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 contai ns two metal centers: a Ni-X-[Fe4S4](2+/1+) cluster (C-center) that se rves as the CO-oxidation site and a standard [Fe4S4](2+/1+) cluster (B -center) that mediates electron flow from the C-center to external ele ctron accepters. Four states of the C-center were previously identifie d in electron paramagnetic resonance (EPR) and Mossbauer studies. In t his report, EPR-redox titrations demonstrate that the fully oxidized, diamagnetic form of the C-center (C-ox) undergoes a one-electron reduc tion 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 e xhibit EPR-active states or reduced Fe4S4 clusters at potentials more positive than -350 mV.