Voltammetry of a flavocytochrome c(3): The lowest potential heme modulatesfumarate reduction rates

Iron-induced flavocytochrome c(3), Ifc(3), from Shewanella frigidimarina NC IMB400, derivatized with a 2-pyridyl disulfide label, self-assembles on gol d electrodes as a functional array whose fumarate reductase activity as vie wed by direct electrochemistry is indistinguishable from that of lie, adsor bed on gold or graphite electrodes. The enhanced stability of the labeled p rotein's array permits analysis at a rotating electrode and limiting cataly tic currents fit well to a Michaelis-Menten description of enzyme kinetics with K-M = 56 +/- 20 mu M, pH 7.5, comparable to that obtained in solution assays. At fumarate concentrations above 145 mu M cyclic voltammetry shows the catalytic response to contain two features. The position and width of t he lower potential component centered on -290 mV and corresponding to a one -electron wave implicates the oxidation state of the lowest potential heme of Ifc(3) as a defining feature in the mechanism of fumarate reduction at h igh turnover rates. We propose the operation of dual pathways for electron transfer to the active site of Ifc(3) with the lowest potential heme acting as an electron relay on one of these pathways.