1
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Zhou Z, Arroum T, Luo X, Kang R, Lee YJ, Tang D, Hüttemann M, Song X. Diverse functions of cytochrome c in cell death and disease. Cell Death Differ 2024; 31:387-404. [PMID: 38521844 PMCID: PMC11043370 DOI: 10.1038/s41418-024-01284-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2023] [Revised: 03/13/2024] [Accepted: 03/18/2024] [Indexed: 03/25/2024] Open
Abstract
The redox-active protein cytochrome c is a highly positively charged hemoglobin that regulates cell fate decisions of life and death. Under normal physiological conditions, cytochrome c is localized in the mitochondrial intermembrane space, and its distribution can extend to the cytosol, nucleus, and extracellular space under specific pathological or stress-induced conditions. In the mitochondria, cytochrome c acts as an electron carrier in the electron transport chain, facilitating adenosine triphosphate synthesis, regulating cardiolipin peroxidation, and influencing reactive oxygen species dynamics. Upon cellular stress, it can be released into the cytosol, where it interacts with apoptotic peptidase activator 1 (APAF1) to form the apoptosome, initiating caspase-dependent apoptotic cell death. Additionally, following exposure to pro-apoptotic compounds, cytochrome c contributes to the survival of drug-tolerant persister cells. When translocated to the nucleus, it can induce chromatin condensation and disrupt nucleosome assembly. Upon its release into the extracellular space, cytochrome c may act as an immune mediator during cell death processes, highlighting its multifaceted role in cellular biology. In this review, we explore the diverse structural and functional aspects of cytochrome c in physiological and pathological responses. We summarize how posttranslational modifications of cytochrome c (e.g., phosphorylation, acetylation, tyrosine nitration, and oxidation), binding proteins (e.g., HIGD1A, CHCHD2, ITPR1, and nucleophosmin), and mutations (e.g., G41S, Y48H, and A51V) affect its function. Furthermore, we provide an overview of the latest advanced technologies utilized for detecting cytochrome c, along with potential therapeutic approaches related to this protein. These strategies hold tremendous promise in personalized health care, presenting opportunities for targeted interventions in a wide range of conditions, including neurodegenerative disorders, cardiovascular diseases, and cancer.
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Affiliation(s)
- Zhuan Zhou
- Department of Surgery, UT Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Tasnim Arroum
- Center for Molecular Medicine and Genetics, Wayne State University, Detroit, MI, 48201, USA
| | - Xu Luo
- Eppley Institute for Research in Cancer and Allied Diseases, Fred and Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, NE, 68198, USA
| | - Rui Kang
- Department of Surgery, UT Southwestern Medical Center, Dallas, TX, 75390, USA
| | - Yong J Lee
- Department of Biomedical Sciences, Cedars-Sinai Medical Center, 8700 Beverly Blvd, Los Angeles, CA, 90048, USA
| | - Daolin Tang
- Department of Surgery, UT Southwestern Medical Center, Dallas, TX, 75390, USA.
| | - Maik Hüttemann
- Center for Molecular Medicine and Genetics, Wayne State University, Detroit, MI, 48201, USA.
- Department of Biochemistry, Microbiology, and Immunology, Wayne State University, Detroit, MI, 48201, USA.
| | - Xinxin Song
- Department of Surgery, UT Southwestern Medical Center, Dallas, TX, 75390, USA.
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2
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Martin WJ, McClelland LJ, Nold SM, Boshae KL, Bowler BE. Effect of proline content and histidine ligation on the dynamics of Ω-loop D and the peroxidase activity of iso-1-cytochrome c. J Inorg Biochem 2024; 252:112474. [PMID: 38176365 DOI: 10.1016/j.jinorgbio.2023.112474] [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: 10/16/2023] [Revised: 12/20/2023] [Accepted: 12/20/2023] [Indexed: 01/06/2024]
Abstract
To study how proline residues affect the dynamics of Ω-loop D (residues 70 to 85) of cytochrome c, we prepared G83P and G83A variants of yeast iso-1-cytochrome c (iso-1-Cytc) in the presence and absence of a K73H mutation. Ω-loop D is important in controlling both the electron transfer function of Cytc and the peroxidase activity of Cytc used in apoptosis because it provides the Met80 heme ligand. The G83P and G83A mutations have no effect on the global stability of iso-1-Cytc in presence or absence of the K73H mutation. However, both mutations destabilize the His73-mediated alkaline conformer relative to the native state. pH jump stopped-flow experiments show that the dynamics of the His73-mediated alkaline transition are significantly enhanced by the G83P mutation. Gated electron transfer studies show that the enhanced dynamics result from an increased rate of return to the native state, whereas the rate of loss of Met80 ligation is unchanged by the G83P mutation. Thus, the G83P substitution does not stiffen the conformation of the native state. Because bis-His heme ligation occurs when Cytc binds to cardiolipin-containing membranes, we studied the effect of His73 ligation on the peroxidase activity of Cytc, which acts as an early signal in apoptosis by causing oxygenation of cardiolipin. We find that the His73 alkaline conformer suppresses the peroxidase activity of Cytc. Thus, the bis-His ligated state of Cytc formed upon binding to cardiolipin is a negative effector for the peroxidase activity of Cytc early in apoptosis.
