Characterization of the metal centers of the corrinoid/iron-sulfur component of the CO dehydrogenase enzyme complex from Methanosarcina thermophila by EPR spectroscopy and spectroelectrochemistry

The multienzyme carbon monoxide dehydrogenase complex from Methanosarcina thermophila contains at least two protein components: a CO-oxidizing nickel/ iron-sulfur (Ni/Fe-S) component and a cobalt-containing corrinoid/iron-sulfur component (Co/Fe-S). The CO dehydrogenase complex has been shown to synthesize acetyl-CoA from CoA, CH₃I, and CO as well as to cleave acetyl-CoA into its methyl, carbonyl, and CoA components as the first step in the catabolism of acetyl-CoA to methane and CO₂. Presumed to serve as an acceptor of the methyl group of acetyl-CoA en route to methane, the Co/Fe-S component contains iron, acid-labile sulfur, and a corrinoid cofactor (factor III) that is the site of methylation. Using EPR spectroscopy and spectroelectrochemistry, we characterized the cobalt and Fe-S centers of the Co/Fe-S component. The redox and EPR properties of the metal centers in the isolated Co/Fe-S component are similar to those of the Co/Fe-S component in the CO dehydrogenase enzyme complex, a result that indicates that any protein-protein interaction between components in the complex has little influence on the properties of the metal centers. The corrinoid is maintained in the base-off state with a formal equilibrium reduction potential (E′₀) at pH 7.8 of -486 mV for the Co^2+/1+ couple that facilitates reduction of the Co²⁺ state by ∼12 kcal/mol relative to base-on cobamides. The Co/Fe-S component also contains a [4Fe-4S]^2+/1+ cluster with an E′₀ at pH 7.8 of -502 mV, which is nearly isopotential with the Co^2+/1+ couple of the cobamide.