Kinetics of Reduction of Fe(III) Complexes by Outer Membrane Cytochromes MtrC and OmcA of Shewanella oneidensis MR-1

Journal Article · Fri Sep 12 00:00:00 EDT 2008 · Applied and Environmental Microbiology

DOI:https://doi.org/10.1128/AEM.01454-08· OSTI ID:946380

Wang, Zheming ^[1]; Liu, Chongxuan ^[1]; Wang, Xuelin ^[1]; Marshall, Matthew J. ^[1]; Zachara, John M. ^[1]; Rosso, Kevin M. ^[1]; Dupuis, Michel ^[1]; Fredrickson, Jim K. ^[1]; Heald, Steve M. ^[2]; Shi, Liang ^[1]

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1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Argonne National Lab. (ANL), Argonne, IL (United States)

Shewanella Oneidensis MR-1 possesses up to 42 c-type cytochromes with heme content varying between 1 to as many as 37. Among them, the outer-membrane cytochromes, particularly MtrC and OmcA, are suspected to function as terminal reductases and are responsible for its enzymatic catalysis capability. So far, the mechanisms of metal reduction by these outer-membrane cytochromes are unknown. In this work, we report the study of reduction kinetics of a series of Fe(III) complexes with citrate, NTA and EDTA by abiotically reduced MtrC and OmcA using a stopped-flow technique in combination with theoretical computation methods within the framework of the electron transfer theory of Marcus and speciation calculations based on the current thermodynamic database. Stopped-flow kinetic data showed that the reaction was very fast and appeared to proceed in two stages, a fast stage that completes in much less than a second and a slower stage afterwards. For a given complex, the reaction is faster by reduction with MtrC than OmcA, while for a given protein, the reaction completes in the decreasing order of Fe-EDTA > Fe-NTA > Fe-citrate. All the stopped-flow kinetic curves could be modeled by two parallel second-order bimolecular redox reactions with second-order rate constants ranging from 0.872 µM^-1s^-1 for the fast reaction between MtrC with Fe-EDTA complex to 0.012 µM^-1s^-1 for the slow reaction between OmcA and Fe-citrate complex. Speciation calculations indicated that at both metal:ligand ratios, 1:1.5 and 1:10, a single dominant ferric complex was responsible for the observed reaction for each ligand and, therefore, the observed dual-reaction pathways was attributed to the differences in the reduction behavior among various heme groups within each protein. The results of redox potential calculations with known thermodynamic data show only small differences on the scale of a few millivolts among the three complexes, suggested that the observed differences in reaction rate cannot be explained by the overall redox reaction free energy. In contrast, reorganization energies calculated based on DFT-COSMO model are substantially different between the complexes, with a larger reorganization energy and therefore a larger activation energy associated with the citrate complex, and progressively smaller ones for the NTA and EDTA complexes. In combination with approximate electronic coupling terms, the theoretical results show good agreement with the observed trend and implicate the reorganization energy as the key factor in the kinetic reaction.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 946380
• Report Number(s): PNNL-SA-57287; AEMIDF; 25692; KC0303020; TRN: US200903%%399
• Journal Information: Applied and Environmental Microbiology, Vol. 74, Issue 21; ISSN 0099-2240
• Publisher: American Society for Microbiology
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
ACTIVATION ENERGY
CATALYSIS
CITRATES
CYTOCHROMES
EDTA
ELECTRON TRANSFER
FREE ENERGY
HEME
KINETICS
MEMBRANES
OXIDOREDUCTASES
REACTION KINETICS
REDOX POTENTIAL
REDOX REACTIONS
THERMODYNAMICS
Shewanella Oneidensis
outer-membrane cytochromes
electron transfer theory
MtrC and OmcA
Environmental Molecular Sciences Laboratory