Respiratory metabolic processes, such as those used by animals, plants, and aerobic microbes, typically use soluble electron acceptors that easily permeate to the cytoplasmic membrane, like molecular oxygen. However, some microbes evolved conductive pathways extending outside the cell to enable respiration using insoluble minerals as electron acceptors, allowing them to thrive in anaerobic (oxygen free) conditions. This process, known as extracellular electron transfer (EET), holds global importance as it plays a part in biogeochemical cycles, including the reduction of Fe(III) and Mn(IV), bioremediation, methane production and oxidation. These activities impact climate change, corrosion, and electricity generation in microbial electrochemical devices. A model anaerobic bacterium, Geobacter sulfurreducens, produces appendages that mediate EET as part of its respiratory metabolism in oxygen-free environments. In the past five years, with the aid of cryo-EM, we have discovered that G. sulfurreducens produces at least three polymerized multiheme cytochrome protein nanowires: OmcS, OmcE, and OmcZ. Each of these filaments is composed of cytochrome monomers with bis-histidine-coordinated hemes forming a chain along the filament axis.Through this research, we aimed to understand the structure-property relationships in these conductive filaments, particularly those that directly support long-range electron transport, and to compare these features among the different filaments. Notably, despite OmcS and OmcE cytochrome protein nanowires having nearly identical heme alignment, there is no sequence homology between them. Additionally, examining the structures of OmcS and OmcE reveals that the hemes are coordinated across the subunit-subunit interface by one of the axial ligand histidines from neighboring subunits. On the other hand, the third cytochrome protein nanowire, OmcZ, lacks such an interfacial heme, and one heme per subunit is offset from the axial chain and is largely exposed to the solvent. A meta-analysis of all heme pairs within multiheme c-type cytochrome structures in the Protein Data Bank shows that the heme pairs in the cytochrome nanowires is common to heme pairs in most multiheme c-type cytochromes involved in electron transfer processes.
Spectroscopic comparisons between OmcS, OmcE, and OmcZ filaments show a range of heme coordination environments, consistent with published results of other multiheme c-type cytochromes, and with the electrochemical behavior of these wires. Kinetic measurements of OmcS and OmcE show that for many electron acceptors studied, these two wires show very different rates of oxidation, suggesting distinct physiological roles despite their similar heme packing arrangement. The new findings suggest that cytochrome filaments evolved to pack heme as densely as biosynthetically possible for rapid electron transfer along the wire axis, while the cytochrome protein scaffold plays a role in selecting for different electron acceptors in the surrounding environment for anaerobic respiration. Results of this work advance our understanding of the biological structures that support long-range electronic conductivity, highlighting their structural, chemical, and physical features that make them effective in long-range electron transfer.
