1
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Modularity of membrane-bound charge-translocating protein complexes. Biochem Soc Trans 2021; 49:2669-2685. [PMID: 34854900 DOI: 10.1042/bst20210462] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2021] [Revised: 11/02/2021] [Accepted: 11/15/2021] [Indexed: 02/05/2023]
Abstract
Energy transduction is the conversion of one form of energy into another; this makes life possible as we know it. Organisms have developed different systems for acquiring energy and storing it in useable forms: the so-called energy currencies. A universal energy currency is the transmembrane difference of electrochemical potential (Δμ~). This results from the translocation of charges across a membrane, powered by exergonic reactions. Different reactions may be coupled to charge-translocation and, in the majority of cases, these reactions are catalyzed by modular enzymes that always include a transmembrane subunit. The modular arrangement of these enzymes allows for different catalytic and charge-translocating modules to be combined. Thus, a transmembrane charge-translocating module can be associated with different catalytic subunits to form an energy-transducing complex. Likewise, the same catalytic subunit may be combined with a different membrane charge-translocating module. In this work, we analyze the modular arrangement of energy-transducing membrane complexes and discuss their different combinations, focusing on the charge-translocating module.
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2
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Sarewicz M, Pintscher S, Pietras R, Borek A, Bujnowicz Ł, Hanke G, Cramer WA, Finazzi G, Osyczka A. Catalytic Reactions and Energy Conservation in the Cytochrome bc1 and b6f Complexes of Energy-Transducing Membranes. Chem Rev 2021; 121:2020-2108. [PMID: 33464892 PMCID: PMC7908018 DOI: 10.1021/acs.chemrev.0c00712] [Citation(s) in RCA: 61] [Impact Index Per Article: 20.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Indexed: 12/16/2022]
Abstract
This review focuses on key components of respiratory and photosynthetic energy-transduction systems: the cytochrome bc1 and b6f (Cytbc1/b6f) membranous multisubunit homodimeric complexes. These remarkable molecular machines catalyze electron transfer from membranous quinones to water-soluble electron carriers (such as cytochromes c or plastocyanin), coupling electron flow to proton translocation across the energy-transducing membrane and contributing to the generation of a transmembrane electrochemical potential gradient, which powers cellular metabolism in the majority of living organisms. Cytsbc1/b6f share many similarities but also have significant differences. While decades of research have provided extensive knowledge on these enzymes, several important aspects of their molecular mechanisms remain to be elucidated. We summarize a broad range of structural, mechanistic, and physiological aspects required for function of Cytbc1/b6f, combining textbook fundamentals with new intriguing concepts that have emerged from more recent studies. The discussion covers but is not limited to (i) mechanisms of energy-conserving bifurcation of electron pathway and energy-wasting superoxide generation at the quinol oxidation site, (ii) the mechanism by which semiquinone is stabilized at the quinone reduction site, (iii) interactions with substrates and specific inhibitors, (iv) intermonomer electron transfer and the role of a dimeric complex, and (v) higher levels of organization and regulation that involve Cytsbc1/b6f. In addressing these topics, we point out existing uncertainties and controversies, which, as suggested, will drive further research in this field.
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Affiliation(s)
- Marcin Sarewicz
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Sebastian Pintscher
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Rafał Pietras
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Arkadiusz Borek
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Łukasz Bujnowicz
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Guy Hanke
- School
of Biological and Chemical Sciences, Queen
Mary University of London, London E1 4NS, U.K.
| | - William A. Cramer
- Department
of Biological Sciences, Purdue University, West Lafayette, Indiana 47907 United States
| | - Giovanni Finazzi
- Laboratoire
de Physiologie Cellulaire et Végétale, Université Grenoble Alpes, Centre National Recherche Scientifique,
Commissariat Energie Atomique et Energies Alternatives, Institut National
Recherche l’agriculture, l’alimentation et l’environnement, 38054 Grenoble Cedex 9, France
| | - Artur Osyczka
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
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3
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Pintscher S, Pietras R, Sarewicz M, Osyczka A. Electron sweep across four b-hemes of cytochrome bc1 revealed by unusual paramagnetic properties of the Qi semiquinone intermediate. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2018; 1859:459-469. [DOI: 10.1016/j.bbabio.2018.03.010] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/09/2017] [Revised: 02/08/2018] [Accepted: 03/18/2018] [Indexed: 01/05/2023]
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4
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Borek A, Ekiert R, Osyczka A. Functional flexibility of electron flow between quinol oxidation Q o site of cytochrome bc 1 and cytochrome c revealed by combinatory effects of mutations in cytochrome b, iron-sulfur protein and cytochrome c 1. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2018; 1859:754-761. [PMID: 29705394 DOI: 10.1016/j.bbabio.2018.04.010] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/17/2018] [Revised: 04/16/2018] [Accepted: 04/24/2018] [Indexed: 01/07/2023]
Abstract
Transfer of electron from quinol to cytochrome c is an integral part of catalytic cycle of cytochrome bc1. It is a multi-step reaction involving: i) electron transfer from quinol bound at the catalytic Qo site to the Rieske iron-sulfur ([2Fe-2S]) cluster, ii) large-scale movement of a domain containing [2Fe-2S] cluster (ISP-HD) towards cytochrome c1, iii) reduction of cytochrome c1 by reduced [2Fe-2S] cluster, iv) reduction of cytochrome c by cytochrome c1. In this work, to examine this multi-step reaction we introduced various types of barriers for electron transfer within the chain of [2Fe-2S] cluster, cytochrome c1 and cytochrome c. The barriers included: impediment in the motion of ISP-HD, uphill electron transfer from [2Fe-2S] cluster to heme c1 of cytochrome c1, and impediment in the catalytic quinol oxidation. The barriers were introduced separately or in various combinations and their effects on enzymatic activity of cytochrome bc1 were compared. This analysis revealed significant degree of functional flexibility allowing the cofactor chains to accommodate certain structural and/or redox potential changes without losing overall electron and proton transfers capabilities. In some cases inhibitory effects compensated one another to improve/restore the function. The results support an equilibrium model in which a random oscillation of ISP-HD between the Qo site and cytochrome c1 helps maintaining redox equilibrium between all cofactors of the chain. We propose a new concept in which independence of the dynamics of the Qo site substrate and the motion of ISP-HD is one of the elements supporting this equilibrium and also is a potential factor limiting the overall catalytic rate.
