Abstract
The variety of solid surfaces to and from which microbes can deliver electrons by extracellular electron transport (EET) processes via outer-membrane c-type cytochromes (OM c-Cyts) expands the importance of microbial respiration in natural environments and industrial applications. Here, we demonstrate that the bifurcated EET pathway of OM c-Cyts sustains the diversity of the EET surface in Shewanella oneidensis MR-1 via specific binding with cell-secreted flavin mononucleotide (FMN) and riboflavin (RF). Microbial current production and whole-cell differential pulse voltammetry revealed that RF and FMN enhance EET as bound cofactors in a similar manner. Conversely, FMN and RF were clearly differentiated in the EET enhancement by gene-deletion of OM c-Cyts and the dependency of the electrode potential and pH. These results indicate that RF and FMN have specific binding sites in OM c-Cyts and highlight the potential roles of these flavin-cytochrome complexes in controlling the rate of electron transfer to surfaces with diverse potential and pH.
Highlights
Cell-secreted Flavins Bound to Membrane Cytochromes Dictate Electron Transfer Reactions to Surfaces with Diverse Charge and pH
To investigate the redox state of the RF responsible for activating electron transport (EET) processes in S. oneidensis MR-1 cells, we conducted differential pulse voltammetry (DPV) measurements followed by the electrochemical cultivation of MR-1 grown on an ITO electrode in the presence of either RF or flavin mononucleotide (FMN)
To confirm that RF associates with the OmcA protein and thereby enhances current production in S. oneidensis MR-1, we examined whether RF could enhance the EET activity of the OmcA protein remaining in a DmtrC mutant strain
Summary
Cell-secreted Flavins Bound to Membrane Cytochromes Dictate Electron Transfer Reactions to Surfaces with Diverse Charge and pH. Together with electron paramagnetic resonance (EPR) detection of the free radical species in Sq, FMN was confirmed to mediate the redox reaction of Ox/Sq at the interface between the cells and electrodes (Fig. 1c) This alteration in the redox reaction provided much more thermodynamically favorable electron transfer kinetics than did the two-electron process of free-form FMN in the shuttling mechanism, demonstrating that the one-electron reaction of FMN provides a major pathway for flavin-mediated EET processes. This alteration in the redox signal was not observed in the absence of electron source or in a mutant lacking the MtrC protein, which has been reported to have a possible flavinbinding site[7,26], indicating that FMN enhances the EET process as a cofactor in MtrC protein with reduced hemes (Fig. 1e)
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