Role of the [4Fe-4S] Cluster in Reductive Activation of the Cobalt Center of the Corrinoid Iron−Sulfur Protein from Clostridium thermoaceticum during Acetate Biosynthesis†

The corrinoid iron−sulfur protein (CFeSP) from Clostridium thermoaceticum functions as a methyl carrier in the Wood−Ljungdahl pathway of acetyl-CoA synthesis. The small subunit (33 kDa) contains cobalt in a corrinoid cofactor, and the large subunit (55 kDa) contains a [4Fe-4S] cluster. The cobalt center is methylated by methyltetrahydrofolate (CH3-H4folate) to form a methylcobalt intermediate and, subsequently, is demethylated by carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS). The work described here demonstrates that the [4Fe-4S] cluster is required to facilitate the reactivation of oxidatively inactivated Cob(II)amide to the active Co(I) state. Site-directed mutagenesis of the large subunit gene was used to change residue 20 from cysteine to alanine, which resulted in formation of a cluster with EPR and redox properties consistent with those of [3Fe-4S] clusters. The midpoint potential of the cluster in the C20A variant was ∼500 mV more positive than that of the [4Fe-4S] cluster in the nat...