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Schütz B, Seidel J, Sturm G, Einsle O, Gescher J. Investigation of the electron transport chain to and the catalytic activity of the diheme cytochrome c peroxidase CcpA of Shewanella oneidensis. Appl Environ Microbiol 2011; 77:6172-80. [PMID: 21742904 PMCID: PMC3165401 DOI: 10.1128/aem.00606-11] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2011] [Accepted: 06/30/2011] [Indexed: 11/20/2022] Open
Abstract
Bacterial diheme c-type cytochrome peroxidases (BCCPs) catalyze the periplasmic reduction of hydrogen peroxide to water. The gammaproteobacterium Shewanella oneidensis produces the peroxidase CcpA under a number of anaerobic conditions, including dissimilatory iron-reducing conditions. We wanted to understand the function of this protein in the organism and its putative connection to the electron transport chain to ferric iron. CcpA was isolated and tested for peroxidase activity, and its structural conformation was analyzed by X-ray crystallography. CcpA exhibited in vitro peroxidase activity and had a structure typical of diheme peroxidases. It was produced in almost equal amounts under anaerobic and microaerophilic conditions. With 50 mM ferric citrate and 50 μM oxygen in the growth medium, CcpA expression results in a strong selective advantage for the cell, which was detected in competitive growth experiments with wild-type and ΔccpA mutant cells that lack the entire ccpA gene due to a markerless deletion. We were unable to reduce CcpA directly with CymA, MtrA, or FccA, which are known key players in the chain of electron transport to ferric iron and fumarate but identified the small monoheme ScyA as a mediator of electron transport between CymA and BCCP. To our knowledge, this is the first detailed description of a complete chain of electron transport to a periplasmic c-type cytochrome peroxidase. This study furthermore reports the possibility of establishing a specific electron transport chain using c-type cytochromes.
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Affiliation(s)
- Björn Schütz
- Institut für Biologie II, Mikrobiologie, Universität Freiburg, Schänzlestr. 1, D-79104 Freiburg, Germany
| | - Julian Seidel
- Institut für organische Chemie und Biochemie, Biochemie, Universität Freiburg, Albertstr. 21, D-79104 Freiburg, Germany
| | - Gunnar Sturm
- Institut für angewandte Biowissenschaften, Angewandte Biologie, Karlsruher Institut für Technologie, Fritz-Haber-Weg 2, D-76131 Karlsruhe, Germany
| | - Oliver Einsle
- Institut für organische Chemie und Biochemie, Biochemie, Universität Freiburg, Albertstr. 21, D-79104 Freiburg, Germany
| | - Johannes Gescher
- Institut für angewandte Biowissenschaften, Angewandte Biologie, Karlsruher Institut für Technologie, Fritz-Haber-Weg 2, D-76131 Karlsruhe, Germany
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102
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Bird LJ, Bonnefoy V, Newman DK. Bioenergetic challenges of microbial iron metabolisms. Trends Microbiol 2011; 19:330-40. [DOI: 10.1016/j.tim.2011.05.001] [Citation(s) in RCA: 264] [Impact Index Per Article: 20.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2011] [Revised: 04/30/2011] [Accepted: 05/03/2011] [Indexed: 11/24/2022]
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103
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Qian Y, Paquete CM, Louro RO, Ross DE, Labelle E, Bond DR, Tien M. Mapping the iron binding site(s) on the small tetraheme cytochrome of Shewanella oneidensis MR-1. Biochemistry 2011; 50:6217-24. [PMID: 21682327 DOI: 10.1021/bi2005015] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
In the model microbe Shewanella oneidensis, multi-heme proteins are utilized for respiratory metabolism where metals serve as the terminal electron acceptor. Among those is the periplasm-localized small tetraheme cytochrome (STC). STC has been extensively characterized structurally and electrochemically to which electron flow in and out of the protein has been modeled. However, until the present work, no kinetic studies have been performed to probe the route of electron flow or to determine the iron-binding site on STC. Using iron chelated by EDTA, NTA, or citrate, we have used chemical modification, site-directed mutagenesis along with isothermal titration calorimetry (ITC), and stopped-flow measurements to identify the iron binding site of STC. Chemical modifications of STC revealed that carboxyl groups on STC are involved in binding of EDTA-Fe(3+). Scanning mutagenesis was performed on Asp and Glu to probe the putative iron-binding site on STC. Two STC mutants (D21N; D80N) showed ∼70% decrease in observed electron transfer rate constant with EDTA-Fe(3+) from transient-state kinetic measurements. The impaired reactivity of STC (D80N/D21N) with EDTA-Fe(3+) was further confirmed by a significant decrease (>10-fold) in iron binding affinity.
