Distinct electron conductance regimes in bacterial decaheme cytochromes
In the absence of soluble oxidants that can enter the cell, Shewanella oneidensis MR-1 can gain energy by extracellular electron transfer to solid surfaces. The c-type cytochromes of the Mtr transmembrane complex facilitate this process by effectively forming a multi-heme wire for electron transport across the cellular outer membrane. S. oneidensis includes two paralog complexes, MtrABC and MtrDEF, where the outer membrane decaheme cytochromes MtrC and MtrF act as external terminals for electron transfer. The details of the electron conductance mechanism in these proteins, which feature an intriguing arrangement of heme-c cofactors, have not yet been fully elucidated. It is unclear whether the dominant mechanism entails electron or hole transfer, and why two Mtr pathways exist. We investigated electron and hole transfer mechanisms in MtrC and MtrF, and found that the two processes proceed via distinct energetic landscapes. Comparison of the computed redox potentials with previous voltammetry experiments in distinct environments, i.e. isolated and electrode-bound conditions of PFV, or in the membrane complex (in vivo) suggests that these systems could function in different regimes depending on the environment. Our results indicate that MtrC and MtrF are not significantly different in terms of electron conductance, thus pointing out that any differences in expression levels of the two systems in vivo are not due to their efficiencies in electron transport. The calculations also further validate previous experimental findings regarding the potential binding sites for flavins in these cytochromes.