Molecular-scale interactions between microbes and earth mineral materials are central to understanding electron transfer mechanisms and microbial ecosystem responses in subsurface environments. Electron transfer by dissimilatory metal-reducing bacteria is facilitated by a series of c-type cytochromes associated with the bacterial cell envelope. The outer membrane protein OmcA is located on the cell surface of the metal reducing bacterium Shewanella oneidensis MR-1. This 85 kDa decaheme c-type cytochrome functions as a terminal reductase that relays electrons generated by the bacteria’s metabolism to extracellular acceptors that include solid metal oxides such as hematite. Although pH is known to be among the factors that can influence electron transfer, details of its effects remain poorly understood, e.g., with regard to electron transfer rates as a function of charge density. Here we report mechanistic insights into electron transfer from cytochromes to mineral surfaces obtained by varying pH in bulk solutions. Our results demonstrate that OmcA assumes a preferred orientation on the iron oxide surface (hematite), presumably to facilitate electron transfer. We show that the pH can affect both the redox behavior of the catalytic centers of the protein, and the interactions of the protein with the metal oxide substrate. Our results highlight the pH dependency of catalytic current, due to the ability of OmcA to adsorb to, and remain catalytically active, transferring electrons to, hematite. We will present data from neutron reflectometry experiments delineating conformational changes of the OmcA at the protein-mineral interface as a function of chemical gradient. The present results provide a structural basis for redox processes at protein-mineral interfaces and will help pave the way to tackling more complex processes at microbe-mineral interfaces in their natural environments.

Interaction of bacterial cytochromes and iron oxide minerals

253rd National Meeting (2017)

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