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Affiliation(s)
- Yufeng Qian
- Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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104
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Okamoto A, Nakamura R, Hashimoto K. In-vivo identification of direct electron transfer from Shewanella oneidensis MR-1 to electrodes via outer-membrane OmcA–MtrCAB protein complexes. Electrochim Acta 2011. [DOI: 10.1016/j.electacta.2011.03.076] [Citation(s) in RCA: 68] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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105
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Cyclic voltammetric analysis of the electron transfer of Shewanella oneidensis MR-1 and nanofilament and cytochrome knock-out mutants. Bioelectrochemistry 2011; 81:74-80. [DOI: 10.1016/j.bioelechem.2011.02.006] [Citation(s) in RCA: 139] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2010] [Revised: 02/04/2011] [Accepted: 02/15/2011] [Indexed: 11/18/2022]
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106
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Abstract
Some bacterial species are able to utilize extracellular mineral forms of iron and manganese as respiratory electron acceptors. In Shewanella oneidensis this involves decaheme cytochromes that are located on the bacterial cell surface at the termini of trans-outer-membrane electron transfer conduits. The cell surface cytochromes can potentially play multiple roles in mediating electron transfer directly to insoluble electron sinks, catalyzing electron exchange with flavin electron shuttles or participating in extracellular intercytochrome electron exchange along "nanowire" appendages. We present a 3.2-Å crystal structure of one of these decaheme cytochromes, MtrF, that allows the spatial organization of the 10 hemes to be visualized for the first time. The hemes are organized across four domains in a unique crossed conformation, in which a staggered 65-Å octaheme chain transects the length of the protein and is bisected by a planar 45-Å tetraheme chain that connects two extended Greek key split β-barrel domains. The structure provides molecular insight into how reduction of insoluble substrate (e.g., minerals), soluble substrates (e.g., flavins), and cytochrome redox partners might be possible in tandem at different termini of a trifurcated electron transport chain on the cell surface.
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107
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Extracellular reduction of hexavalent chromium by cytochromes MtrC and OmcA of Shewanella oneidensis MR-1. Appl Environ Microbiol 2011; 77:4035-41. [PMID: 21498755 DOI: 10.1128/aem.02463-10] [Citation(s) in RCA: 101] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
To characterize the roles of cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 in Cr(VI) reduction, the effects of deleting the mtrC and/or omcA gene on Cr(VI) reduction and the cellular locations of reduced Cr(III) precipitates were investigated. Compared to the rate of reduction of Cr(VI) by the wild type (wt), the deletion of mtrC decreased the initial rate of Cr(VI) reduction by 43.5%, while the deletion of omcA or both mtrC and omcA lowered the rate by 53.4% and 68.9%, respectively. In wt cells, Cr(III) precipitates were detected by transmission electron microscopy in the extracellular matrix between the cells, in association with the outer membrane, and inside the cytoplasm. No extracellular matrix-associated Cr(III) precipitates, however, were found in the cytochrome mutant cell suspension. In mutant cells without either MtrC or OmcA, most Cr(III) precipitates were found in association with the outer membrane, while in mutant cells lacking both MtrC and OmcA, most Cr(III) precipitates were found inside the cytoplasm. Cr(III) precipitates were also detected by scanning election microscopy on the surfaces of the wt and mutants without MtrC or OmcA but not on the mutant cells lacking both MtrC and OmcA, demonstrating that the deletion of mtrC and omcA diminishes the extracellular formation of Cr(III) precipitates. Furthermore, purified MtrC and OmcA reduced Cr(VI) with apparent k(cat) values of 1.2 ± 0.2 (mean ± standard deviation) and 10.2 ± 1 s(-1) and K(m) values of 34.1 ± 4.5 and 41.3 ± 7.9 μM, respectively. Together, these results consistently demonstrate that MtrC and OmcA are the terminal reductases used by S. oneidensis MR-1 for extracellular Cr(VI) reduction where OmcA is a predominant Cr(VI) reductase.