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Affiliation(s)
- Arkadiusz Borek
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Robert Ekiert
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Artur Osyczka
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland.
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5
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Bhaduri S, Stadnytskyi V, Zakharov SD, Hasan SS, Bujnowicz Ł, Sarewicz M, Savikhin S, Osyczka A, Cramer WA. Pathways of Transmembrane Electron Transfer in Cytochrome bc Complexes: Dielectric Heterogeneity and Interheme Coulombic Interactions. J Phys Chem B 2017; 121:975-983. [DOI: 10.1021/acs.jpcb.6b11709] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Affiliation(s)
| | | | | | | | - Ł. Bujnowicz
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics, and
Biotechnology, Jagiellonian University, Kraków 31-007, Poland
| | - M. Sarewicz
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics, and
Biotechnology, Jagiellonian University, Kraków 31-007, Poland
| | | | - A. Osyczka
- Department
of Molecular Biophysics, Faculty of Biochemistry, Biophysics, and
Biotechnology, Jagiellonian University, Kraków 31-007, Poland
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6
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Kuleta P, Sarewicz M, Postila P, Róg T, Osyczka A. Identifying involvement of Lys251/Asp252 pair in electron transfer and associated proton transfer at the quinone reduction site of Rhodobacter capsulatus cytochrome bc1. BIOCHIMICA ET BIOPHYSICA ACTA 2016; 1857:1661-8. [PMID: 27421232 PMCID: PMC5001787 DOI: 10.1016/j.bbabio.2016.07.003] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/20/2016] [Revised: 06/22/2016] [Accepted: 07/10/2016] [Indexed: 11/23/2022]
Abstract
Describing dynamics of proton transfers in proteins is challenging, but crucial for understanding processes which use them for biological functions. In cytochrome bc1, one of the key enzymes of respiration or photosynthesis, proton transfers engage in oxidation of quinol (QH2) and reduction of quinone (Q) taking place at two distinct catalytic sites. Here we evaluated by site-directed mutagenesis the contribution of Lys251/Asp252 pair (bacterial numbering) in electron transfers and associated with it proton uptake to the quinone reduction site (Qi site). We showed that the absence of protonable group at position 251 or 252 significantly changes the equilibrium levels of electronic reactions including the Qi-site mediated oxidation of heme bH, reverse reduction of heme bH by quinol and heme bH/Qi semiquinone equilibrium. This implicates the role of H-bonding network in binding of quinone/semiquinone and defining thermodynamic properties of Q/SQ/QH2 triad. The Lys251/Asp252 proton path is disabled only when both protonable groups are removed. With just one protonable residue from this pair, the entrance of protons to the catalytic site is sustained, albeit at lower rates, indicating that protons can travel through parallel routes, possibly involving water molecules. This shows that proton paths display engineering tolerance for change as long as all the elements available for functional cooperation secure efficient proton delivery to the catalytic site.
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Affiliation(s)
- Patryk Kuleta
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University in Krakow, Gronostajowa 7, 30-387 Kraków, Poland
| | - Marcin Sarewicz
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University in Krakow, Gronostajowa 7, 30-387 Kraków, Poland
| | - Pekka Postila
- Structural Bioinformatics Laboratory, Biochemistry, Faculty of Science and Engineering, Åbo Akademi University, Turku, Finland
| | - Tomasz Róg
- Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland; Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland
| | - Artur Osyczka
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University in Krakow, Gronostajowa 7, 30-387 Kraków, Poland.
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7
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Pintscher S, Kuleta P, Cieluch E, Borek A, Sarewicz M, Osyczka A. Tuning of Hemes b Equilibrium Redox Potential Is Not Required for Cross-Membrane Electron Transfer. J Biol Chem 2016; 291:6872-81. [PMID: 26858251 PMCID: PMC4807273 DOI: 10.1074/jbc.m115.712307] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2015] [Indexed: 11/22/2022] Open
Abstract
In biological energy conversion, cross-membrane electron transfer often involves an assembly of two hemes b. The hemes display a large difference in redox midpoint potentials (ΔEm_b), which in several proteins is assumed to facilitate cross-membrane electron transfer and overcome a barrier of membrane potential. Here we challenge this assumption reporting on heme b ligand mutants of cytochrome bc1 in which, for the first time in transmembrane cytochrome, one natural histidine has been replaced by lysine without loss of the native low spin type of heme iron. With these mutants we show that ΔEm_b can be markedly increased, and the redox potential of one of the hemes can stay above the level of quinone pool, or ΔEm_b can be markedly decreased to the point that two hemes are almost isopotential, yet the enzyme retains catalytically competent electron transfer between quinone binding sites and remains functional in vivo. This reveals that cytochrome bc1 can accommodate large changes in ΔEm_b without hampering catalysis, as long as these changes do not impose overly endergonic steps on downhill electron transfer from substrate to product. We propose that hemes b in this cytochrome and in other membranous cytochromes b act as electronic connectors for the catalytic sites with no fine tuning in ΔEm_b required for efficient cross-membrane electron transfer. We link this concept with a natural flexibility in occurrence of several thermodynamic configurations of the direction of electron flow and the direction of the gradient of potential in relation to the vector of the electric membrane potential.
