Comparison of iron-molybdenum cofactor-deficient nitrogenase MoFe proteins by x-ray absorption spectroscopy. Implications for P-cluster biosynthesis

Abstract

Nitrogenase, the enzyme system responsible for biological nitrogen fixation, is believed to utilize two unique metalloclusters in catalysis. There is considerable interest in understanding how these metalloclusters are assembled in vivo. It has been presumed that immature iron-molybdenum cofactor-deficient nitrogenase MoFe proteins contain the P-cluster, although no biosynthetic pathway for the assembly of this complex cluster has been identified as yet. Through the comparison by iron K-edge x-ray absorption edge and extended fine structure analyses of cofactor-deficient MoFe proteins resulting from nifH and nifB deletion strains of Azotobacter vinelandii, a novel [Fe-S] cluster is identified in the ΔnifH MoFe protein. The iron-iron scattering displayed by the ΔnifH MoFe protein is more similar to that of a standard [Fe4S4]-containing protein than that of the ΔnifB MoFe protein, which is shown to contain a "normal" P-cluster. The iron-sulfur scattering of the ΔnifH MoFe protein, however, indicates differences in its cluster from an [Fe4S4] (Cys)4 site that may be consistent with the presence of either oxygenic or nitrogenic ligation. Based on these results, models for the [Fe-S] center in the ΔnifH MoFe protein are constructed, the most likely of which consist of two separate [Fe4S4] sites, each with some non-cysteinyl coordination. This type of model suggests that the P-cluster is formed by the condensation of two [Fe4S4] fragments, possibly concomitant with Fe protein (NifH)-induced conformational change.