Voltammetry of a flavocytochrome c₃: The lowest potential heme modulates fumarate reduction rates

Iron-induced flavocytochrome C₃, IfC₃, from Shewanella frigidimarina NCIMB400, derivatized with a 2-pyridyl disulfide label, self-assembles on gold electrodes as a functional array whose fumarate reductase activity as viewed by direct electrochemistry is indistinguishable from that of IfC₃ adsorbed on gold or graphite electrodes. The enhanced stability of the labeled protein's array permits analysis at a rotating electrode and limiting catalytic currents fit well to a Michaelis-Menten description of enzyme kinetics with K(M) = 56 ± 20 μM, pH 7.5, comparable to that obtained in solution assays. At fumarate concentrations above 145 μM cyclic voltammetry shows the catalytic response to contain two features. The position and width of the lower potential component centered on -290 mV and corresponding to a one-electron wave implicates the oxidation state of the lowest potential heme of IfC₃ as a defining feature in the mechanism of fumarate reduction at high turnover rates. We propose the operation of dual pathways for electron transfer to the active site of IfC₃ with the lowest potential heme acting as an electron relay on one of these pathways.