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Affiliation(s)
- Sebastian Pintscher
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Patryk Kuleta
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Ewelina Cieluch
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Arkadiusz Borek
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Marcin Sarewicz
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Artur Osyczka
- From the Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
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8
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Verissimo AF, Shroff NP, Daldal F. During Cytochrome c Maturation CcmI Chaperones the Class I Apocytochromes until the Formation of Their b-Type Cytochrome Intermediates. J Biol Chem 2015; 290:16989-7003. [PMID: 25979338 DOI: 10.1074/jbc.m115.652818] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Indexed: 12/12/2022] Open
Abstract
The c-type cytochromes are electron transfer proteins involved in energy transduction. They have heme-binding (CXXCH) sites that covalently ligate heme b via thioether bonds and are classified into different classes based on their protein folds and the locations and properties of their cofactors. Rhodobacter capsulatus produces various c-type cytochromes using the cytochrome c maturation (Ccm) System I, formed from the CcmABCDEFGHI proteins. CcmI, a component of the heme ligation complex CcmFHI, interacts with the heme-handling protein CcmE and chaperones apocytochrome c2 by binding its C-terminal helix. Whether CcmI also chaperones other c-type apocytochromes, and the effects of heme on these interactions were unknown previously. Here, we purified different classes of soluble and membrane-bound c-type apocytochromes (class I, c2 and c1, and class II c') and investigated their interactions with CcmI and apoCcmE. We report that, in the absence of heme, CcmI and apoCcmE recognized different classes of c-type apocytochromes with different affinities (nM to μM KD values). When present, heme induced conformational changes in class I apocytochromes (e.g. c2) and decreased significantly their high affinity for CcmI. Knowing that CcmI does not interact with mature cytochrome c2 and that heme converts apocytochrome c2 into its b-type derivative, these findings indicate that CcmI holds the class I apocytochromes (e.g. c2) tightly until their noncovalent heme-containing b-type cytochrome-like intermediates are formed. We propose that these intermediates are subsequently converted into mature cytochromes following the covalent ligation of heme via the remaining components of the Ccm complex.
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Affiliation(s)
- Andreia F Verissimo
- From the Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6019
| | - Namita P Shroff
- From the Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6019
| | - Fevzi Daldal
- From the Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6019
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9
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Ekiert R, Czapla M, Sarewicz M, Osyczka A. Hybrid fusions show that inter-monomer electron transfer robustly supports cytochrome bc1 function in vivo. Biochem Biophys Res Commun 2014; 451:270-5. [PMID: 25089001 PMCID: PMC4152375 DOI: 10.1016/j.bbrc.2014.07.117] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2014] [Accepted: 07/24/2014] [Indexed: 10/25/2022]
Abstract
Electronic connection between Qo and Qi quinone catalytic sites of dimeric cytochrome bc1 is a central feature of the energy-conserving Q cycle. While both the intra- and inter-monomer electron transfers were shown to connect the sites in the enzyme, mechanistic and physiological significance of the latter remains unclear. Here, using a series of mutated hybrid cytochrome bc1-like complexes, we show that inter-monomer electron transfer robustly sustains the function of the enzyme in vivo, even when the two subunits in a dimer come from different species. This indicates that minimal requirement for bioenergetic efficiency is to provide a chain of cofactors for uncompromised electron flux between the catalytic sites, while the details of protein scaffold are secondary.
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Affiliation(s)
- Robert Ekiert
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Monika Czapla
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Marcin Sarewicz
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
| | - Artur Osyczka
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland.
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10
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Daidone I, Amadei A, Zaccanti F, Borsari M, Bortolotti CA. How the Reorganization Free Energy Affects the Reduction Potential of Structurally Homologous Cytochromes. J Phys Chem Lett 2014; 5:1534-40. [PMID: 26270092 DOI: 10.1021/jz5005208] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Differences in the reduction potential E(0) among structurally similar metalloproteins cannot always be fully explained on the basis of their 3-D structures. We investigate the molecular determinants to E(0) using the mixed quantum mechanics/molecular mechanics approach named perturbed matrix method (PMM); after comparison with experimental values to assess the reliability of our calculations, we investigate the relationship between the change in free energy upon reduction ΔA(0) and the reorganization energy. We find that the reduction potential of cytochromes can be regarded as the result of the sum of two terms, one being mostly dependent on the energy fluctuations within a limited range around the mean transition energy and the second being mostly dependent linearly on the difference Δλ = λred - λox of the reorganization free energies for the ox → red (λred) and for the red → ox (λox) relaxations.
