Structural data on the nitrogenase complex of Azotobacter vinelandii, Av1•(Av2) 2 , stabi lized by MgADP•AlF 4 -and, in particular, the structure and properties of a P cluster involved in the nitrogenase ATPase reaction, were analyzed. The ATP binding site and all nitrogenase metal clusters are arranged in one plane, the distances between the closest partners being 14-15 Å. The ATP binding site in the Fe protein, which decreases the half reduction poten tial (E m ) of the [4Fe-4S] cluster in Av2 to -0.43 V, does not affect the potentials of the P cluster and Fe-Mo cofactor (FeMoco). Amino acids 74-95 in the β subunit of Av1 "en velop" the P cluster in Av1; therefore, the phosphate intermediate of the ATPase reaction of nitrogenase occurs apparently in the direct contact with the P cluster. By increasing the acceptor properties of the P cluster, this intermediate may favor the electron transfer from the Fe protein to the P cluster, thus bringing it into the super reduced state.