The Catalytic Redox Machine a NioFeoS Cluster

The CO dehydrogenase reaction has been extensively studied in Rho-dospirillum rubrum and C. thermoaceticum. For studying this reaction, R. rubrum offers the advantage that it lacks Cluster A and the entire acetyl-CoA synthase subunit, which also contains Ni. In addition, a Ni-deficient protein eontaining all Fe components of the holoenzyme ean be isolated. Furthermore, the erystal structure of this protein will be known in the next few months. 

The C-cluster of carbon monoxide dehydrogenase is the active site for the oxidation of CO to CO 2. This conclusion is based on rapid kinetic studies in which changes in the spectra of Cluster C undergo changes at rates commensurate with the rate of CO oxidation (Kumar et al., 1993). In addition, cyanide, which is a relatively specific inhibitor of CO oxidation, binds specifically to Cluster C (Anderson et al., 1993). 

FIGURE 2. Proposed catalytic mechanism of CO oxidation. Results indicate that, at high CO concentrations, two mol of CO hind to generate a four-electron reduced enzyme during each catalytic cycle. At low CO concentrations, the upper cycle is the predominant reaction, since electron transfer to Cluster B would then occur faster than binding of CO to the cred2 form of the enzyme. Modified from (Seravalli et al., 1997). 

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