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Affiliation(s)
- William J Martin
- Department of Chemistry & Biochemistry, University of Montana, Missoula, MT 59812, United States
| | - Levi J McClelland
- Department of Chemistry & Biochemistry, University of Montana, Missoula, MT 59812, United States; Division of Biological Sciences, University of Montana, Missoula, MT 59812, United States; Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, MT 59812, United States
| | - Shiloh M Nold
- Department of Chemistry & Biochemistry, University of Montana, Missoula, MT 59812, United States
| | - Kassandra L Boshae
- Department of Chemistry & Biochemistry, University of Montana, Missoula, MT 59812, United States
| | - Bruce E Bowler
- Department of Chemistry & Biochemistry, University of Montana, Missoula, MT 59812, United States; Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, MT 59812, United States.
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3
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Nutho B, Samsri S, Pornsuwan S. Structural Dynamics of the Precatalytic State of Human Cytochrome c upon T28C, G34C, and A50C Mutations: A Molecular Dynamics Simulation Perspective. ACS OMEGA 2023; 8:15229-15238. [PMID: 37151554 PMCID: PMC10157674 DOI: 10.1021/acsomega.3c00220] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/12/2023] [Accepted: 04/06/2023] [Indexed: 05/09/2023]
Abstract
The native structure of cytochrome c (cytc) contains hexacoordinate heme iron with His18 and Met80 residues ligated at the axial sites. Mutations of cytc at Ω-loops have been investigated in modulating the peroxidase activity and, hence, related to the initiation of the apoptotic pathway. Our previous experimental data reported on the peroxidase activity of the cysteine-directed mutants at different parts of the Ω-loop of human cytc (hCytc), that is, T28C, G34C, and A50C. In this work, we performed 1 μs molecular dynamics (MD) simulations to elucidate the detailed structural and dynamic changes upon these mutations, particularly at the proximal Ω-loop. The structures of hCytc were modeled in the hexacoordinated form, which was referred to as the "precatalytic state". The results showed that the structural features of the G34C mutant were more distinctive than those of other mutants. G34C mutation caused local destabilization and flexibility at the proximal Ω-loop (residues 12-28) and an extended distance between this Ω-loop region and heme iron. Besides, analysis of the orientation of the Arg38 side chain of the G34C mutant revealed the Arg38 conformer facing away from the heme iron. The obtained MD results also suggested structural diversity of the precatalytic states for the three hCytc mutants, specifically the effect of G34C mutation on the flexibility of the proximal Ω-loops. Therefore, our MD simulations combined with previous experimental data provide detailed insights into the structural basis of hCytc that could contribute to its pro-apoptotic function.
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Affiliation(s)
- Bodee Nutho
- Department
of Pharmacology, Faculty of Science, Mahidol
University, Bangkok 10400, Thailand
| | - Sasiprapa Samsri
- Department
of Chemistry and Center of Excellence for Innovation in Chemistry,
Faculty of Science, Mahidol University, Bangkok 10400, Thailand
| | - Soraya Pornsuwan
- Department
of Chemistry and Center of Excellence for Innovation in Chemistry,
Faculty of Science, Mahidol University, Bangkok 10400, Thailand
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4
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Lei H, Kelly AD, Bowler BE. Alkaline State of the Domain-Swapped Dimer of Human Cytochrome c: A Conformational Switch for Apoptotic Peroxidase Activity. J Am Chem Soc 2022; 144:21184-21195. [PMID: 36346995 PMCID: PMC9743720 DOI: 10.1021/jacs.2c08325] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
A 2.08 Å structure of an alkaline conformer of the domain-swapped dimer of K72A human cytochrome c (Cytc) crystallized at pH 9.9 is presented. In the structure, Lys79 is ligated to the heme. All other domain-swapped dimer structures of Cytc have water bound to this coordination site. Part of Ω-loop D (residues 70-85) forms a flexible linker between the subunits in other Cytc domain-swapped dimer structures but instead converts to a helix in the alkaline conformer of the dimer combining with the C-terminal helix to form two 26-residue helices that bracket both sides of the dimer. The alkaline transition of the K72A human dimer monitored at both 625 nm (high spin heme) and 695 nm (Met80 ligation) yields midpoint pH values of 6.6 and 7.6, respectively, showing that the Met80 → Lys79 and high spin to low spin transitions are distinct. The dimer peroxidase activity increases rapidly below pH 7, suggesting that population of the high spin form of the heme is what promotes peroxidase activity. Comparison of the structures of the alkaline dimer and the neutral pH dimer shows that the neutral pH conformer has a better electrostatic surface for binding to a cardiolipin-containing membrane and provides better access for small molecules to the heme iron. Given that the pH of mitochondrial cristae ranges from 6.9 to 7.2, the alkaline transition of the Cytc dimer could provide a conformational switch to tune the peroxidase activity of Cytc that oxygenates cardiolipin in the early stages of apoptosis.