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108
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Firer-Sherwood MA, Bewley KD, Mock JY, Elliott SJ. Tools for resolving complexity in the electron transfer networks of multiheme cytochromes c. Metallomics 2011; 3:344-8. [PMID: 21327265 DOI: 10.1039/c0mt00097c] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
Examining electron transfer between two proteins with identical spectroscopic signatures is a challenging task. It is supposed that several multiheme cytochromes in Shewanella oneidensis form a molecular "wire" through which electrons are transported across the cellular space and a direct study of this transient protein-protein interaction has not yet been reported. In this study, we present variations on catalytic protein film voltammetry and an anaerobic affinity chromatography assay to demonstrate unidirectional electron transfer between proposed protein pairs. Through use of these techniques, we are able to confirm the transient interactions between these cytochromes, supporting the model of electron transfer that is present in the literature.
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109
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Ross DE, Flynn JM, Baron DB, Gralnick JA, Bond DR. Towards electrosynthesis in shewanella: energetics of reversing the mtr pathway for reductive metabolism. PLoS One 2011; 6:e16649. [PMID: 21311751 PMCID: PMC3032769 DOI: 10.1371/journal.pone.0016649] [Citation(s) in RCA: 205] [Impact Index Per Article: 15.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2010] [Accepted: 12/22/2010] [Indexed: 11/29/2022] Open
Abstract
Bioelectrochemical systems rely on microorganisms to link complex oxidation/reduction reactions to electrodes. For example, in Shewanella oneidensis strain MR-1, an electron transfer conduit consisting of cytochromes and structural proteins, known as the Mtr respiratory pathway, catalyzes electron flow from cytoplasmic oxidative reactions to electrodes. Reversing this electron flow to drive microbial reductive metabolism offers a possible route for electrosynthesis of high value fuels and chemicals. We examined electron flow from electrodes into Shewanella to determine the feasibility of this process, the molecular components of reductive electron flow, and what driving forces were required. Addition of fumarate to a film of S. oneidensis adhering to a graphite electrode poised at −0.36 V versus standard hydrogen electrode (SHE) immediately led to electron uptake, while a mutant lacking the periplasmic fumarate reductase FccA was unable to utilize electrodes for fumarate reduction. Deletion of the gene encoding the outer membrane cytochrome-anchoring protein MtrB eliminated 88% of fumarate reduction. A mutant lacking the periplasmic cytochrome MtrA demonstrated more severe defects. Surprisingly, disruption of menC, which prevents menaquinone biosynthesis, eliminated 85% of electron flux. Deletion of the gene encoding the quinone-linked cytochrome CymA had a similar negative effect, which showed that electrons primarily flowed from outer membrane cytochromes into the quinone pool, and back to periplasmic FccA. Soluble redox mediators only partially restored electron transfer in mutants, suggesting that soluble shuttles could not replace periplasmic protein-protein interactions. This work demonstrates that the Mtr pathway can power reductive reactions, shows this conduit is functionally reversible, and provides new evidence for distinct CymA:MtrA and CymA:FccA respiratory units.
