Abstract
The outer membrane protein OmcA is an 85 kDa decaheme c-type cytochrome located on the surface of the dissimilatory metal reducing bacterium Shewanella oneidensis MR-1. It is assumed to mediate electron shuttling, generated by the bacteria’s metabolism, to extracellular acceptors that include solid metal oxides such as hematite (α-Fe2O3). To investigate the mechanism by which OmcA interacts with hematite, we purified OmcA and characterized its solution structure by small angle X-ray scattering (SAXS) and its interaction with hematite by neutron reflectometry (NR). SAXS results showed that OmcA is a monomer that adopts a flat ellipsoidal shape with a dimension of 3.4×9.0×6.5 nm3. Changes in redox state affect OmcA conformation. In addition, OmcA interacts with small organic ligands known to act as electron shuttle molecules, such as flavin mononucleotide (FMN), resulting in the formation of high molecular weight assemblies. A model system, developed using NR to study the interaction of OmcA with hematite, shows that OmcA forms a well-defined monomolecular layer on hematite surfaces. This allows OmcA to preferentially interact with hematite in a conformation that maximizes its contact area with the mineral surface. Overall, these results provide experimental and quantitative evidence for OmcA reduction of solid metal oxides involving both direct and indirect mechanisms.