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Affiliation(s)
- Isabella Daidone
- †Department of Physical and Chemical Sciences, University of L'Aquila, via Vetoio (Coppito 1), 67010 L'Aquila, Italy
| | - Andrea Amadei
- ‡Department of Chemical Sciences and Technologies, University of Rome "Tor Vergata", via della Ricerca Scientifica 1, 00133 Rome, Italy
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11
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Xia D, Esser L, Tang WK, Zhou F, Zhou Y, Yu L, Yu CA. Structural analysis of cytochrome bc1 complexes: implications to the mechanism of function. BIOCHIMICA ET BIOPHYSICA ACTA 2013; 1827:1278-94. [PMID: 23201476 PMCID: PMC3593749 DOI: 10.1016/j.bbabio.2012.11.008] [Citation(s) in RCA: 119] [Impact Index Per Article: 10.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/30/2012] [Revised: 11/13/2012] [Accepted: 11/19/2012] [Indexed: 01/18/2023]
Abstract
The cytochrome bc1 complex (bc1) is the mid-segment of the cellular respiratory chain of mitochondria and many aerobic prokaryotic organisms; it is also part of the photosynthetic apparatus of non-oxygenic purple bacteria. The bc1 complex catalyzes the reaction of transferring electrons from the low potential substrate ubiquinol to high potential cytochrome c. Concomitantly, bc1 translocates protons across the membrane, contributing to the proton-motive force essential for a variety of cellular activities such as ATP synthesis. Structural investigations of bc1 have been exceedingly successful, yielding atomic resolution structures of bc1 from various organisms and trapped in different reaction intermediates. These structures have confirmed and unified results of decades of experiments and have contributed to our understanding of the mechanism of bc1 functions as well as its inactivation by respiratory inhibitors. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.
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Affiliation(s)
- Di Xia
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA.
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12
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Czapla M, Cieluch E, Borek A, Sarewicz M, Osyczka A. Catalytically-relevant electron transfer between two hemes bL in the hybrid cytochrome bc1-like complex containing a fusion of Rhodobacter sphaeroides and capsulatus cytochromes b. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2013; 1827:751-60. [PMID: 23428397 PMCID: PMC4330944 DOI: 10.1016/j.bbabio.2013.02.007] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 10/18/2012] [Revised: 02/06/2013] [Accepted: 02/11/2013] [Indexed: 11/24/2022]
Abstract
To address mechanistic questions about the functioning of dimeric cytochrome bc1 new genetic approaches have recently been developed. They were specifically designed to enable construction of asymmetrically-mutated variants suitable for functional studies. One approach exploited a fusion of two cytochromes b that replaced the separate subunits in the dimer. The fusion protein, built from two copies of the same cytochrome b of purple bacterium Rhodobacter capsulatus, served as a template to create a series of asymmetrically-mutated cytochrome bc1-like complexes (B–B) which, through kinetic studies, disclosed several important principles of dimer engineering. Here, we report on construction of another fusion protein complex that adds a new tool to investigate dimeric function of the enzyme through the asymmetrically mutated forms of the protein. This complex (BS–B) contains a hybrid protein that combines two different cytochromes b: one coming from R. capsulatus and the other — from a closely related species, R. sphaeroides. With this new fusion we addressed a still controversial issue of electron transfer between the two hemes bL in the core of dimer. Kinetic data obtained with a series of BS–B variants provided new evidence confirming the previously reported observations that electron transfer between those two hemes occurs on a millisecond timescale, thus is a catalytically-relevant event. Both types of the fusion complexes (B–B and BS–B) consistently implicate that the heme-bL–bL bridge forms an electronic connection available for inter-monomer electron transfer in cytochrome bc1.
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Affiliation(s)
| | | | | | | | - Artur Osyczka
- Corresponding author. Tel.: + 48 12 664 6348; fax: + 48 12 664 69 02.
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13
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Bortolotti CA, Amadei A, Aschi M, Borsari M, Corni S, Sola M, Daidone I. The reversible opening of water channels in cytochrome c modulates the heme iron reduction potential. J Am Chem Soc 2012; 134:13670-8. [PMID: 22873369 DOI: 10.1021/ja3030356] [Citation(s) in RCA: 61] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Dynamic protein-solvent interactions are fundamental for life processes, but their investigation is still experimentally very demanding. Molecular dynamics simulations up to hundreds of nanoseconds can bring to light unexpected events even for extensively studied biomolecules. This paper reports a combined computational/experimental approach that reveals the reversible opening of two distinct fluctuating cavities in Saccharomyces cerevisiae iso-1-cytochrome c. Both channels allow water access to the heme center. By means of a mixed quantum mechanics/molecular dynamics (QM/MD) theoretical approach, the perturbed matrix method (PMM), that allows to reach long simulation times, changes in the reduction potential of the heme Fe(3+)/Fe(2+) couple induced by the opening of each cavity are calculated. Shifts of the reduction potential upon changes in the hydration of the heme propionates are observed. These variations are relatively small but significant and could therefore represent a tool developed by cytochrome c for the solvent driven, fine-tuning of its redox functionality.