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Affiliation(s)
- Daniel E. Ross
- The BioTechnology Institute, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
| | - Jeffrey M. Flynn
- The BioTechnology Institute, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
| | - Daniel B. Baron
- The BioTechnology Institute, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
| | - Jeffrey A. Gralnick
- The BioTechnology Institute, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
- Department of Microbiology, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
| | - Daniel R. Bond
- The BioTechnology Institute, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
- Department of Microbiology, University of Minnesota-Twin Cities, St. Paul, Minnesota, United States of America
- * E-mail:
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110
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Yuan Y, Zhou S, Xu N, Zhuang L. Electrochemical characterization of anodic biofilms enriched with glucose and acetate in single-chamber microbial fuel cells. Colloids Surf B Biointerfaces 2011; 82:641-6. [DOI: 10.1016/j.colsurfb.2010.10.015] [Citation(s) in RCA: 70] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2010] [Revised: 09/02/2010] [Accepted: 10/07/2010] [Indexed: 11/25/2022]
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111
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Qian F, Morse DE. Miniaturizing microbial fuel cells. Trends Biotechnol 2011; 29:62-9. [DOI: 10.1016/j.tibtech.2010.10.003] [Citation(s) in RCA: 111] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2010] [Revised: 10/11/2010] [Accepted: 10/13/2010] [Indexed: 01/22/2023]
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112
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Qian F, Wang G, Li Y. Solar-driven microbial photoelectrochemical cells with a nanowire photocathode. NANO LETTERS 2010; 10:4686-91. [PMID: 20939571 DOI: 10.1021/nl102977n] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/12/2023]
Abstract
We report a self-biased, solar-driven microbial photoelectrochemical cell (solar MPC) that can produce sustainable energy through coupling the microbial catalysis of biodegradable organic matter with solar energy conversion. The solar MPC consists of a p-type cuprous oxide nanowire-arrayed photocathode and an electricigen (Shewanella oneidensis MR-1)-colonizing anode, which can harvest solar energy and bioenergy, respectively. The photocathode and bioanode are interfaced by matching the redox potentials of bacterial cells and the electronic bands of semiconductor nanowires. We successfully demonstrated substantial current generation of 200 μA from the MPC device based on the synergistic effect of the bioanode (projected area of 20 cm2) and photocathode (projected area of 4 cm2) at zero bias under white light illumination of 20 mW/cm2. We identified the transition of rate-limiting step from the photocathode to the bioanode with increasing light intensities. The solar MPC showed self-sustained operation for more than 50 h in batch-fed mode under continuous light illumination. The ability to tune the synergistic effect between microbial cells and semiconductor nanowire systems could open up new opportunities for microbial/nanoelectronic hybrid devices with unique applications in energy conversion, environmental protection, and biomedical research.
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Affiliation(s)
- Fang Qian
- Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States.
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113
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Coursolle D, Gralnick JA. Modularity of the Mtr respiratory pathway of Shewanella oneidensis strain MR-1. Mol Microbiol 2010; 77:995-1008. [PMID: 20598084 DOI: 10.1111/j.1365-2958.2010.07266.x] [Citation(s) in RCA: 110] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Four distinct pathways predicted to facilitate electron flow for respiration of externally located substrates are encoded in the genome of Shewanella oneidensis strain MR-1. Although the pathways share a suite of similar proteins, the activity of only two of these pathways has been described. Respiration of extracellular substrates requires a mechanism to facilitate electron transfer from the quinone pool in the cytoplasmic membrane to terminal reductase enzymes located on the outer leaflet of the outer membrane. The four pathways share MtrA paralogues, a periplasmic electron carrier cytochrome, and terminal reductases similar to MtrC for reduction of metals, flavins and electrodes or to DmsAB for reduction of dimethyl sulphoxide (DMSO). The promiscuity of respiratory electron transfer reactions catalysed by these pathways has made studying strains lacking single proteins difficult. Here, we present a comprehensive analysis of MtrA and MtrC paralogues in S. oneidensis to define the roles of these proteins in respiration of insoluble iron oxide, soluble iron citrate, flavins and DMSO. We present evidence that some periplasmic electron carrier components and terminal reductases in these pathways can provide partial compensation in the absence of the primary component, a phenomenon described as modularity, and discuss biochemical and evolutionary implications.
