Evidence for a Ligand CO That Is Required for Catalytic Activity of CO Dehydrogenase from Rhodospirillum rubrum

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Abstract
Radiolabeling studies support the existence of a nonsubstrate CO ligand (CO(L)) to the Fe atom of the proposed [FeNi] cluster of carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum. Purified CODH has variable amounts of CO(L) dissociated depending on the extent of handling of the proteins. This dissociated CO(L) can be restored by incubation of CODH with CO, resulting in a 30-40% increase in initial activity relative to as-isolated purified CODH. A similar amount of CO(L) binding is observed when as-isolated purified CODH is incubated with (14)CO: approximately 0.33 mol of CO binds per 1 mol of CODH. Approximately 1 mol of CO was released from CO-preincubated CODH upon denaturation of the protein. No CO could be detected upon denaturation of CODH that had been incubated with cyanide. CO(L) binds to both Ni-containing and Ni-deficient CODH, indicating that CO(L) is liganded to the Fe atom of the proposed [FeNi] center. Furthermore, the Ni in the CO(L)-deficient CODH can be removed by treatment with a Ni-specific chelator, dimethylglyoxime. CO preincubation protects the dimethylglyoxime-labile Ni, indicating that CO(L) is also involved in the stability of Ni in the proposed [FeNi] center.