Spectroelectrochemical characterization of the metal centers in carbon monoxide dehydrogenase (CODH) and nickel-deficient CODH from Rhodospirillum rubrum

Carbon-monoxide dehydrogenase (CODH) from Rhodospirillum rubrum contains two metal centers: a Ni-X-[Fe4S4](2+/1+) cluster (C-center) that serves as the CO-oxidation site and a standard [Fe4S4](2+/1+) cluster (B-center) that mediates electron flow from the C-center to external electron accepters. Four states of the C-center were previously identified in electron paramagnetic resonance (EPR) and Mossbauer studies. In this report, EPR-redox titrations demonstrate that the fully oxidized, diamagnetic form of the C-center (C-ox) undergoes a one-electron reduction to the C-red1 state (g(av) = 1.87) with a midpoint potential of -110 mV. The reduction of C-ox to C-red1 is shown to coincide with the reduction of an [Fe4S4](2+/1+) cluster in redox-titration experiments monitored by UV-visible spectroscopy. Nickel-deficient CODH, which is devoid of nickel yet contains both [Fe4S4](2+/1+) clusters, does not exhibit EPR-active states or reduced Fe4S4 clusters at potentials more positive than -350 mV.