The MoFe protein is anα2β2 heterotetramer (Mr≈ 250,000) that contains two pairs of novel metalloclusters, called P-cluster ([8Fe-7S]) and FeMo-cofactor ([7Fe-Mo-9S-homocitrate-X]). One of each type of metal cluster is contained in an αβ-unit (11, 12), and thus each MoFe protein consists of two catalytic units (Figure 1). FeMo-cofactor is the site of substrate binding and reduction (Figure 3). This cofactor consists of a transition metal-sulfur framework and one molecule of (R)-homocitrate (41). A [4Fe-3S] subcluster is connected to a [3Fe-Mo-3S] subcluster by the atom X at one corner and three bridging inorganic sulfides (6). (R)-Homocitrate is coordinated to the Mo atom through its 2-hydroxy and 2-carboxyl groups. The presence of the light atom (X) at the center of FeMo-cofactor was detected in a high-resolution (1.16 Å) structure of the MoFe protein (6). The electron density for X is most consistent with N, C, or O, but efforts to identify this atom have yet to be successful (42–44). FeMo-cofactor is anchored to the MoFe protein byα-275Cys to an Fe atom at one end and α-442His to the Mo atom at the other end.