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Affiliation(s)
| | - Allison D. Kelly
- Department of Chemistry and Biochemistry and Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, USA
| | - Bruce E. Bowler
- Department of Chemistry and Biochemistry and Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, USA
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5
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Cytochrome c Interaction with Cardiolipin Plays a Key Role in Cell Apoptosis: Implications for Human Diseases. Symmetry (Basel) 2022. [DOI: 10.3390/sym14040767] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
In the cell cytochrome, c performs different functions depending on the environment in which it acts; therefore, it has been classified as a multifunction protein. When anchored to the outer side of the inner mitochondrial membrane, native cytochrome c acts as a Schweitzer-StennerSchweitzer-Stenner that transfers electrons from cytochrome c reductase to cytochrome c oxidase in the respiratory chain. On the other hand, to interact with cardiolipin (one of the phospholipids making up the mitochondrial membrane) and form the cytochrome c/cardiolipin complex in the apoptotic process, the protein reorganizes its structure into a non-native state characterized by different asymmetry. The formation of the cytochrome c/cardiolipin complex is a fundamental step of the apoptotic pathway, since the structural rearrangement induces peroxidase activity in cytochrome c, the subsequent permeabilization of the membrane, and the release of the free protein into the cytoplasm, where cytochrome c activates the apoptotic process. Apoptosis is closely related to the pathogenesis of neoplastic, neurodegenerative and cardiovascular diseases; in this contest, the biosynthesis and remodeling of cardiolipin are crucial for the regulation of the apoptotic process. Since the role of cytochrome c as a promoter of apoptosis strictly depends on the non-native conformation(s) that the protein acquires when bound to the cardiolipin and such event leads to cytochrome c traslocation into the cytosol, the structural and functional properties of the cytochrome c/cardiolipin complex in cell fate will be the focus of the present review.
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6
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Feng Y, Liu XC, Li L, Gao SQ, Wen GB, Lin YW. Naturally Occurring I81N Mutation in Human Cytochrome c Regulates Both Inherent Peroxidase Activity and Interactions with Neuroglobin. ACS OMEGA 2022; 7:11510-11518. [PMID: 35415373 PMCID: PMC8992277 DOI: 10.1021/acsomega.2c01256] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/02/2022] [Accepted: 03/14/2022] [Indexed: 05/24/2023]
Abstract
Human cytochrome c (hCyt c) is a crucial heme protein and plays an indispensable role in energy conversion and intrinsic apoptosis pathways. The sequence and structure of Cyt c were evolutionarily conserved and only a few naturally occurring mutants were detected in humans. Among those variable sites, position 81 was proposed to act as a peroxidase switch in the initiation stages of apoptosis. In this study, we show that Ile81 not only suppresses the intrinsic peroxidase activity but also is essential for Cyt c to interact with neuroglobin (Ngb), a potential protein partner. The kinetic assays showed that the peroxidase activity of the naturally occurring variant I81N was enhanced up to threefold under pH 5. The local stability of the Ω-loop D (residues 70-85) in the I81N variant was decreased. Moreover, the Alphafold2 program predicted that Ile81 forms stable contact with human Ngb. Meanwhile, the Ile81 to Asn81 missense mutation abolishes the interaction interface, resulting in a ∼40-fold decrease in binding affinity. These observations provide an insight into the structure-function relationship of the conserved Ile81 in vertebrate Cyt c.
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Affiliation(s)
- Yu Feng
- School
of Chemistry and Chemical Engineering, University
of South China, Hengyang 421001, China
| | - Xi-Chun Liu
- School
of Chemistry and Chemical Engineering, University
of South China, Hengyang 421001, China
| | - Lianzhi Li
- School
of Chemistry and Chemical Engineering, Liaocheng
University, Liaocheng 252059, China
| | - Shu-Qin Gao
- Key
Lab of Protein Structure and Function of Universities in Hunan Province, University of South China, Hengyang 421001, China
| | - Ge-Bo Wen
- Key
Lab of Protein Structure and Function of Universities in Hunan Province, University of South China, Hengyang 421001, China
| | - Ying-Wu Lin
- School
of Chemistry and Chemical Engineering, University
of South China, Hengyang 421001, China
- Key
Lab of Protein Structure and Function of Universities in Hunan Province, University of South China, Hengyang 421001, China
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7
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Effect on intrinsic peroxidase activity of substituting coevolved residues from Ω-loop C of human cytochrome c into yeast Iso-1-cytochrome c. J Inorg Biochem 2022; 232:111819. [DOI: 10.1016/j.jinorgbio.2022.111819] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2021] [Revised: 03/25/2022] [Accepted: 04/02/2022] [Indexed: 11/15/2022]
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8
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Samsri S, Pornsuwan S. Influence of cysteine-directed mutations at the Ω-loops on peroxidase activity of human cytochrome c. Arch Biochem Biophys 2021; 709:108980. [PMID: 34224685 DOI: 10.1016/j.abb.2021.108980] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2021] [Revised: 06/08/2021] [Accepted: 06/22/2021] [Indexed: 11/19/2022]
Abstract
Cytochrome c (Cytc) is a multifunctional protein associated with electron shuttling in the inner membrane of mitochondria and also involving in the apoptotic pathway. It has been identified that mutations located in the flexible central 40-57 Ω-loop including the naturally occurring G41S, Y48H, and A51V mutants, which are found in patients with thrombocytopenia 4, a platelet disorder, alter the structural properties of human Cytc (hCytc) that associated to enhanced peroxidase activity. In this work we compared the cysteine-directed mutants of hCytc located in three different parts of Ω-loops, i.e., T28C and G34C (proximal Ω-loop), and A50C (central Ω-loop), with respect to the wild-type (WT) hCytc. The mutants and WT hCytc were structurally characterized by circular dichroism, heating and chemical denaturations, and fluorescence spectroscopy. The flexibility at the cysteine mutated sites was directly determined by site-directed spin-labeling Electron Spin Resonance. Alkaline transitions were determined by pH titration and the alkaline conformers were related to peroxidase activity of all hCytc proteins. Structural and dynamic characterizations were rationally correlated to the modulation of peroxidase activity in these mutants in comparison to the WT hCytc. We found that the cysteine mutations at residues T28 and G34, both located in the same region of Ω-loop, developed different conformations and dynamical properties that lead to different effects on the rates of peroxidase activity (G34C was ~2.6 folds higher), whereas the rate of G34C was closer to that of A50C mutant. The results implied that the flexibility and local structures of the proximal Ω-loop could also play an important role in modulating the peroxidase activity which can be associated to apoptosis.
