Comparative structure-potentio-spectroscopy of the Shewanella outer membrane multiheme cytochromes

Many species of bacteria can generate energy in the anoxic subsurface by directly coupling intracellular oxidative reactions to the reduction of extracellular metal oxides. Coupling these processes requires electron transfer networks that extend from the inside of the cell, across the outer membrane to the extracellular terminal electron acceptors. The best described of these networks is from Shewanella oneidensis MR-1, where four structures of outer membrane multiheme cytochromes (OMMCs) have been determined. These OMMCs contain 10–11 bis-histidine-ligated c-type hemes and are directly involved in the reduction of iron and manganese oxides at the cell surface. The heme ligands for some of these structures have been characterised using electron paramagnetic resonance (EPR), the redox properties have been mapped by protein film electrochemistry (PFE) and more recently molecular dynamic simulations have been used to obtain microscopic redox potentials for individual heme groups. This review maps these different experimental techniques onto the structures, providing insight into the intramolecular electron transfer pathways of OMMCs, revealing future directions for study.