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Affiliation(s)
- Carlo Augusto Bortolotti
- Department of Life Sciences, University of Modena and Reggio Emilia, Via Campi 183, 41125, Modena, Italy
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14
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Kyndt JA, Fitch JC, Berry RE, Stewart MC, Whitley K, Meyer TE, Walker FA, Cusanovich MA. Tyrosine triad at the interface between the Rieske iron-sulfur protein, cytochrome c1 and cytochrome c2 in the bc1 complex of Rhodobacter capsulatus. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2012; 1817:811-8. [PMID: 22306765 DOI: 10.1016/j.bbabio.2012.01.013] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/26/2011] [Revised: 01/20/2012] [Accepted: 01/20/2012] [Indexed: 11/25/2022]
Abstract
A triad of tyrosine residues (Y152-154) in the cytochrome c(1) subunit (C1) of the Rhodobacter capsulatus cytochrome bc(1) complex (BC1) is ideally positioned to interact with cytochrome c(2) (C2). Mutational analysis of these three tyrosines showed that, of the three, Y154 is the most important, since its mutation to alanine resulted in significantly reduced levels, destabilization, and inactivation of BC1. A second-site revertant of this mutant that regained photosynthetic capacity was found to have acquired two further mutations-A181T and A200V. The Y152Q mutation did not change the spectral or electrochemical properties of C1, and showed wild-type enzymatic C2 reduction rates, indicating that this mutation did not introduce major structural changes in C1 nor affect overall activity. Mutations Y153Q and Y153A, on the other hand, clearly affect the redox properties of C1 (e.g. by lowering the midpoint potential as much as 117 mV in Y153Q) and the activity by 90% and 50%, respectively. A more conservative Y153F mutant on the other hand, behaves similarly to wild-type. This underscores the importance of an aromatic residue at position Y153, presumably to maintain close packing with P184, which modeling indicates is likely to stabilize the sixth heme ligand conformation.
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Affiliation(s)
- John A Kyndt
- Department of Chemistry and Biochemistry, University of Arizona, Tucson, AZ 85721, USA.
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15
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Czapla M, Borek A, Sarewicz M, Osyczka A. Enzymatic activities of isolated cytochrome bc₁-like complexes containing fused cytochrome b subunits with asymmetrically inactivated segments of electron transfer chains. Biochemistry 2012; 51:829-35. [PMID: 22233445 PMCID: PMC3269193 DOI: 10.1021/bi2016316] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
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Homodimeric structure of cytochrome bc1, a common component of biological energy conversion
systems, builds
in four catalytic quinone oxidation/reduction sites and four chains
of cofactors (branches) that, connected by a centrally located bridge,
form a symmetric H-shaped electron transfer system. The mechanism
of operation of this complex system is under constant debate. Here,
we report on isolation and enzymatic examination of cytochrome bc1-like complexes containing fused cytochrome b subunits in which asymmetrically introduced mutations
inactivated individual branches in various combinations. The structural
asymmetry of those forms was confirmed spectroscopically. All the
asymmetric forms corresponding to cytochrome bc1 with partial or full inactivation of one monomer retain high
enzymatic activity but at the same time show a decrease in the maximum
turnover rate by a factor close to 2. This strongly supports the model
assuming independent operation of monomers. The cross-inactivated
form corresponding to cytochrome bc1 with
disabled complementary parts of each monomer retains the enzymatic
activity at the level that, for the first time on isolated from membranes
and purified to homogeneity preparations, demonstrates that intermonomer
electron transfer through the bridge effectively sustains the enzymatic
turnover. The results fully support the concept that electrons freely
distribute between the four catalytic sites of a dimer and that any
path connecting the catalytic sites on the opposite sides of the membrane
is enzymatically competent. The possibility to examine enzymatic properties
of isolated forms of asymmetric complexes constructed using the cytochrome b fusion system extends the array of tools available for
investigating the engineering of dimeric cytochrome bc1 from the mechanistic and physiological perspectives.
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Affiliation(s)
- Monika Czapla
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, Kraków, Poland
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16
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Czapla M, Borek A, Sarewicz M, Osyczka A. Fusing two cytochromes b of Rhodobacter capsulatus cytochrome bc1 using various linkers defines a set of protein templates for asymmetric mutagenesis. Protein Eng Des Sel 2011; 25:15-25. [PMID: 22119789 PMCID: PMC3276305 DOI: 10.1093/protein/gzr055] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Cytochrome bc1 (mitochondrial complex III), one of the key enzymes of biological energy conversion, is a functional homodimer in which each monomer contains three catalytic subunits: cytochrome c1, the iron–sulfur subunit and cytochrome b. The latter is composed of eight transmembrane α-helices which, in duplicate, form a hydrophobic core of a dimer. We show that two cytochromes b can be fused into one 16-helical subunit using a number of different peptide linkers that vary in length but all connect the C-terminus of one cytochrome with the N-terminus of the other. The fusion proteins replace two cytochromes b in the dimer defining a set of available protein templates for introducing mutations that allow breaking symmetry of a dimer. A more detailed comparison of the form with the shortest, 3 amino acid, linker to the form with 12 amino acid linker established that both forms display similar level of structural plasticity to accommodate several, but not all, asymmetric patterns of mutations that knock out individual segments of cofactor chains. While the system based on a fused gene does not allow for the assessments of the functionality of electron-transfer paths in vivo, the family of proteins with fused cytochrome b offers attractive model for detailed investigations of molecular mechanism of catalysis at in vitro/reconstitution level.