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Affiliation(s)
- Sasiprapa Samsri
- Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand
| | - Soraya Pornsuwan
- Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok, Thailand.
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9
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Altered structure and dynamics of pathogenic cytochrome c variants correlate with increased apoptotic activity. Biochem J 2021; 478:669-684. [PMID: 33480393 DOI: 10.1042/bcj20200793] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2020] [Revised: 01/17/2021] [Accepted: 01/21/2021] [Indexed: 01/16/2023]
Abstract
Mutation of cytochrome c in humans causes mild autosomal dominant thrombocytopenia. The role of cytochrome c in platelet formation, and the molecular mechanism underlying the association of cytochrome c mutations with thrombocytopenia remains unknown, although a gain-of-function is most likely. Cytochrome c contributes to several cellular processes, with an exchange between conformational states proposed to regulate changes in function. Here, we use experimental and computational approaches to determine whether pathogenic variants share changes in structure and function, and to understand how these changes might occur. Three pathogenic variants (G41S, Y48H, A51V) cause an increase in apoptosome activation and peroxidase activity. Molecular dynamics simulations of these variants, and two non-naturally occurring variants (G41A, G41T), indicate that increased apoptosome activation correlates with the increased overall flexibility of cytochrome c, particularly movement of the Ω loops. Crystal structures of Y48H and G41T complement these studies which overall suggest that the binding of cytochrome c to apoptotic protease activating factor-1 (Apaf-1) may involve an 'induced fit' mechanism which is enhanced in the more conformationally mobile variants. In contrast, peroxidase activity did not significantly correlate with protein dynamics. Thus, the mechanism by which the variants increase peroxidase activity is not related to the conformational dynamics of the native hexacoordinate state of cytochrome c. Recent molecular dynamics data proposing conformational mobility of specific cytochrome c regions underpins changes in reduction potential and alkaline transition pK was not fully supported. These data highlight that conformational dynamics of cytochrome c drive some but not all of its properties and activities.
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10
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Steele HBB, Elmer-Dixon MM, Rogan JT, Ross JBA, Bowler BE. The Human Cytochrome c Domain-Swapped Dimer Facilitates Tight Regulation of Intrinsic Apoptosis. Biochemistry 2020; 59:2055-2068. [PMID: 32428404 PMCID: PMC7291863 DOI: 10.1021/acs.biochem.0c00326] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Oxidation of cardiolipin (CL) by cytochrome c (cytc) has been proposed to initiate the intrinsic pathway of apoptosis. Domain-swapped dimer (DSD) conformations of cytc have been reported both by our laboratory and by others. The DSD is an alternate conformer of cytc that could oxygenate CL early in apoptosis. We demonstrate here that the cytc DSD has a set of properties that would provide tighter regulation of the intrinsic pathway. We show that the human DSD is kinetically more stable than horse and yeast DSDs. Circular dichroism data indicate that the DSD has a less asymmetric heme environment, similar to that seen when the monomeric protein binds to CL vesicles at high lipid:protein ratios. The dimer undergoes the alkaline conformational transition near pH 7.0, 2.5 pH units lower than that of the monomer. Data from fluorescence correlation spectroscopy and fluorescence anisotropy suggest that the alkaline transition of the DSD may act as a switch from a high affinity for CL nanodiscs at pH 7.4 to a much lower affinity at pH 8.0. Additionally, the peroxidase activity of the human DSD increases 7-fold compared to that of the monomer at pH 7 and 8, but by 14-fold at pH 6 when mixed Met80/H2O ligation replaces the lysine ligation of the alkaline state. We also present data that indicate that cytc binding shows a cooperative effect as the concentration of cytc is increased. The DSD appears to have evolved into a pH-inducible switch that provides a means to control activation of apoptosis near pH 7.0.
