Electron transfer in multiheme cytochromes of Shewanella oneidensis MR-1: CymA and the dissimilatory metal reduction pathway
Sherwood, Mackenzie A. Firer
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Shewanella oneidensis is a facultative, gram-negative microbe that, in the absence of oxygen, can use a wide variety of terminal electron acceptors including iron, manganese, uranium, nitrite, nitrate, sulfate, fumarate, and DMSO. The anaerobic versatility is believed to be the result of a highly branched electron transfer pathway involving many redox-active proteins. Shewanella is capable of dissimilatory metal reduction (DMR) of insoluble iron and manganese oxides, in which electrons are transferred from the cell's interior to its exterior. Several multiheme c -type cytochromes comprise a pathway for this electron transfer. These cytochromes, specifically the tetraheme protein, CymA, and the decaheme protein, MtrA, are the primary focus of this thesis. The current model of electron transfer indicates that electrons originate in the cytoplasmic membrane from the menaquinol pool, and are transferred into the periplasm by CymA. From here the pathway branches and electrons are transferred into several potential periplasmic targets, including MtrA. MtrA may then transfer electrons directly or indirectly to MtrC and OmcA, which have been shown to reduce exogenous electron acceptors such as iron oxides. Recently, it has been suggested that MtrA and MtrC dock with 13-barrel protein, MtrB and transfer electrons through the porin sheath. Here, the DMR pathway has been studied with respect to four aims: (1) purification and characterization of the multiheme cytochromes through the use of non-catalytic protein film voltammetry (PFV), (2) structural analysis of MtrA by small angle X-ray scattering (SAXS), (3) investigation of protein-protein interactions via catalytic PFV and anaerobic affinity chromatography, and (4) exploration of heme cofactor function within the tetraheme cytochrome, CymA and MtrA by characterizing heme knockout mutants of the two proteins. We demonstrate that these proteins interact to form an electron transfer pathway from the cytoplasm to terminal electron acceptors on the outside of the cell through a "wire" of heme cofactors. Additionally, the data support the model that MtrA can span a large portion of the peri plasmic space to act as an intermediary by accepting electrons from CymA and subsequently docking with MtrB to transfer electrons to MtrC.
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