The beta subunit generates CO from CO(2), while the alpha subunit (this protein) combines the CO with CoA and a methyl group to form acetyl-CoA. The methyl group, which is incorporated into acetyl-CoA, is transferred to the alpha subunit by a corrinoid iron-sulfur protein. acetyl-CoA + Co(I)-[corrinoid Fe-S protein] + H(+) = CO + CoA + methyl-Co(III)-[corrinoid Fe-S protein] Binds 2 nickel ions per subunit. Binds 1 [4Fe-4S] cluster per subunit. Tetramer of two alpha and two beta chains.