Electron transport mechanism of periplasmic methyl orange reduction mediated by CctA in Shewanella oneidensis

Abstract:

Extracellular electron transfer (EET) of the electrochemically active microbe, Shewanella oneidensis, has promising prospects of application in pollutant degradation, environmental remediation, bioelectrochemical sensing, and energy utilization. The small tetrahaem cytochrome (CctA) is one of the most abundant proteins in the periplasmic space of S. oneidensis. Although CctA is involved in multiple redox processes, the knowledge on the behavior and mechanism of CctA in EET is limited. [Objective] To investigate the role of CctA in the EET with azo dyes as electron acceptors in the periplasmic space of the model strain S. oneidensis MR-1, and further enrich the knowledge about the mechanism of anaerobic respiration of S. oneidensis. [Methods] We took methyl orange (MO) as the electron acceptor to explore the characteristics of periplasmic MO reduction in ∆mtr (mtr-deleted strain) and investigate the role of CctA in EET by gene knockout and complementation. [Results] In the absence of Mtr complexes, CctA mediates the electron transfer for the periplasmic MO reduction. The reduction rate of MO had a positive correlation with the expression level of recombinant CctA at low concentrations, while higher levels of recombinant CctA did not further improve the reduction rate. The results of protein film voltammetry (PFV) distinguished CctA significantly from high-potential redox proteins in the periplasmic space, suggesting its involvement in a low-potential MO reduction pathway. [Conclusion] We uncovered the unique electron transfer behavior of CctA in periplasmic MO reduction from molecular dynamics, bringing valuable information for understanding EET, designing or modifying extracellular redox systems by means of synthetic biology, and facilitating the application of bioelectrochemistry in pollutant degradation.