Reveals a [Ni-4Fe-5S] Cluster

The homodimeric nickel-containing CO dehydrogenase from the anaerobic bacterium Carboxydothermus hydrogenoformans catalyzes the oxidation of CO to CO2. A crystal structure of the reduced enzyme has been solved at 1.6 angstrom resolution. This structure represents the prototype for Ni-containing CO dehydrogenases from anaerobic bacteria and archaea. It contains Þve metal clusters of which clusters B, B9, and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters. The active-site clusters C and C9 are novel, asymmetric [Ni-4Fe-5S] clusters. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry. The capacity to oxidize carbon monoxide (CO) is a central metabolic feature of several groups of bacteria and archaea. CO is used as a growth substrate by the aerobic carbox-idotrophic bacteria, species of the anaerobic acetogenic bacteria, sulfate-reducing bacteria and archaea, phototrophic bacteria, methano-genic archaea, and hydrogenogenic bacteria (1–4). Carboxydothermus hydrogenofor-mans, the most extensively studied represen-tative of the hydrogenogenic bacteria, cou-ples the oxidation of CO to CO2 with the reduction of protons to H2 in the energy