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Affiliation(s)
- Monika Czapla
- Department of Molecular Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Kraków, Poland
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17
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Kokhan O, Shinkarev VP, Wraight CA. Binding of imidazole to the heme of cytochrome c1 and inhibition of the bc1 complex from Rhodobacter sphaeroides: II. Kinetics and mechanism of binding. J Biol Chem 2010; 285:22522-31. [PMID: 20448037 PMCID: PMC2903381 DOI: 10.1074/jbc.m110.128082] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2010] [Revised: 04/30/2010] [Indexed: 11/06/2022] Open
Abstract
The kinetics of imidazole (Im) and N-methylimidazole (MeIm) binding to oxidized cytochrome (cyt) c(1) of detergent-solubilized bc(1) complex from Rhodobacter sphaeroides are described. The rate of formation of the cyt c(1)-Im complex exhibited three separated regions of dependence on the concentration of imidazole: (i) below 8 mM Im, the rate increased with concentration in a parabolic manner; (ii) above 20 mM, the rate leveled off, indicating a rate-limiting conformational step with lifetime approximately 1 s; and (iii) at Im concentrations above 100 mM, the rate substantially increased again, also parabolically. In contrast, binding of MeIm followed a simple hyperbolic concentration dependence. The temperature dependences of the binding and release kinetics of Im and MeIm were also measured and revealed very large activation parameters for all reactions. The complex concentration dependence of the Im binding rate is not consistent with the popular model for soluble c-type cytochromes in which exogenous ligand binding is preceded by spontaneous opening of the heme cleft, which becomes rate-limiting at high ligand concentrations. Instead, binding of ligand to the heme is explained by a model in which an initial and superficial binding facilitates access to the heme by disruption of hydrogen-bonded structures in the heme domain. For imidazole, two separate pathways of heme access are indicated by the distinct kinetics at low and high concentration. The structural basis for ligand entry to the heme cleft is discussed.
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Affiliation(s)
| | | | - Colin A. Wraight
- From the Center for Biophysics and Computational Biology and
- the Department of Biochemistry, University of Illinois, Urbana, Illinois 61801
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18
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Cieluch E, Pietryga K, Sarewicz M, Osyczka A. Visualizing changes in electron distribution in coupled chains of cytochrome bc(1) by modifying barrier for electron transfer between the FeS cluster and heme c(1). BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2009; 1797:296-303. [PMID: 19917265 PMCID: PMC2807467 DOI: 10.1016/j.bbabio.2009.11.003] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/22/2009] [Revised: 11/09/2009] [Accepted: 11/10/2009] [Indexed: 11/24/2022]
Abstract
Cytochrome c1 of Rhodobacter (Rba.) species provides a series of mutants which change barriers for electron transfer through the cofactor chains of cytochrome bc1 by modifying heme c1 redox midpoint potential. Analysis of post-flash electron distribution in such systems can provide useful information about the contribution of individual reactions to the overall electron flow. In Rba. capsulatus, the non-functional low-potential forms of cytochrome c1 which are devoid of the disulfide bond naturally present in this protein revert spontaneously by introducing a second-site suppression (mutation A181T) that brings the potential of heme c1 back to the functionally high levels, yet maintains it some 100 mV lower from the native value. Here we report that the disulfide and the mutation A181T can coexist in one protein but the mutation exerts a dominant effect on the redox properties of heme c1 and the potential remains at the same lower value as in the disulfide-free form. This establishes effective means to modify a barrier for electron transfer between the FeS cluster and heme c1 without breaking disulfide. A comparison of the flash-induced electron transfers in native and mutated cytochrome bc1 revealed significant differences in the post-flash equilibrium distribution of electrons only when the connection of the chains with the quinone pool was interrupted at the level of either of the catalytic sites by the use of specific inhibitors, antimycin or myxothiazol. In the non-inhibited system no such differences were observed. We explain the results using a kinetic model in which a shift in the equilibrium of one reaction influences the equilibrium of all remaining reactions in the cofactor chains. It follows a rather simple description in which the direction of electron flow through the coupled chains of cytochrome bc1 exclusively depends on the rates of all reversible partial reactions, including the Q/QH2 exchange rate to/from the catalytic sites.
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Affiliation(s)
- Ewelina Cieluch
- Department of Biophysics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, ul. Gronostajowa 7, 30-307 Kraków, Poland
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19
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Quinone and non-quinone redox couples in Complex III. J Bioenerg Biomembr 2008; 40:493-9. [PMID: 18975063 DOI: 10.1007/s10863-008-9174-6] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2008] [Accepted: 07/17/2008] [Indexed: 10/21/2022]
Abstract
The Q cycle mechanism proposed by Peter Mitchell in the 1970's explicitly considered the modification of ubiquinone two-electron redox properties upon binding to Complex III to match the thermodynamics of the other single-electron redox cofactors in the complex, and guide electron transfer to support the generation of a proton electro-chemical gradient across native membranes. A better understanding of the engineering of Complex III is coming from a now moderately well defined thermodynamic description of the redox components as a function of pH, including the Qi/heme b(H) cluster. The redox properties of the most obscure component, Qo, is finally beginning to be resolved.