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Affiliation(s)
- Dan Coursolle
- BioTechnology Institute and Department of Microbiology, University of Minnesota-Twin Cities, St. Paul, MN 55108, USA
| | - Jeffrey A Gralnick
- BioTechnology Institute and Department of Microbiology, University of Minnesota-Twin Cities, St. Paul, MN 55108, USA
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114
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Bodemer GJ, Antholine WA, Basova LV, Saffarini D, Pacheco AA. The effect of detergents and lipids on the properties of the outer-membrane protein OmcA from Shewanella oneidensis. J Biol Inorg Chem 2010; 15:749-58. [DOI: 10.1007/s00775-010-0643-0] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2009] [Accepted: 02/24/2010] [Indexed: 11/27/2022]
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115
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Reardon CL, Dohnalkova AC, Nachimuthu P, Kennedy DW, Saffarini DA, Arey BW, Shi L, Wang Z, Moore D, McLean JS, Moyles D, Marshall MJ, Zachara JM, Fredrickson JK, Beliaev AS. Role of outer-membrane cytochromes MtrC and OmcA in the biomineralization of ferrihydrite by Shewanella oneidensis MR-1. GEOBIOLOGY 2010; 8:56-68. [PMID: 20002197 DOI: 10.1111/j.1472-4669.2009.00226.x] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
In an effort to improve the understanding of electron transfer mechanisms at the microbe-mineral interface, Shewanella oneidensis MR-1 mutants with in-frame deletions of outer-membrane cytochromes (OMCs), MtrC and OmcA, were characterized for the ability to reduce ferrihydrite (FH) using a suite of microscopic, spectroscopic, and biochemical techniques. Analysis of purified recombinant proteins demonstrated that both cytochromes undergo rapid electron exchange with FH in vitro with MtrC displaying faster transfer rates than OmcA. Immunomicroscopy with cytochrome-specific antibodies revealed that MtrC co-localizes with iron solids on the cell surface while OmcA exhibits a more diffuse distribution over the cell surface. After 3-day incubation of MR-1 with FH, pronounced reductive transformation mineral products were visible by electron microscopy. Upon further incubation, the predominant phases identified were ferrous phosphates including vivianite [Fe(3)(PO(4))(2)x8H(2)O] and a switzerite-like phase [Mn(3),Fe(3)(PO(4))(2)x7H(2)O] that were heavily colonized by MR-1 cells with surface-exposed outer-membrane cytochromes. In the absence of both MtrC and OmcA, the cells ability to reduce FH was significantly hindered and no mineral transformation products were detected. Collectively, these results highlight the importance of the outer-membrane cytochromes in the reductive transformation of FH and support a role for direct electron transfer from the OMCs at the cell surface to the mineral.
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Affiliation(s)
- C L Reardon
- Biological Sciences Division, Pacific Northwest National Laboratory, Richland, WA, USA
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116
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117
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Paquete CM, Louro RO. Molecular details of multielectron transfer: the case of multiheme cytochromes from metal respiring organisms. Dalton Trans 2009; 39:4259-66. [PMID: 20422082 DOI: 10.1039/b917952f] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Shewanella are facultative anaerobic bacteria of remarkable respiratory versatility that includes the dissimilatory reduction of metal ores. They contain a large number of multiheme c-type cytochromes that play a significant role in various anaerobic respiratory processes. Of all the cytochromes found in Shewanella, only the two most abundant periplasmic cytochromes, the small tetraheme cytochrome (STC) and flavocytochrome c(3) (Fcc(3)) have been structurally characterized. For these two proteins the molecular bases for their redox properties were determined using spectroscopic methods based on paramagnetic NMR, that allow the contribution of specific hemes to be discriminated. In this perspective these results are reviewed in the context of the continuing effort to understand the molecular mechanisms of electron transfer in the respiratory chains of these organisms.
