Solution structure and dynamics of the outer membrane cytochrome OmcF from Geobacter sulfurreducens

Gene knock-out studies on Geobacter sulfurreducens cells showed that the outer membrane-associated monoheme cytochrome OmcF is involved in respiratory pathways leading to the extracellular reduction of Fe(III) and U(VI). In addition, microarray analysis of an OmcF-deficient mutant revealed that many of the genes with decreased transcript level were those whose expression is up-regulated in cells grown with a graphite electrode as electron acceptor, suggesting that OmcF also regulates the electron transfer to electrode surfaces and the concomitant electricity production by G. sulfurreducens in microbial fuel cells. ¹⁵N,¹³C–labeled OmcF was produced and NMR spectroscopy was used to determine the solution structure of the protein in the fully reduced state and the pH-dependent conformational changes. In addition, ¹⁵N relaxation NMR experiments were used to characterize the overall and internal backbone dynamics of OmcF. The structure obtained is well-defined, with an average pairwise root mean square deviation of 0.37 Å for the backbone atoms and 0.98 Å for all heavy atoms. For the first time a solution structure and the protein motions were determined for an outer membrane cytochrome from G. sulfurreducens, which constitutes an important step to understand the extracellular electron transfer mechanism in Geobacter cells.