Figure 31.4 The 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 MoFe cofactors. Each P cluster contains eight iron atoms (green) and seven sulfides linked to the protein by six cysteinate residues. Each MoFe cofactor contains one molybdenum atom, seven iron atoms, nine sulfides, a central atom, and a homocitrate and is linked to the protein by one cysteinate residue and one histidine residue.