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118
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Characterization of an electron conduit between bacteria and the extracellular environment. Proc Natl Acad Sci U S A 2009; 106:22169-74. [PMID: 20018742 DOI: 10.1073/pnas.0900086106] [Citation(s) in RCA: 292] [Impact Index Per Article: 19.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning beta-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment.
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119
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The Mtr respiratory pathway is essential for reducing flavins and electrodes in Shewanella oneidensis. J Bacteriol 2009; 192:467-74. [PMID: 19897659 DOI: 10.1128/jb.00925-09] [Citation(s) in RCA: 289] [Impact Index Per Article: 19.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The Mtr respiratory pathway of Shewanella oneidensis strain MR-1 is required to effectively respire both soluble and insoluble forms of oxidized iron. Flavins (riboflavin and flavin mononucleotide) recently have been shown to be excreted by MR-1 and facilitate the reduction of insoluble substrates. Other Shewanella species tested accumulated flavins in supernatants to an extent similar to that of MR-1, suggesting that flavin secretion is a general trait of the species. External flavins have been proposed to act as both a soluble electron shuttle and a metal chelator; however, at biologically relevant concentrations, our results suggest that external flavins primarily act as electron shuttles for MR-1. Using deletion mutants lacking various Mtr-associated proteins, we demonstrate that the Mtr extracellular respiratory pathway is essential for the reduction of flavins and that decaheme cytochromes found on the outer surface of the cell (MtrC and OmcA) are required for the majority of this activity. Given the involvement of external flavins in the reduction of electrodes, we monitored current production by Mtr respiratory pathway mutants in three-electrode bioreactors under controlled flavin concentrations. While mutants lacking MtrC were able to reduce flavins at 50% of the rate of the wild type in cell suspension assays, these strains were unable to grow into productive electrode-reducing biofilms. The analysis of mutants lacking OmcA suggests a role for this protein in both electron transfer to electrodes and attachment to surfaces. The parallel phenotypes of Mtr mutants in flavin and electrode reduction blur the distinction between direct contact and the redox shuttling strategies of insoluble substrate reduction by MR-1.
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120
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Periplasmic electron transfer via the c-type cytochromes MtrA and FccA of Shewanella oneidensis MR-1. Appl Environ Microbiol 2009; 75:7789-96. [PMID: 19837833 DOI: 10.1128/aem.01834-09] [Citation(s) in RCA: 101] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Dissimilatory microbial reduction of insoluble Fe(III) oxides is a geochemically and ecologically important process which involves the transfer of cellular, respiratory electrons from the cytoplasmic membrane to insoluble, extracellular, mineral-phase electron acceptors. In this paper evidence is provided for the function of the periplasmic fumarate reductase FccA and the decaheme c-type cytochrome MtrA in periplasmic electron transfer reactions in the gammaproteobacterium Shewanella oneidensis. Both proteins are abundant in the periplasm of ferric citrate-reducing S. oneidensis cells. In vitro fumarate reductase FccA and c-type cytochrome MtrA were reduced by the cytoplasmic membrane-bound protein CymA. Electron transfer between CymA and MtrA was 1.4-fold faster than the CymA-catalyzed reduction of FccA. Further experiments showing a bidirectional electron transfer between FccA and MtrA provided evidence for an electron transfer network in the periplasmic space of S. oneidensis. Hence, FccA could function in both the electron transport to fumarate and via MtrA to mineral-phase Fe(III). Growth experiments with a DeltafccA deletion mutant suggest a role of FccA as a transient electron storage protein.
