Mind the gap: long range charge transfer across the periplasm of Shewanella oneidensis

Abstract

Shewanella oneidensis a gram negative proteobacteria, is capable of the dissimilatory metal reduction (DMR) of insoluble iron and manganese oxides, through the deployment of multiple strategies, including using multiple multi‐heme c‐type cytochromes that electrochemically wire the cellular interior to the exterior. In order to harness the remarkably biological chemistry of Shewanella in bioenergy and microbial fuel cells, we examined the molecular details of long‐range electron transfer within and between Shewanella multi‐heme cytochromes that are implicated in the DMR pathway. To probe the model of Mtr cytochrome interactions, we have developed new analytical methods to quantify the directionality and stoichiometry of redox reactions in the Shewanella Mtr cytochromes. We have modified traditional PFE methods to probe the protein:protein interactions that may be at work in the Shewanella DMR pathway, revealing an unforeseen unidirectionality for electron transfer. These data, coupled to structural studies of MtrA using small angle x‐ray scattering (SAXS) and analytical ultracentrifugation (AUC), elucidate a model of long‐range electron transfer that occurs from the quinol pool of Shewanella to the cell surface, as mediated by a series of multi‐heme cytochromes c. For each multi‐heme cytochrome, the high number of redox cofactors and their similar reduction potentials, convolutes the routine assignment of specific macroscopic reduction potentials to individual hemes. Here, novel Mtr cytochrome mutants where predicted heme ligands have been mutated from His to Met/Ala mutagenically “map” the nature of each heme center. The electrochemical, spectroscopic and physiological impact of such mutations will be presented.