Redox Linked Coordination Change In OmcA: Implications for Dissimilatory Fe(III) Reduction

OmcA is a bis-his ligated decaheme c type cytochrome with an unknown crystal structure expressed in the outer-membrane of the metal reducing bacteria Shewenella oneidensis in response to reducing conditions. Although metal reducing bacteria play an important role in bioremediating toxic and heavy metals, their mechanism of electron transfer to solids is not yet completely understood. We hypothesized that the sorption behavior of OmcA is redox dependent due to its purported role in electron transfer to metal-oxides. Fe K-XANES and EXAFS data for the reduced and oxidized OmcA in solution, oxidized OmcA dissolved in 6 M GdnHCl (Guanidinium-HCl) and oxidized OmcA sorbed to SnO2 surfaces were collected and analyzed.

Fe K-XANES and EXAFS spectra for oxidized OmcA showed evidence for a sorption induced change in the coordination environment of heme Fe. Similar to our results for the equine mono-heme cytochrome c, the heme Fe coordination of sorbed OmcA was similar to the denatured OmcA. Denatured oxidized OmcA however, disintegrated, based on its XANES and EXAFS spectral similarities to Fe metal foil. OmcA behavior was markedly different from the behavior of the equine cytochrome c which, on denaturation changed from its native his-met ligation to a bis-his ligation. Based on XANES and EXAFS spectral similarities of sorbed oxidized OmcA with denatured OmcA, our results indicate that oxidized OmcA is not designed to interact with metal-oxide minerals. If OmcA is indeed the electron shuttle to metal-oxide surfaces, we expect reduced OmcA to sorb readily in order to transfer electrons; similarly, the oxidized OmcA should be designed to desorb. Fe K-XANES and EXAFS spectra for dissolved OmcA showed redox linked changes. suggesting that the sorption behavior of reduced and oxidized OmcA would differ consistent with the demands of its role in dissimilatory Fe(III) reduction.