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Affiliation(s)
- Harmen B. B. Steele
- Department of Chemistry & Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, Montana 59812, United States
| | - Margaret M. Elmer-Dixon
- Department of Chemistry & Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, Montana 59812, United States
| | - James T. Rogan
- Department of Chemistry & Biochemistry, University of Montana, Missoula, Montana 59812, United States
| | - J. B. Alexander Ross
- Department of Chemistry & Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, Montana 59812, United States
| | - Bruce E. Bowler
- Department of Chemistry & Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure & Dynamics, University of Montana, Missoula, Montana 59812, United States
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11
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Wheel and Deal in the Mitochondrial Inner Membranes: The Tale of Cytochrome c and Cardiolipin. OXIDATIVE MEDICINE AND CELLULAR LONGEVITY 2020; 2020:6813405. [PMID: 32377304 PMCID: PMC7193304 DOI: 10.1155/2020/6813405] [Citation(s) in RCA: 25] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/29/2019] [Accepted: 02/28/2020] [Indexed: 12/15/2022]
Abstract
Cardiolipin oxidation and degradation by different factors under severe cell stress serve as a trigger for genetically encoded cell death programs. In this context, the interplay between cardiolipin and another mitochondrial factor—cytochrome c—is a key process in the early stages of apoptosis, and it is a matter of intense research. Cytochrome c interacts with lipid membranes by electrostatic interactions, hydrogen bonds, and hydrophobic effects. Experimental conditions (including pH, lipid composition, and post-translational modifications) determine which specific amino acid residues are involved in the interaction and influence the heme iron coordination state. In fact, up to four binding sites (A, C, N, and L), driven by different interactions, have been reported. Nevertheless, key aspects of the mechanism for cardiolipin oxidation by the hemeprotein are well established. First, cytochrome c acts as a pseudoperoxidase, a process orchestrated by tyrosine residues which are crucial for peroxygenase activity and sensitivity towards oxidation caused by protein self-degradation. Second, flexibility of two weakest folding units of the hemeprotein correlates with its peroxidase activity and the stability of the iron coordination sphere. Third, the diversity of the mode of interaction parallels a broad diversity in the specific reaction pathway. Thus, current knowledge has already enabled the design of novel drugs designed to successfully inhibit cardiolipin oxidation.
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12
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Comparison of the structural dynamic and mitochondrial electron-transfer properties of the proapoptotic human cytochrome c variants, G41S, Y48H and A51V. J Inorg Biochem 2020; 203:110924. [DOI: 10.1016/j.jinorgbio.2019.110924] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2019] [Revised: 11/08/2019] [Accepted: 11/12/2019] [Indexed: 01/02/2023]
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13
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Chea EE, Deredge DJ, Jones LM. Insights on the Conformational Ensemble of Cyt C Reveal a Compact State during Peroxidase Activity. Biophys J 2019; 118:128-137. [PMID: 31810655 DOI: 10.1016/j.bpj.2019.11.011] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2019] [Revised: 10/29/2019] [Accepted: 11/11/2019] [Indexed: 12/19/2022] Open
Abstract
Cytochrome c (cyt c) is known for its role in the electron transport chain but transitions to a peroxidase-active state upon exposure to oxidative species. The peroxidase activity ultimately results in the release of cyt c into the cytosol for the engagement of apoptosis. The accumulation of oxidative modifications that accompany the onset of the peroxidase function are well-characterized. However, the concurrent structural and conformational transitions of cyt c remain undercharacterized. Fast photochemical oxidation of proteins (FPOP) coupled with mass spectrometry is a protein footprinting technique used to structurally characterize proteins. FPOP coupled with native ion mobility separation shows that exposure to H2O2 results in the accumulation of a compact state of cyt c. Subsequent top-down fragmentation to localize FPOP modifications reveals changes in heme coordination between conformers. A time-resolved functional assay suggests that this compact conformer is peroxidase active. Altogether, combining FPOP, ion mobility separation, and top-down and bottom-up mass spectrometry allows us to discern individual conformations in solution and obtain a better understanding of the conformational ensemble and structural transitions of cyt c as it transitions from a respiratory role to a proapoptotic role.
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Affiliation(s)
- Emily E Chea
- Department of Pharmaceutical Sciences, University of Maryland Baltimore, Baltimore, Maryland
| | - Daniel J Deredge
- Department of Pharmaceutical Sciences, University of Maryland Baltimore, Baltimore, Maryland
| | - Lisa M Jones
- Department of Pharmaceutical Sciences, University of Maryland Baltimore, Baltimore, Maryland.
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14
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Lei H, Bowler BE. Naturally Occurring A51V Variant of Human Cytochrome c Destabilizes the Native State and Enhances Peroxidase Activity. J Phys Chem B 2019; 123:8939-8953. [PMID: 31557440 DOI: 10.1021/acs.jpcb.9b05869] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
The A51V variant of human cytochrome c is linked to thrombocytopenia 4 (THC4), a condition that causes decreased blood platelet counts. A 1.82 Å structure of the A51V variant shows only minor changes in tertiary structure relative to the wild-type (WT) protein. Guanidine hydrochloride denaturation demonstrates that the global stability of the A51V variant is 1.3 kcal/mol less than that of the WT protein. The midpoint pH, pH1/2, of the alkaline transition of the A51V variant is 1 unit less than that of the WT protein. Stopped-flow pH jump experiments show that the A51V substitution affects the triggering ionization for one of two kinetically distinguishable alkaline conformers and enhances the accessibility of a high-spin heme transient. The pH1/2 for acid unfolding of the A51V variant is 0.7 units higher than for that of the WT protein. Consistent with the greater accessibility of non-native conformers for the A51V variant, the kcat values for its peroxidase activity increase by 6- to 15-fold in the pH range of 5-8 versus those of the WT protein. These data along with previously reported data for the other THC4-linked variants, G41S and Y48H, underscore the role of Ω-loop C (residues 40-57) in modulating the peroxidase activity of cytochrome c early in apoptosis.
