OmcS oxidoreductase is an enzyme found in some species of bacteria, including Geobacter sulfurreducens, where it catalyzes the transfer of electrons. It is a multiheme c-Type cytochrome localized outside of the cell along the pili of some exoelectrogenic bacterial species, serving as mediator of extracellular electron transfer from pili to Fe(III) oxides and other extracellular electron acceptors.
OmcS may also facilitate temporary electron storage, acting as capacitor, allowing Geobacter species to move towards distant terminal electron acceptors. OmcS has six low-spin bis-histidinyl hexacoordinated heme groups, and the OmcS gene is the most highly up-regulated of all genes in the Geobacter sulfurreducens KN400 strain when cultivated in a Microbial Fuel Cell, as compared to the PCA strain.