Export and role of flavin electron shuttles in Shewanella oneidensis Strain MR-1

Abstract

Our planet is home to microbes with an impressive diversity of metabolic capabilities. For instance, dissimilatory metal reducing bacteria (DMRB) can respire minerals, heavy metals, and electrodes. Electrode respiration is important both for investigating DMRB and biotechnology applications including biosensors, bioenergy, and wastewater treatment. Placing electrodes in wastewater allows for electricity production and oxidation of carbon sources to carbon dioxide. Shewanella oneidensis MR-1, a model DMRB, transfers electrons generated from metabolism across the inner and outer membranes to insoluble external electron acceptors. This feat is accomplished using an electron conduit consisting of multi-heme cytochromes and flavin electron shuttles. The electron conduit in Shewanella evolved to move electrons out of the cell, but in controlled environments, the conduit is functionally reversible, meaning electrons can flow either out of or into the cell. In a process termed electrosynthesis, electrons are transferred from an electrode into a cell to direct metabolism to produce specific metabolites. A useful way to study electrosynthesis in cells is to use three-electrode bioreactors. The working electrode in a bioreactor is poised at an oxidizing or reducing potential and continuously measures electrons entering or leaving the electrode. When electrodes were used to donate electrons to attached Shewanella cells, intracellular ATP was produced. Mutants defective in generating or using proton motive force were unable to produce wild-type levels of ATP. If significant quantities of ATP can be produced by reverse electron flow, it will be a critical step towards fixing carbon dioxide into metabolic precursors and eventually into biofuels.