevant neurotransmitter: sodium symporters (NSS), and, it exhibits a hallmark 5-TM inverted repeat ("LeuT-fold"), a fold recently discovered to also exist in several secondary transporter families, underscoring its general role in transporter function. Constructing the transport cycle of "LeuT-fold" transporters requires detailed structural and dynamic descriptions of the outward-facing (OF) and inward-facing (IF) states, as well as the intermediate states. To this end, we have modeled the structurally unknown IF state of LeuT, based on the known crystal structures of the OF state of LeuT and the IF state of vSGLT, a "LeuT-fold" transporter. The detailed methodology developed for the study combines structure-based alignment, threading, targeted MD and equilibrium MD, and can be applied to other proteins. The resulting IF-state models maintain the secondary structural features of LeuT. Water penetration and solvent accessibility calculations show that TM1, TM3, TM6 and TM8 line the substrate binding/unbinding pathway with TM10 and its pseudosymmetric partner, TM5, participating in the extracellular and intracellular halves of the lumen, respectively. We report conformational hotspots where notable changes in interactions occur between the IF and OF states. We observe Na2 exiting the LeuT-substrate- complex in the IF state, mainly due to TM1 bending. Inducing a transition in only one of the two pseudosymmetric domains, while allowing the second to respond dynamically, is found to be sufficient to induce the formation of the IF state. We also propose that TM2 and TM7 may be facilitators of TM1 and TM6 motion. Thus, this study not only presents a novel modeling methodology applied to obtain the IF state of LeuT, but also describes structural elements involved in a possibly general transport mechanism in transporters adopting the "LeuT-fold".
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Affiliation(s)
- Saher Afshan Shaikh
- Department of Biochemistry and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America
| | - Emad Tajkhorshid
- Department of Biochemistry and Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America
- College of Medicine and Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America
- * E-mail:
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Agelis G, Roumelioti P, Resvani A, Durdagi S, Androutsou ME, Kelaidonis K, Vlahakos D, Mavromoustakos T, Matsoukas J. An efficient synthesis of a rationally designed 1,5 disubstituted imidazole AT(1) angiotensin II receptor antagonist: reorientation of imidazole pharmacophore groups in losartan reserves high receptor affinity and confirms docking studies. J Comput Aided Mol Des 2010; 24:749-58. [PMID: 20623162 DOI: 10.1007/s10822-010-9371-3] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2010] [Accepted: 06/17/2010] [Indexed: 11/25/2022]
Abstract
A new 1,5 disubstituted imidazole AT(1) Angiotensin II (AII) receptor antagonist related to losartan with reversion of butyl and hydroxymethyl groups at the 2-, 5-positions of the imidazole ring was synthesized and evaluated for its antagonist activity (V8). In vitro results indicated that the reorientation of butyl and hydroxymethyl groups on the imidazole template of losartan retained high binding affinity to the AT(1) receptor concluding that the spacing of the substituents at the 2,5- positions is of primary importance. The docking studies are confirmed by binding assay results which clearly show a comparable binding score of the designed compound V8 with that of the prototype losartan. An efficient, regioselective and cost effective synthesis renders the new compound as an attractive candidate for advanced toxicological evaluation and a drug against hypertension.
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Affiliation(s)
- George Agelis
- Department of Chemistry, University of Patras, Patras 26500, Greece.
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29
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Ion/substrate-dependent conformational dynamics of a bacterial homolog of neurotransmitter:sodium symporters. Nat Struct Mol Biol 2010; 17:822-9. [PMID: 20562855 DOI: 10.1038/nsmb.1854] [Citation(s) in RCA: 165] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2010] [Accepted: 04/16/2010] [Indexed: 11/08/2022]
Abstract
Crystallographic, computational and functional analyses of LeuT have revealed details of the molecular architecture of Na(+)-coupled transporters and the mechanistic nature of ion/substrate coupling, but the conformational changes that support a functional transport cycle have yet to be described fully. We have used site-directed spin labeling and electron paramagnetic resonance (EPR) analysis to capture the dynamics of LeuT in the region of the extracellular vestibule associated with the binding of Na(+) and leucine. The results outline the Na(+)-dependent formation of a dynamic outward-facing intermediate that exposes the primary substrate binding site and the conformational changes that occlude this binding site upon subsequent binding of the leucine substrate. Furthermore, the binding of the transport inhibitors tryptophan, clomipramine and octyl-glucoside is shown to induce structural changes that distinguish the resulting inhibited conformation from the Na(+)/leucine-bound state.
