MoFe protein. This protein is a heterotetramer composed of two α subunits (red) and two β subunits (blue). Notice that the protein contains two copies each of two types of clusters: P clusters and FeMo cofactors. Each P cluster contains eight iron atoms (green) and seven sulfides linked to the protein by six cysteinate residues. Each FeMo cofactor contains one molybdenum atom, seven iron atoms, nine sulfides, the interstitial carbon atom, and a homocitrate, and is linked to the protein by one cysteinate residue and one histidine residue.
[Drawn from 1M1N.pdb.]