Effect of Ionic Strength on the Rate of Extracellular Electron Transport in Shewanella oneidensis MR‐1 through Bound‐Flavin Semiquinones

Cell‐secreted flavin binds to outer‐membrane c ‐type cytochromes (OM c ‐Cyts) as a redox cofactor in Shewanella oneidensis MR‐1, generating a semiquinone (Sq) state to enhance the rate of extracellular electron‐transport (EET) process by several orders of magnitude. Here, as ionic strength ( I s ) is a major factor in stabilizing bound Sq in flavoproteins, we examined the influence of I s on the flavin affinity in OM c‐ Cyts to promote Sq formation for enhancing the rate of the EET process. Estimated dissociation constants showed that an increase in I s induces threefold higher Sq formation in OM c‐ Cyts. However, the higher I s neither resulted in the larger current production nor current enhancement by flavin addition. Strong I s dependency for the redox potential of heme centers in OM c ‐Cyts suggests that I s not only controls the stability of Sq, but also alters coupling constants among redox centers in OM c ‐Cyts through structural changes.