A conformational role for NifW in the maturation of molybdenum nitrogenase P-cluster

Reduction of dinitrogen by molybdenum nitrogenase relies on complex metalloclusters: the [8Fe:7S] P-cluster and the [7Fe:9S:Mo:C:homocitrate] FeMo-cofactor. Although both clusters bear topological similarities and require the reductive fusion of [4Fe:4S] sub-clusters to achieve their respective assemblies, P-clusters are assembled directly on the NifD₂K₂ polypeptide prior to the insertion of FeMo-co, which is fully assembled separately from NifD₂K₂. P-cluster maturation involves the iron protein NifH₂ as well as several accessory proteins, whose role has not been elucidated. In the present work, two NifD₂K₂ species bearing immature P-clusters were isolated from an Azotobacter vinelandii strain in which the genes encoding NifH and the accessory protein NifZ were deleted, and characterized by X-ray absorption spectroscopy and EPR. These analyses showed that both NifD₂K₂ complexes harbor clusters that are electronically and structurally similar, with each NifDK unit containing two [4Fe:4S]^2+/+ clusters. Binding of the accessory protein NifW parallels a decrease in the distance between these clusters, as well as a subtle change in their coordination. These results support a conformational role for NifW in P-cluster biosynthesis, bringing the two [4Fe:4S] precursors closer prior to their fusion, which may be crucial in challenging cellular contexts.