Carbon monoxide dehydrogenase from Clostridium thermoaceticum: quaternary structure, stoichiometry of its SDS-induced dissociation, and characterization of the faster-migrating form.

The molecular mass (M(r)) of the nickel- and iron-sulfur-containing enzyme CO dehydrogenase from Clostridium thermoaceticum was determined by sedimentation equilibrium ultracentrifugation to be 300,000 +/- 30,000 Da. Since the enzyme is known to contain equal numbers of two types of subunits (M(r) = 82,000 Da for alpha and 73,000 Da for beta), this indicates an alpha 2 beta 2 quaternary structure. The enzyme was previously thought to have an alpha 3 beta 3 structure because it migrates through calibrated size-exclusion chromatographic columns with an apparent M(r) of about 420,000 Da. The disproportionately fast migration rate suggests that the enzyme is nonspherical. SDS induces the dissociation of an alpha subunit, yielding a stable species called FM-CODH. FM-CODH had a molecular mass of 210,000 +/- 30,000 Da, indicating an alpha 1 beta 2 structure. It contained 2.1 +/- 0.3 Ni and 16 +/- 3 Fe per alpha 1 beta 2, exhibited S-->Fe charge-transfer transitions typical of Fe-S proteins, and afforded the gav = 1.82, 1.86, and 1.94 EPR signals. Quantitation of the 1.82 and (1.94 +/- 1.86) signals afforded 0.35 and 1.9 spin/alpha 1 beta 2, respectively. FM-CODH samples exhibited CO oxidation activity, but little CO/acetyl-CoA exchange activity. Some FM-CODH samples exhibited CO oxidation activities as high as native enzyme. These results, along with the quantified spin intensities of the EPR signals, indicate that FM-CODH contains the B- and C-clusters and suggest that these clusters are located in the beta subunit. The alpha subunit that dissociated during formation of FM-CODH is not required for CO oxidation activity. FM-CODH is either devoid of A-clusters, or if such clusters are present, they have lost their ability to exhibit substantial NiFeC signals and CO/acetyl-CoA exchange activity. Incubating FM-CODH and alpha yielded a species that migrated through polyacrylamide gels at the same rate as native enzyme, and had a molecular mass indicating an alpha 2 beta 2 structure. Thus, the SDS-induced dissociation of the enzyme appears to be reversible.