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Affiliation(s)
- Haotian Lei
- Department of Chemistry and Biochemistry, Center for Bimolecular Structure and Dynamics , University of Montana , Missoula , Montana 59812 , United States
| | - Bruce E Bowler
- Department of Chemistry and Biochemistry, Center for Bimolecular Structure and Dynamics , University of Montana , Missoula , Montana 59812 , United States
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15
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Deng Y, Weaver ML, Hoke KR, Pletneva EV. A Heme Propionate Staples the Structure of Cytochrome c for Methionine Ligation to the Heme Iron. Inorg Chem 2019; 58:14085-14106. [PMID: 31589413 DOI: 10.1021/acs.inorgchem.9b02111] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Ligand-switch reactions at the heme iron are common in biological systems, but their mechanisms and the features of the polypeptide fold that support dual ligation are not well understood. In cytochrome c (cyt c), two low-stability loops (Ω-loop C and Ω-loop D) are connected by the heme propionate HP6. At alkaline pH, the native Met80 ligand from Ω-loop D switches to a Lys residue from the same loop. Deprotonation of an as yet unknown group triggers the alkaline transition. We have created the two cyt c variants T49V/K79G and T78V/K79G with altered connections of these two loops to HP6. Electronic absorption, NMR, and EPR studies demonstrate that at pH 7.4 ferric forms of these variants are Lys-ligated, whereas ferrous forms maintain the native Met80 ligation. Measurements of protein stability, cyclic voltammetry, pH-jump and gated electron-transfer kinetics have revealed that these Thr to Val substitutions greatly affect the alkaline transition in both ferric and ferrous proteins. The substitutions modify the stability of the Met-ligated species and reduction potentials of the heme iron. The kinetics of ligand-switch processes are also altered, and analyses of these effects implicate redox-dependent differences in metal-ligand interactions and the role of the protein dynamics, including cross-talk between the two Ω-loops. With the two destabilized variants, it is possible to map energy levels for the Met- and Lys-ligated species in both ferric and ferrous proteins and assess the role of the protein scaffold in redox-dependent preferences for these two ligands. The estimated shift in the heme iron reduction potential upon deprotonation of the "trigger" group is consistent with those associated with deprotonation of an HP, suggesting that HP6, on its own or as a part of a hydrogen-bonded cluster, is a likely "trigger" for the Met to Lys ligand switch.
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Affiliation(s)
- Yunling Deng
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
| | - Madeline L Weaver
- Department of Chemistry and Biochemistry , Berry College , Mount Berry , Georgia 30149 , United States
| | - Kevin R Hoke
- Department of Chemistry and Biochemistry , Berry College , Mount Berry , Georgia 30149 , United States
| | - Ekaterina V Pletneva
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
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Cytochrome c: An extreme multifunctional protein with a key role in cell fate. Int J Biol Macromol 2019; 136:1237-1246. [DOI: 10.1016/j.ijbiomac.2019.06.180] [Citation(s) in RCA: 88] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2019] [Revised: 06/24/2019] [Accepted: 06/24/2019] [Indexed: 01/25/2023]
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17
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Engilberge S, Rennie ML, Crowley PB. Calixarene capture of partially unfolded cytochrome c. FEBS Lett 2019; 593:2112-2117. [PMID: 31254353 DOI: 10.1002/1873-3468.13512] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2019] [Revised: 06/20/2019] [Accepted: 06/20/2019] [Indexed: 12/18/2022]
Abstract
Supramolecular receptors such as water-soluble calixarenes are in development as 'molecular glues' for protein assembly. Here, we obtained cocrystals of sulfonato-calix[6]arene (sclx6 ) and yeast cytochrome c (cytc) in the presence of imidazole. A crystal structure at 2.65 Å resolution reveals major structural rearrangement and disorder in imidazole-bound cytc. The largest protein-calixarene interface involves 440 Å2 of the protein surface with key contacts at Arg13, Lys73, and Lys79. These lysines participate in alkaline transitions of cytc and are part of Ω-loop D, which is substantially restructured in the complex with sclx6 . The structural modification also includes Ω-loop C, which is disordered (residues 41-55 inclusive). These results suggest the possibility of using supramolecular scaffolds to trap partially disordered proteins.
