PNNL

Abstract

As key components of the electron transfer (ET) pathways used for dissimilatory reduction of solid iron [Fe(III)] and manganese [Mn(IV)] (hydr)oxides, outer membrane cytochromes MtrC and OmcA of Shewanella oneidensis MR-1 and OmcE and OmcS of Geobacter sulfurreducens mediate ET reactions extracellularly. Cell surface-exposed MtrC and OmcA can transfer electrons directly to the metal oxides. S. oneidensis MR-1 cells also secrete flavins that can facilitate ET to the oxides. The secreted flavins are thought to serve either as chelators that form soluble Fe(III)/Mn(IV)-flavin complexes or as electron shuttles that ferry the electrons from cell-associated ET proteins to the metal oxides. Cell-surface localization may also permit MtrC and OmcA to transfer electrons extracellularly to either flavin-chelated Fe(III)/Mn(IV) or oxidized flavins. OmcE and OmcS are proposed to be located on the Geobacter cell surface where they are believed to function as the intermediates to relay electrons to type IV pili, which are then hypothesized to transfer electrons directly to the metal oxides. Thus, cell surface-localization positions these outer membrane cytochromes to transfer electrons to Fe(III)/Mn(IV) oxides external to the bacterial cells either directly, indirectly, or both, demonstrating a common strategy shared by Shewanella and Geobacter for extracellular reduction of the oxides.