Figure 6:

The catalytic FeMo cofactor of Mo nitrogenase. This complex iron-sulfur cluster prominently contains molybdenum at an apical position as a heterometal, bidentally coordinated by a homocitrate molecule. It obtains its highly symmetric structure through the insertion of a central carbide (formally C⁴⁻) that originates from S-adenosyl methionine. The [Mo:7Fe:9S:C]:homocitrate cluster is coordinated only by two residues of the NifD subunit, Cys 275 and His 442, and all eight metal ions are coordinatively saturated. Figure made from PDB entry 3U7Q.