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Affiliation(s)
- Sylvain Engilberge
- School of Chemistry, National University of Ireland Galway, Galway, Ireland
| | - Martin L Rennie
- School of Chemistry, National University of Ireland Galway, Galway, Ireland
| | - Peter B Crowley
- School of Chemistry, National University of Ireland Galway, Galway, Ireland
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Lei H, Nold SM, Motta LJ, Bowler BE. Effect of V83G and I81A Substitutions to Human Cytochrome c on Acid Unfolding and Peroxidase Activity below a Neutral pH. Biochemistry 2019; 58:2921-2933. [DOI: 10.1021/acs.biochem.9b00295] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Affiliation(s)
- Haotian Lei
- Department of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, United States
| | - Shiloh M. Nold
- Department of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
| | - Luis Jung Motta
- Department of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
| | - Bruce E. Bowler
- Department of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, United States
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Yin V, Mian SH, Konermann L. Lysine carbonylation is a previously unrecognized contributor to peroxidase activation of cytochrome c by chloramine-T. Chem Sci 2019; 10:2349-2359. [PMID: 30881663 PMCID: PMC6385661 DOI: 10.1039/c8sc03624a] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/14/2018] [Accepted: 12/30/2018] [Indexed: 12/31/2022] Open
Abstract
The peroxidase activity of cytochrome c (cyt c) plays a key role during apoptosis. Peroxidase catalysis requires a vacant Fe coordination site, i.e., cyt c must undergo an activation process involving structural changes that rupture the native Met80-Fe contact. A common strategy for dissociating this bond is the conversion of Met80 to sulfoxide (MetO). It is widely believed that this MetO formation in itself is sufficient for cyt c activation. This notion originates from studies on chloramine-T-treated cyt c (CT-cyt c) which represents a standard model for the peroxidase activated state. CT-cyt c is considered to be a "clean" species that has undergone selective MetO formation, without any other modifications. Using optical, chromatographic, and mass spectrometry techniques, the current work demonstrates that CT-induced activation of cyt c is more complicated than previously thought. MetO formation alone results in only marginal peroxidase activity, because dissociation of the Met80-Fe bond triggers alternative ligation scenarios where Lys residues interfere with access to the heme. We found that CT causes not only MetO formation, but also carbonylation of several Lys residues. Carbonylation is associated with -1 Da mass shifts that have gone undetected in the CT-cyt c literature. Proteoforms possessing both MetO and Lys carbonylation exhibit almost fourfold higher peroxidase activity than those with MetO alone. Carbonylation abrogates the capability of Lys to coordinate the heme, thereby freeing up the distal site as required for an active peroxidase. Previous studies on CT-cyt c may have inadvertently examined carbonylated proteoforms, potentially misattributing effects of carbonylation to solely MetO formation.
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Affiliation(s)
- Victor Yin
- Department of Chemistry and Department of Biochemistry , The University of Western Ontario , London , Ontario N6A 5B7 , Canada .
| | - Safee H Mian
- Department of Chemistry and Department of Biochemistry , The University of Western Ontario , London , Ontario N6A 5B7 , Canada .
| | - Lars Konermann
- Department of Chemistry and Department of Biochemistry , The University of Western Ontario , London , Ontario N6A 5B7 , Canada .
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Tomášková N, Varhač R, Lysáková V, Musatov A, Sedlák E. Peroxidase activity of cytochrome c in its compact state depends on dynamics of the heme region. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2018; 1866:1073-1083. [DOI: 10.1016/j.bbapap.2018.09.003] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/03/2018] [Revised: 09/06/2018] [Accepted: 09/10/2018] [Indexed: 10/28/2022]
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Deng Y, Zhong F, Alden SL, Hoke KR, Pletneva EV. The K79G Mutation Reshapes the Heme Crevice and Alters Redox Properties of Cytochrome c. Biochemistry 2018; 57:5827-5840. [PMID: 30142276 DOI: 10.1021/acs.biochem.8b00650] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The two roles of cytochrome c (cyt c), in oxidative phosphorylation and apoptosis, critically depend on redox properties of its heme iron center. The K79G mutant has served as a parent protein for a series of mutants of yeast iso-1 cyt c. The mutation preserves the Met80 coordination to the heme iron, as found in WT* (K72A/C102S), and many spectroscopic properties of K79G and WT* are indistinguishable. The K79G mutation does not alter the global stability, fold, rate of Met80 dissociation, or thermodynamics of the alkaline transition (p Ka) of the protein. However, the reduction potential of the heme iron decreases; further, the p KH of the trigger group and the rate of the Met-to-Lys ligand exchange associated with the alkaline transition decrease, suggesting changes in the environment of the heme. The rates of electron self-exchange and bimolecular electron transfer (ET) with positively charged inorganic complexes increase, as does the intrinsic peroxidase activity. Analysis of the reaction rates suggests that there is increased accessibility of the heme edge in K79G and supports the importance of the Lys79 site for bimolecular ET reactions of cyt c, including those with some of its native redox partners. Structural modeling rationalizes the observed effects to arise from changes in the volume of the heme pocket and solvent accessibility of the heme group. Kinetic and structural analyses of WT* characterize the properties of the heme crevice of this commonly employed reference variant. This study highlights the important role of Lys79 for defining functional redox properties of cyt c.
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Affiliation(s)
- Yunling Deng
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
| | - Fangfang Zhong
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
| | - Stephanie L Alden
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
| | - Kevin R Hoke
- Department of Chemistry and Biochemistry , Berry College , Mount Berry , Georgia 30149 , United States
| | - Ekaterina V Pletneva
- Department of Chemistry , Dartmouth College , Hanover , New Hampshire 03755 , United States
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Schweitzer-Stenner R. Relating the multi-functionality of cytochrome c to membrane binding and structural conversion. Biophys Rev 2018; 10:1151-1185. [PMID: 29574621 PMCID: PMC6082307 DOI: 10.1007/s12551-018-0409-4] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2017] [Accepted: 02/23/2018] [Indexed: 12/16/2022] Open
Abstract
Cytochrome c is known as an electron-carrying protein in the respiratory chain of mitochondria. Over the last 20 years, however, alternative functions of this very versatile protein have become the focus of research interests. Upon binding to anionic lipids such as cardiolipin, the protein acquires peroxidase activity. Multiple lines of evidence suggest that this requires a conformational change of the protein which involves partial unfolding of its tertiary structure. This review summarizes the current state of knowledge of how cytochrome c interacts with cardiolipin-containing surfaces and how this affects its structure and function. In this context, we delineate partially conflicting results regarding the affinity of cytochrome c binding to cardiolipin-containing liposomes of different size and its influence on the structure of the protein and the morphology of the membrane.
