Purification and Characterization of OmcZ, an Outer-Surface, Octaheme c -Type Cytochrome Essential for Optimal Current Production by Geobacter sulfurreducens

Abstract

Previous studies have demonstrated that Geobacter sulfurreducens requires the c-type cytochrome OmcZ, which is present in large (OmcZ L ; 50-kDa) and small (OmcZ S ; 30-kDa) forms, for optimal current production in microbial fuel cells. This protein was further characterized to aid in understanding its role in current production. Subcellular-localization studies suggested that OmcZ S was the predominant extracellular form of OmcZ. N-and C-terminal amino acid sequence analysis of purified OmcZ S and molecular weight measurements indicated that OmcZ S is a cleaved product of OmcZ L retaining all 8 hemes, including 1 heme with the unusual c-type heme-binding motif CX 14 CH. The purified OmcZ S was remarkably thermally stable (thermaldenaturing temperature, 94.2°C). Redox titration analysis revealed that the midpoint reduction potential of OmcZ S is approximately ؊220 mV (versus the standard hydrogen electrode [SHE]) with nonequivalent heme groups that cover a large reduction potential range (؊420 to ؊60 mV). OmcZ S transferred electrons in vitro to a diversity of potential extracellular electron acceptors, such as Fe(III) citrate, U(VI), Cr(VI), Au(III), Mn(IV) oxide, and the humic substance analogue anthraquinone-2,6-disulfonate, but not Fe(III) oxide. The biochemical properties and extracellular localization of OmcZ suggest that it is well suited for promoting electron transfer in current-producing biofilms of G. sulfurreducens.

Long-range electron transport in Geobacter sulfurreducens biofilms is redox gradient-driven

“…Such biofilms are electrically conductive, inferred by the ability of cells not in direct contact with the anode surface to contribute to current generation (22), and confirmed by two-electrode conductivity measurements (20,23). Geobacter cells possess an abundance of multiheme c-type cytochromes on their outer membrane, in the ECM, and along PilApili (24)(25)(26)(27)(28)(29)(30)(31)(32)(33). Cyclic voltammetry (19,(34)(35)(36)(37) and spectroelectrochemical measurements (38)(39)(40)(41)(42)(43) of actively respiring Geobacter biofilms are consistent with long-range electron transport mediated by sequential electron transfer reactions through a network of bound redox cofactors comprised of ECM c-type cytochromes terminating with electron transfer to the anode surface.…”

Proteins involved in electron transfer to Fe(III) and Mn(IV) oxides by Geobacter sulfurreducens and Geobacter uraniireducens

“…Gene deletion studies have suggested that the outer membrane surface c-type cytochrome OmcZ (GSU2076) is important for growth on current-harvesting anodes (Inoue et al, 2010Nevin et al, 2009;Richter et al, 2009). omcZ was upregulated during growth on Mn(IV) oxide but not Fe(III) oxide (Table 4).…”

“…For example, whole-genome microarray comparisons of gene transcript abundance in current-producing biofilms versus biofilms reducing fumarate revealed that the octahaem c-type cytochrome OmcZ was one of the most highly expressed genes in current-producing biofilms . Subsequent gene deletion studies demonstrated that OmcZ was required for effective current production Richter et al, 2009) and further characterization demonstrated that OmcZ is an outer surface protein that is specifically localized at the biofilm-anode interface in current-producing biofilms (Inoue et al, 2010. In a previous study, Fe(III) reduction pathways were investigated by comparing gene expression in G. sulfurreducens growing with Fe(III) chelated with citrate as the electron acceptor and cells growing by fumarate reduction .…”

The microbiology of microbial electrolysis cells

“…Furthermore, direct electron transfer differs when the extracellular electron acceptor is soluble. For instance, OmcS and pili are not required for reduction of Fe(III) citrate by Geobacter whereas OmcZ is 11,28 .…”