Acceleration of Extracellular Electron Transfer by Alternative Redox-Active Molecules to Riboflavin for Outer-Membrane Cytochrome c of Shewanella oneidensis MR-1

摘要

Extracellular electron transfer (EET) between outer membrane c-type cytochromes (OM c-Cyts) of Shewanella oneidensis MR-1 and anodic and cathodic electrodes is markedly enhanced by the presence of riboflavin that operates as a redox active center in OM c-Cyt protein OmcA. Here, to obtain insight into the EET mechanism via bound riboflavin in OmcA, we replaced riboflavin with two flavin-like polycyclic molecules, safranin (Saf) and anthraquinone-l-sulfonate (alpha-AQS), and examined the influence of the interaction with OmcA and the potential of these redox molecules for the rate of cathodic EET by MR-1 in vivo. The results of electrochemical assays with wild-type and mutant strains of MR-1 lacking OmcA showed that both Saf and alpha-AQS increased the cathodic current production for fumarate reduction at -0.45 V versus SHE as a bound cofactor in OmcA. Dissociation constant and enhancement factor analyses of Saf and alpha-AQS suggested that the N(5) atom in the isoalloxazine ring of riboflavin is important for the affinity with OmcA and that a positive redox potential is critical for a high rate of EET. The present results suggest that the molecular design of the redox-active center in OM c-Cyts may allow control of the rate of EET in EET-capable bacteria.