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Deacon OM, Svistunenko DA, Moore GR, Wilson MT, Worrall JA. Naturally Occurring Disease-Related Mutations in the 40–57 Ω-Loop of Human Cytochrome c Control Triggering of the Alkaline Isomerization. Biochemistry 2018; 57:4276-4288. [DOI: 10.1021/acs.biochem.8b00520] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Affiliation(s)
- Oliver M. Deacon
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K
| | - Dimitri A. Svistunenko
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K
| | - Geoffrey R. Moore
- School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K
| | - Michael T. Wilson
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K
| | - Jonathan A.R. Worrall
- School of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K
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Schwenger A, Jurkowski TP, Richert C. Capturing and Stabilizing Folded Proteins in Lattices Formed with Branched Oligonucleotide Hybrids. Chembiochem 2018; 19:1523-1530. [DOI: 10.1002/cbic.201800145] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2018] [Indexed: 02/05/2023]
Affiliation(s)
- Alexander Schwenger
- Institut für Organische ChemieUniversität Stuttgart Pfaffenwaldring 55 70569 Stuttgart Germany
| | - Tomasz P. Jurkowski
- Institut für Biochemie und Technische BiochemieUniversität Stuttgart Allmandring 31 70569 Stuttgart Germany
| | - Clemens Richert
- Institut für Organische ChemieUniversität Stuttgart Pfaffenwaldring 55 70569 Stuttgart Germany
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Lei H, Bowler BE. Humanlike substitutions to Ω-loop D of yeast iso-1-cytochrome c only modestly affect dynamics and peroxidase activity. J Inorg Biochem 2018. [PMID: 29530594 DOI: 10.1016/j.jinorgbio.2018.02.022] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Structural studies of yeast iso-1-cytochrome c (L.J. McClelland, T.-C. Mou, M.E. Jeakins-Cooley, S.R. Sprang, B.E. Bowler, Proc. Natl. Acad. Sci. U.S.A. 111 (2014) 6648-6653) show that modest movement of Ω-loop D (residues 70-85, average RMSD versus the native structure: 0.81 Å) permits loss of Met80-heme ligation creating an available coordination site to catalyze the peroxidase activity mediated by cytochrome c early in apoptosis. However, Ala81 and Gly83 move significantly (RMSDs of 2.18 and 1.26 Å, respectively). Ala81 and Gly83 evolve to Ile and Val, respectively, in human cytochrome c and peroxidase activity decreases 25-fold relative to the yeast protein at pH 7. To test the hypothesis that these residues evolved to restrict the peroxidase activity of cytochrome c, A81I and G83V variants of yeast iso-1-cytochrome c were prepared. For both variants, the apparent pKa of the alkaline transition increases by 0.2 to 0.3 relative to the wild type (WT) protein and the rate of opening the heme crevice is slowed. The cooperativity of acid unfolding is decreased for the G83V variant. At pH 7 and 8, the catalytic rate constant, kcat, for the peroxidase activity of both variants decreases relative to WT, consistent with the effects on alkaline isomerization. Below pH 7, the loss in the cooperativity of acid unfolding causes kcat for peroxidase activity to increase for the G83V variant relative to WT. Neither variant decreases kcat to the level of the human protein, indicating that other residues also contribute to the low peroxidase activity of human cytochrome c.
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Affiliation(s)
- Haotian Lei
- Department of Chemistry and Biochemistry, University of Montana, Missoula, MT 59812, United States; Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States
| | - Bruce E Bowler
- Department of Chemistry and Biochemistry, University of Montana, Missoula, MT 59812, United States; Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, MT 59812, United States.
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Yin V, Shaw GS, Konermann L. Cytochrome c as a Peroxidase: Activation of the Precatalytic Native State by H2O2-Induced Covalent Modifications. J Am Chem Soc 2017; 139:15701-15709. [DOI: 10.1021/jacs.7b07106] [Citation(s) in RCA: 47] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Affiliation(s)
- Victor Yin
- Department of Chemistry and Department
of Biochemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
| | - Gary S. Shaw
- Department of Chemistry and Department
of Biochemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
| | - Lars Konermann
- Department of Chemistry and Department
of Biochemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
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Elmer-Dixon MM, Bowler BE. Site A-Mediated Partial Unfolding of Cytochrome c on Cardiolipin Vesicles Is Species-Dependent and Does Not Require Lys72. Biochemistry 2017; 56:4830-4839. [DOI: 10.1021/acs.biochem.7b00694] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Affiliation(s)
- Margaret M. Elmer-Dixon
- Department
of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center
for Bimolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, United States
| | - Bruce E. Bowler
- Department
of Chemistry and Biochemistry, University of Montana, Missoula, Montana 59812, United States
- Center
for Bimolecular Structure and Dynamics, University of Montana, Missoula, Montana 59812, United States
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