Direct electrochemistry of mitochondrial cytochrome c and three membrane-bound cytochromes from dissimilatory iron reducing bacteria on hematite and ITO electrodes

Dissimilatory iron-reducing bacteria (DIRB) are a phylogenetically and physiologically diverse clade that can reduce metals such as Fe(III) and Mn(IV) for respiration. They can also reductively immobilize contaminants such as Cr(VI) and U(VI). This electron transfer process is thought to occur in several ways, one of which involves terminal electron transfer from a c-type cytochrome to a solid mineral surface. Determining the electrochemical behavior of isolated membrane-bound proteins has been limited, and no direct electrochemistry on natural mineral surfaces has been conducted previously. In order to facilitate electrochemical characterization the behavior of the well-studied mitochondrial cytochrome-c (Mcc) was determined on natural hematite (and indium tin oxide) electrodes to establish a baseline for comparison with DIRB cytochromes. Mcc is a monoheme cytochrome with histidine-methionine axial heme Fe coordination that has a midpoint potential at 50 to 70 mV vs. Ag/AgCl. Mcc also expresses another conformational state with histidine-histidine axial coordinated heme and with a characterisitic midpoint potential ranging from -350 to -400 mV vs. Ag/AgCl. The electrochemical behavior of three membrane bound cytochromes from three species of DIRB was then tested and compared to Mcc. All three exhibit histidine-histidine axial coordination with a characteristic -350 to -400 mV midpoint potential (vs. Ag/AgCl). One species (Acidiphilium cryptum) exhibited a +50 mV midpoint characteristic of an histidine-methionine axial coordinated heme as well as another conformation at -400 mV. Comparisons of Mcc with three membrane-bound cytochromes showed strong similarities in behavior, including conformational changes in one species.