A Novel Binuclear [CuSMo] Cluster at the Active Site of Carbon Monoxide Dehydrogenase: Characterization by X-ray Absorption Spectroscopy

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Abstract
The structurally characterized molybdoenzyme carbon monoxide dehydrogenase (CODH) catalyzes the oxidation of CO to CO2 in the aerobic bacterium Oligotropha carboxidovorans. The active site of the enzyme was studied by Mo− and Cu−K-edge X-ray absorption spectroscopy. This revealed a bimetallic [CuISMoVI(O)2] cluster in oxidized CODH which was converted into a [CuISMoIV(O)OH(2)] cluster upon reduction. The Cu···Mo distance is 3.70 Å in the oxidized form and is increased to 4.23 Å upon reduction. The bacteria contain CODH species with the complete and functional bimetallic cluster along with enzyme species deficient in Cu and/or bridging S. The latter are precursors in the posttranslational biosynthesis of the metal cluster. Cu-deficient CODH is the most prominent precursor and contains a [HSMo(O)OH(2)] cluster. Se−K-edge X-ray absorption spectroscopy demonstrates that Se is coordinated by two C atoms at 1.94−1.95 Å distance. This is interpreted as a replacement of the S in methionine residues. In contrast to a previous report [Dobbek, H., Gremer, L., Meyer, O., and Huber, R. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 8884−8889] Se was not identified in the active site of CODH.