Analysis of a high redox potential heme in tetraheme cytochrome c3 by direct electrochemistry

Abstract

Cytochrome c3 from Desulfovibrio vulgaris (Miyazaki F), a redox protein, contains four bis-histidine-coordinated hemes and has lower redox potential than other heme proteins. Direct electrochemical measurements of cytochrome c3 were carried out using a pyrolytic graphite edge (PGE) electrode. A low redox potential, already measured by redox titration, and a high redox potential (−245mV vs. Ag/AgCl) were observed at room temperature. The high redox potential of cytochrome c3 was similar to that observed for the loss of an axial ligand at heme. To investigate the loss of the histidine ligand, we explored the electrochemistry of four cytochrome c3 mutants, in which the sixth coordinated histidine was replaced by methionine. The electrochemistry of the cytochrome c3 mutants indicated that only Heme III undergoes loss of its axial histidine ligand.