Redox-dependent structural changes in the nitrogenase P-cluster

The structure of the nitrogenase MoFe-protein from Azotobacter vinelandii has been refined to 2.0 Angstrom resolution in two oxidation states. EPR studies on the crystals indicate that the structures correspond to the spectroscopically assigned oxidized (p(OX)/M-OX) and the native or dithionite-reduced (P-N/M-N) forms of the enzyme, Both MoFe-protein structures are essentially identical, with the exception of the P-cluster. The MoFe-protein P-cluster in each state is found to contain eight Fe and seven S atoms. Interconversion between the two redox states involves movement of two Fe atoms and an exchange of protein coordination for ligands supplied by a central S atom. In the oxidized P-OX state, the cluster is coordinated by the protein through six cysteine ligands, Ser-beta 188 O gamma, and the backbone amide of Cys-alpha 88. In the native P-N state, Ser-beta 188 O gamma and the amide N of Cys-alpha 88 no longer coordinate the cluster due to movement of their coordinated Fe atoms toward the central sulfur. Consequently, this central sulfur adopts a distorted octahedral environment with six surrounding Fe atoms. A previously described model of the P-cluster containing 8Fe-8S likely reflects the inappropriate modeling of a single structure to a mixture of these two P-cluster redox states, These observed redox-mediated structural changes of the P-cluster suggest a role for this cluster in coupling electron transfer and proton transfer in nitrogenase.