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 acceptors. 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.
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