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20
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Ogawa K, Sonoyama T, Takeda T, Ichiki SI, Nakamura S, Kobayashi Y, Uchiyama S, Nakasone K, Takayama SIJ, Mita H, Yamamoto Y, Sambongi Y. Roles of a short connecting disulfide bond in the stability and function of psychrophilic Shewanella violacea cytochrome c (5). Extremophiles 2007; 11:797-807. [PMID: 17657404 DOI: 10.1007/s00792-007-0099-5] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2007] [Accepted: 06/15/2007] [Indexed: 10/23/2022]
Abstract
Cys-59 and Cys-62, forming a disulfide bond in the four-residue loop of Shewanella violacea cytochrome c (5) (SV cytc (5)), contribute to protein stability but not to redox function. These Cys residues were substituted with Ala in SV cytc (5), and the structural and functional properties of the resulting C59A/C62A variant were determined and compared with those of the wild-type. The variant had similar features to those of the wild-type in absorption, circular dichroic, and paramagnetic (1)H NMR spectra. In addition, the redox potentials of the wild-type and variant were essentially the same, indicating that removal of the disulfide bond from SV cytc (5) does not affect the redox function generated in the vicinity of heme. However, calorimetric analysis of the wild-type and variant showed that the mutations caused a drastic decrease in the protein stability through enthalpy, but not entropy. Four residues are encompassed by the SV cytc (5) disulfide bond, which is the shortest one that has been proved to affect protein stability. The protein stability of SV cytc (5) can be controlled without changing the redox function, providing a new strategy for regulating the stability and function of cytochrome c.
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Affiliation(s)
- Keiko Ogawa
- Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Hiroshima, 739-8528, Japan
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21
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Devanathan S, Salamon Z, Tollin G, Fitch JC, Meyer TE, Berry EA, Cusanovich MA. Plasmon waveguide resonance spectroscopic evidence for differential binding of oxidized and reduced Rhodobacter capsulatus cytochrome c2 to the cytochrome bc1 complex mediated by the conformation of the Rieske iron-sulfur protein. Biochemistry 2007; 46:7138-45. [PMID: 17516628 PMCID: PMC2565683 DOI: 10.1021/bi602649u] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The dissociation constants for the binding of Rhodobacter capsulatus cytochrome c2 and its K93P mutant to the cytochrome bc1 complex embedded in a phospholipid bilayer were measured by plasmon waveguide resonance spectroscopy in the presence and absence of the inhibitor stigmatellin. The reduced form of cytochrome c2 strongly binds to reduced cytochrome bc1 (Kd = 0.02 microM) but binds much more weakly to the oxidized form (Kd = 3.1 microM). In contrast, oxidized cytochrome c2 binds to oxidized cytochrome bc1 in a biphasic fashion with Kd values of 0.11 and 0.58 microM. Such a biphasic interaction is consistent with binding to two separate sites or conformations of oxidized cytochrome c2 and/or cytochrome bc1. However, in the presence of stigmatellin, we find that oxidized cytochrome c2 binds to oxidized cytochrome bc1 in a monophasic fashion with high affinity (Kd = 0.06 microM) and reduced cytochrome c2 binds less strongly (Kd = 0.11 microM) but approximately 30-fold more tightly than in the absence of stigmatellin. Structural studies with cytochrome bc1, with and without the inhibitor stigmatellin, have led to the proposal that the Rieske protein is mobile, moving between the cytochrome b and cytochrome c1 components during turnover. In one conformation, the Rieske protein binds near the heme of cytochrome c1, while the cytochrome c2 binding site is also near the cytochrome c1 heme but on the opposite side from the Rieske site, where cytochrome c2 cannot directly interact with Rieske. However, the inhibitor, stigmatellin, freezes the Rieske protein iron-sulfur cluster in a conformation proximal to cytochrome b and distal to cytochrome c1. We conclude from this that the dual conformation of the Rieske protein is primarily responsible for biphasic binding of oxidized cytochrome c2 to cytochrome c1. This optimizes turnover by maximizing binding of the substrate, oxidized cytochrome c2, when the iron-sulfur cluster is proximal to cytochrome b and minimizing binding of the product, reduced cytochrome c2, when it is proximal to cytochrome c1.
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Affiliation(s)
- S Devanathan
- Department of Biochemistry and Molecular Biophysics, University of Arizona, Tucson, Arizona 85721, USA
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22
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Shinkarev VP, Crofts AR, Wraight CA. Spectral analysis of the bc(1) complex components in situ: beyond the traditional difference approach. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2005; 1757:67-77. [PMID: 16386703 DOI: 10.1016/j.bbabio.2005.11.006] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2005] [Revised: 11/08/2005] [Accepted: 11/10/2005] [Indexed: 10/25/2022]
Abstract
The cytochrome (cyt) bc(1) complex (ubiquinol: cytochrome c oxidoreductase) is the central enzyme of mitochondrial and bacterial electron-transport chains. It is rich in prosthetic groups, many of which have significant but overlapping absorption bands in the visible spectrum. The kinetics of the cytochrome components of the bc(1) complex are traditionally followed by using the difference of absorbance changes at two or more different wavelengths. This difference-wavelength (DW) approach has been used extensively in the development and testing of the Q-cycle mechanism of the bc(1) complex in Rhodobacter sphaeroides chromatophores. However, the DW approach does not fully compensate for spectral interference from other components, which can significantly distort both amplitudes and kinetics. Mechanistic elaboration of cyt bc(1) turnover requires an approach that overcomes this limitation. Here, we compare the traditional DW approach to a least squares (LS) analysis of electron transport, based on newly determined difference spectra of all individual components of cyclic electron transport in chromatophores. Multiple sets of kinetic traces, measured at different wavelengths in the absence and presence of specific inhibitors, were analyzed by both LS and DW approaches. Comparison of the two methods showed that the DW approach did not adequately correct for the spectral overlap among the components, and was generally unreliable when amplitude changes for a component of interest were small. In particular, it was unable to correct for extraneous contributions to the amplitudes and kinetics of cyt b(L). From LS analysis of the chromophoric components (RC, c(tot), b(H) and b(L)), we show that while the Q-cycle model remains firmly grounded, quantitative reevaluation of rates, amplitudes, delays, etc., of individual components is necessary. We conclude that further exploration of mechanisms of the bc(1) complex, will require LS deconvolution for reliable measurement of the kinetics of individual components of the complex in situ.