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30
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Aleksandrov A, Thompson D, Simonson T. Alchemical free energy simulations for biological complexes: powerful but temperamental.... J Mol Recognit 2010; 23:117-27. [PMID: 19693787 DOI: 10.1002/jmr.980] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Free energy simulations compare multiple ligand:receptor complexes by "alchemically" transforming one into another, yielding binding free energy differences. Since their introduction in the 1980s, many technical and theoretical obstacles were surmounted, and the method ("MDFE," since molecular dynamics are often used) has matured into a powerful tool. We describe its current status, its effectiveness, and the challenges it faces. MDFE has provided chemical accuracy for many systems but remains expensive, with significant human overhead costs. The bottlenecks have shifted, partly due to increased computer power. To study diverse sets of ligands, force field availability and accuracy can be a major difficulty. Another difficulty is the frequent need to consider multiple states, related to sidechain protonation or buried waters, for example. Sophisticated, automated methods to sample these states are maturing, such as constant pH simulations. Meanwhile, combinations of MDFE and simpler approaches, like continuum dielectric models, can be very effective. As illustrations, we show how, with careful force field parameterization, MDFE accurately predicts binding specificities between complex tetracycline ligands and their targets. We describe substrate binding to the aspartyl-tRNA synthetase enzyme, where many distinct electrostatic states play a role, and a histidine and a Mg(2+) ion act as coupled switches that help enforce a strict preference for the aspartate substrate, relative to several analogs. Overall, MDFE has achieved a predictive status, where novel ligands can be studied and molecular recognition elucidated in depth. It should play an increasing role in the analysis of complex cellular processes and biomolecular engineering.
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Affiliation(s)
- Alexey Aleksandrov
- Laboratoire de Biochimie (CNRS UMR7654), Department of Biology, Ecole Polytechnique, 91128 Palaiseau, France
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31
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Molecular mechanism of ion-ion and ion-substrate coupling in the Na+-dependent leucine transporter LeuT. Biophys J 2008; 95:4613-21. [PMID: 18708457 DOI: 10.1529/biophysj.108.139741] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Ion-coupled transport of neurotransmitter molecules by neurotransmitter:sodium symporters (NSS) play an important role in the regulation of neuronal signaling. One of the major events in the transport cycle is ion-substrate coupling and formation of the high-affinity occluded state with bound ions and substrate. Molecular mechanisms of ion-substrate coupling and the corresponding ion-substrate stoichiometry in NSS transporters has yet to be understood. The recent determination of a high-resolution structure for a bacterial homolog of Na(+)/Cl(-)-dependent neurotransmitter transporters, LeuT, offers a unique opportunity to analyze the functional roles of the multi-ion binding sites within the binding pocket. The binding pocket of LeuT contains two metal binding sites. The first ion in site NA1 is directly coupled to the bound substrate (Leu) with the second ion in the neighboring site (NA2) only approximately 7 A away. Extensive, fully atomistic, molecular dynamics, and free energy simulations of LeuT in an explicit lipid bilayer are performed to evaluate substrate-binding affinity as a function of the ion load (single versus double occupancy) and occupancy by specific monovalent cations. It was shown that double ion occupancy of the binding pocket is required to ensure substrate coupling to Na(+) and not to Li(+) or K(+) cations. Furthermore, it was found that presence of the ion in site NA2 is required for structural stability of the binding pocket as well as amplified selectivity for Na(+) in the case of double ion occupancy.
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Noskov SY, Roux B. Control of ion selectivity in LeuT: two Na+ binding sites with two different mechanisms. J Mol Biol 2008; 377:804-18. [PMID: 18280500 PMCID: PMC4948944 DOI: 10.1016/j.jmb.2008.01.015] [Citation(s) in RCA: 157] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2007] [Revised: 01/07/2008] [Accepted: 01/08/2008] [Indexed: 10/22/2022]
Abstract
The x-ray structure of LeuT, a bacterial homologue of Na(+)/Cl(-)-dependent neurotransmitter transporters, provides a great opportunity to better understand the molecular basis of monovalent cation selectivity in ion-coupled transporters. LeuT possesses two ion binding sites, NA1 and NA2, which are highly selective for Na(+). Extensive all-atom free-energy molecular dynamics simulations of LeuT embedded in an explicit membrane are performed at different temperatures and various occupancy states of the binding sites to dissect the molecular mechanism of ion selectivity. The results show that the two binding sites display robust selectivity for Na(+) over K(+) or Li(+), the competing ions of most similar radii. Of particular interest, the mechanism primarily responsible for selectivity for each of the two binding sites appears to be different. In NA1, selectivity for Na(+) over K(+) arises predominantly from the strong electrostatic field arising from the negatively charged carboxylate group of the leucine substrate coordinating the ion directly. In NA2, which comprises only neutral ligands, selectivity for Na(+) is enforced by the local structural restraints arising from the hydrogen-bonding network and the covalent connectivity of the polypeptide chain surrounding the ion according to a "snug-fit" mechanism.
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Affiliation(s)
- Sergei Y Noskov
- Institute for Biocomplexity and Informatics, Department for Biological Sciences, University of Calgary, 2500 University Drive, Calgary, AB, Canada.
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