Structure of molybdenum-dependent nitrogenase from Azotobacter vinelandii

Left: The two subunits of the homodimeric Fe protein each containing a bound MgADP and the bridging Fe4–S4 cluster.

Each unit binds one P iron–sulfur cluster and one FeMo-co iron–sulfur cluster.

Right: The relative positions and structures of the Fe4–S4 cluster of the Fe protein, and the P cluster and the FeMo cofactor (FeMo-co) of the MoFe protein are shown.

Hydrolysis of bound ATP is thought both to drive the reduction of P cluster by Fe protein and to trigger a conformational change in Fe protein that causes it to dissociate transiently from MoFe protein, assuring unidirectional electron flow.