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Affiliation(s)
- Vladimir P Shinkarev
- Department of Biochemistry, University of Illinois at Urbana -- Champaign, 156 Davenport Hall, 607 South Mathews Avenue, Urbana, IL 61801, USA.
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23
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Osyczka A, Moser CC, Dutton PL. Novel cyanide inhibition at cytochrome c1 of Rhodobacter capsulatus cytochrome bc1. BIOCHIMICA ET BIOPHYSICA ACTA-BIOENERGETICS 2004; 1655:71-6. [PMID: 15100019 DOI: 10.1016/j.bbabio.2003.07.005] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/08/2003] [Revised: 07/14/2003] [Accepted: 07/14/2003] [Indexed: 11/18/2022]
Abstract
Oxidized cytochrome c(1) in photosynthetic bacterium Rhodobacter capsulatus cytochrome bc(1) reversibly binds cyanide with surprisingly high, micromolar affinity. The binding dramatically lowers the redox midpoint potential of heme c(1) and inhibits steady-state turnover activity of the enzyme. As cytochrome c(1), an auxiliary redox center of the high-potential chain of cytochrome bc(1), does not interact directly with the catalytic quinone/quinol binding sites Q(o) and Q(i), cyanide introduces a novel, Q-site independent locus of inhibition. This is the first report of a reversible inhibitor that manipulates the energetics and electron transfers of the high-potential redox chain of cytochrome bc(1), while maintaining quinone substrate catalytic sites in an intact form.
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Affiliation(s)
- Artur Osyczka
- The Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, 1004 Stellar-Chance Building, 422 Curie Boulevard, Philadelphia, PA 19104, USA.
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24
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Osyczka A, Moser CC, Daldal F, Dutton PL. Reversible redox energy coupling in electron transfer chains. Nature 2004; 427:607-12. [PMID: 14961113 DOI: 10.1038/nature02242] [Citation(s) in RCA: 209] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2003] [Accepted: 11/14/2003] [Indexed: 11/09/2022]
Abstract
Reversibility is a common theme in respiratory and photosynthetic systems that couple electron transfer with a transmembrane proton gradient driving ATP production. This includes the intensely studied cytochrome bc1, which catalyses electron transfer between quinone and cytochrome c. To understand how efficient reversible energy coupling works, here we have progressively inactivated individual cofactors comprising cytochrome bc1. We have resolved millisecond reversibility in all electron-tunnelling steps and coupled proton exchanges, including charge-separating hydroquinone-quinone catalysis at the Q(o) site, which shows that redox equilibria are relevant on a catalytic timescale. Such rapid reversibility renders popular models based on a semiquinone in Q(o) site catalysis prone to short-circuit failure. Two mechanisms allow reversible function and safely relegate short-circuits to long-distance electron tunnelling on a timescale of seconds: conformational gating of semiquinone for both forward and reverse electron transfer, or concerted two-electron quinone redox chemistry that avoids the semiquinone intermediate altogether.
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Affiliation(s)
- Artur Osyczka
- The Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania, 19104, USA
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25
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Li J, Darrouzet E, Dhawan IK, Johnson MK, Osyczka A, Daldal F, Knaff DB. Spectroscopic and oxidation-reduction properties of Rhodobacter capsulatus cytochrome c1 and its M183K and M183H variants. BIOCHIMICA ET BIOPHYSICA ACTA 2002; 1556:175-86. [PMID: 12460675 DOI: 10.1016/s0005-2728(02)00360-2] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Two variants of the cytochrome c1 component of the Rhodobacter capsulatus cytochrome bc1 complex, in which Met183 (an axial heme ligand) was replaced by lysine (M183K) or histidine (M183H), have been analyzed. Electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectra of the intact complex indicate that the histidine/methionine heme ligation of the wild-type cytochrome is replaced by histidine/lysine ligation in M183K and histidine/histidine ligation in M183H. Variable amounts of histidine/histidine axial heme ligation were also detected in purified wild-type cytochrome c1 and its M183K variant, suggesting that a histidine outside the CSACH heme-binding domain can be recruited as an alternative ligand. Oxidation-reduction titrations of the heme in purified cytochrome c1 revealed multiple redox forms. Titrations of the purified cytochrome carried out in the oxidative or reductive direction differ. In contrast, titrations of cytochrome c1 in the intact bc1 complex and in a subcomplex missing the Rieske iron-sulfur protein were fully reversible. An Em7 value of -330 mV was measured for the single disulfide bond in cytochrome c1. The origins of heme redox heterogeneity, and of the differences between reductive and oxidative heme titrations, are discussed in terms of conformational changes and the role of the disulfide in maintaining the native structure of cytochrome c1.
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Affiliation(s)
- Jun Li
- Department of Chemistry and Biochemistry and Center for Biotechnology and Genomics, Texas Tech University, Box 41061, Lubbock, TX 79409-1061, USA
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Wastl J, Bendall DS, Howe CJ. Higher plants contain a modified cytochrome c(6). TRENDS IN PLANT SCIENCE 2002; 7:244-245. [PMID: 12049919 DOI: 10.1016/s1360-1385(02)02280-x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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