The beta subunit (this protein) generates CO from CO(2), while the alpha subunit 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. CO + H2O + 2 oxidized [2Fe-2S]-[ferredoxin] = CO2 + 2 H(+) + 2 reduced [2Fe-2S]-[ferredoxin] Binds 1 [Ni-Fe-S] cluster per subunit. Binds 2 [4Fe-4S] clusters per homodimer. Tetramer of two alpha and two beta chains.