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 from microbes to insoluble minerals is facilitated by a series of c-type cytochromes. These mostly membrane-associated redox proteins shuttle electrons from the inner membrane across the periplasmatic space and through the outer membrane to a terminal electron acceptor [e.g. Fe(III)]. Although the key proteins involved in this process have been identified, their mode of interaction with mineral surfaces serving as insoluble electron acceptors is still unknown. It has been shown, that c-type cytochromes are located on the extracellular leaflet of outer membrane to form a stable macromolecular complex and establish contact with a mineral surface. However, no information about their structural arrangement in contact with a mineral surface is currently available. Here we report mechanistic insights into electron transfer from cytochromes to mineral surfaces obtained by varying pH in bulk solutions. Our results demonstrate that cytochrome c 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 cytochrome c to adsorb to, and remain catalytically active, transferring electrons to, hematite. We will present data from neutron reflectometry experiments delineating conformational changes of the cytochrome c at the protein-mineral interface as a function of pH. 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.

The study of interaction of bacterial cytochromes and iron oxide minerals

255th National Meeting (2018)

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