Insights Into Bacterial Cytochrome c Interaction with Metal-Oxide Surfaces Using Optical Waveguide Light Mode Spectroscopy and Quartz Crystal Microbalance: Implications for Dissimilatory Fe(III) Reduction.

Dissimilatory metal or Fe(III) reducing bacteria can use insoluble Fe(III) oxide minerals or other heavy metals [Cr(VI), U(VI), Np(VII)] as terminal electron acceptors, coupling metal reduction with the oxidation of organic compounds to produce energy for growth. Dissmilatory metal reduction is therefore not only important for biogeochemical Fe and Mn cycling but also under study for bioremediation strategies involving reductive immobilization of components of toxic radioactive waste. Here we investigate sorption properties on Al₂O₃ (isostructural with Fe₂O₃) of outer-membrane and periplasmic c-type cytochromes isolated from two model dissimilatory Fe(III) reducing bacterial species namely Shewenella oneidensis and Geobacter sulfurreducens. Cytochromes investigated in this study include outer-membrane OmcA, OmcB and periplasmic Stc from S. oneidensis and outer-membrane OmcB and periplasmic PpcA from G. sulfurreducens. The crystal structure of all the above cytochromes is as yet unknown with the exception of Stc. Optical waveguide lightmode spectroscopy (OWLS) in conjunction with quartz crystal microbalance (QCM) allow for detection of the density of sorbed cytochrome along with its water content thus providing a qualitative understanding of cytochrome c conformational lability while using only nano gram quantities of cytochromes. We also characterized redox dependent sorption behavior of OmcA, Stc from S. oneidensis and OmcB from G. sulfurreducens for the first time. The mechanism of electron transfer to solid surfaces is sorption dependent but not yet completely understood. This study thus provides critical insights into dissimilatory Fe(III) reduction.