The role of a P-cluster in the nitrogenase atpase reaction

Structural data on the nitrogenase complex of Azotobacter vinelandii, Av1·(Av2)2, stabilized by MgADP·AlF4− 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 A. The ATP-binding site in the Fe-protein, which decreases the half-reduction potential (Em) 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 “envelop” 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.