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121
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Baron D, LaBelle E, Coursolle D, Gralnick JA, Bond DR. Electrochemical measurement of electron transfer kinetics by Shewanella oneidensis MR-1. J Biol Chem 2009; 284:28865-73. [PMID: 19661057 DOI: 10.1074/jbc.m109.043455] [Citation(s) in RCA: 176] [Impact Index Per Article: 11.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Shewanella oneidensis strain MR-1 can respire using carbon electrodes and metal oxyhydroxides as electron acceptors, requiring mechanisms for transferring electrons from the cell interior to surfaces located beyond the cell. Although purified outer membrane cytochromes will reduce both electrodes and metals, S. oneidensis also secretes flavins, which accelerate electron transfer to metals and electrodes. We developed techniques for detecting direct electron transfer by intact cells, using turnover and single turnover voltammetry. Metabolically active cells attached to graphite electrodes produced thin (submonolayer) films that demonstrated both catalytic and reversible electron transfer in the presence and absence of flavins. In the absence of soluble flavins, electron transfer occurred in a broad potential window centered at approximately 0 V (versus standard hydrogen electrode), and was altered in single (DeltaomcA, DeltamtrC) and double deletion (DeltaomcA/DeltamtrC) mutants of outer membrane cytochromes. The addition of soluble flavins at physiological concentrations significantly accelerated electron transfer and allowed catalytic electron transfer to occur at lower applied potentials (-0.2 V). Scan rate analysis indicated that rate constants for direct electron transfer were slower than those reported for pure cytochromes (approximately 1 s(-1)). These observations indicated that anodic current in the higher (>0 V) window is due to activation of a direct transfer mechanism, whereas electron transfer at lower potentials is enabled by flavins. The electrochemical dissection of these activities in living cells into two systems with characteristic midpoint potentials and kinetic behaviors explains prior observations and demonstrates the complementary nature of S. oneidensis electron transfer strategies.
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Affiliation(s)
- Daniel Baron
- BioTechnology Institute, University of Minnesota, St. Paul, Minnesota 55108, USA
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Shi L, Richardson DJ, Wang Z, Kerisit SN, Rosso KM, Zachara JM, Fredrickson JK. The roles of outer membrane cytochromes of Shewanella and Geobacter in extracellular electron transfer. ENVIRONMENTAL MICROBIOLOGY REPORTS 2009; 1:220-7. [PMID: 23765850 DOI: 10.1111/j.1758-2229.2009.00035.x] [Citation(s) in RCA: 208] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/11/2023]
Abstract
As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] (hydr)oxides, outer membrane multihaem c-type cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Both MtrC and OmcA are at least partially exposed to the extracellular side of the outer membrane and their translocation across the outer membrane is mediated by bacterial type II secretion system. Purified MtrC and OmcA can bind Fe(III) oxides, such as haematite (α-Fe2 O3 ), and directly transfer electrons to the haematite surface. Bindings of MtrC and OmcA to haematite are probably facilitated by their putative haematite-binding motifs whose conserved sequence is Thr-Pro-Ser/Thr. Purified MtrC and OmcA also exhibit broad operating potential ranges that make it thermodynamically feasible to transfer electrons directly not only to Fe(III) oxides but also to other extracellular substrates with different redox potentials. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as intermediates to relay electrons to type IV pili, which are hypothesized to transfer electrons directly to the metal oxides. Cell surface-localized cytochromes thus are key components mediating extracellular ET reactions in both Shewanella and Geobacter for extracellular reduction of Fe(III) oxides.
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Affiliation(s)
- Liang Shi
- Pacific Northwest National Laboratory, Richland, WA 99354, USA. Schools of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK
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The tetraheme cytochrome from Shewanella oneidensis MR-1 shows thermodynamic bias for functional specificity of the hemes. J Biol Inorg Chem 2008; 14:375-85. [DOI: 10.1007/s00775-008-0455-7] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2008] [Accepted: 11/14/2008] [Indexed: 10/21/2022]
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