Anaerobic CO dehydrogenases catalyze the reversible oxidation of CO to CO2 at a complex Ni-, Fe-, and S-containing metal center called cluster C. We report crystal structures of CO dehydrogenase II from Carboxydothermus hydrogenoformans in three different states. In a reduced state, exogenous CO2 supplied in solution is bound and reductively activated by cluster C. In the intermediate structure, CO2 acts as a bridging ligand between Ni and the asymmetrically coordinated Fe, where it completes the square-planar coordination of the Ni ion. It replaces a water/hydroxo ligand bound to the Fe ion in the other two states. The structures define the mechanism of CO oxidation and CO2 reduction at the Ni-Fe site of cluster C.
The biological redox transformations ofCOz, N2, and Hz are essential processes in global biogeochemical cycles and are catalyzed by enzymes containing complex metal clusters based on iron and sulfi.lr whose detailed function is still poorly understood (l, 2). Carbon monoxide dehydrogenases (CODHases) are the biological catalysts for the reversible oxidation of CO to CO2• with water as the source of oxygen: CO + HzO ---> COz + 2e- + 2ft (Eg. I). Two principal types of
CODHases have been described that differ in their cofactor composition, structure, and stability in the presence of dioxygen: Anaerobic bacteria and archaea use oxygen-sensitive Ni- and Fe-containing CODHases, whereas aerobic, carboxydotrophic bacteria use a Cu-, Mo-. and Fe-containing flavoenzyme (3). The Ni,Fe-CODHases are monoofi.mctional or bifi.lIlctional enzymes associated with Ni,Fe-containing acetyl-coenzyme A (CoA) synnthases (ACS) (4, 5) [for review, see (3, 6)]