The Nitrogenase FeMo‐Cofactor Precursor Formed by NifB Protein: A Diamagnetic Cluster Containing Eight Iron Atoms

The biological activation of N₂ occurs at the FeMo‐cofactor, a 7Fe–9S–Mo–C–homocitrate cluster. FeMo‐cofactor formation involves assembly of a Fe_6–8–S_X–C core precursor, NifB‐co, which occurs on the NifB protein. Characterization of NifB‐co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent 4Fe–4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB‐co and deliver it to the NifEN protein, upon which FeMo‐cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB‐co‐loaded form provided an opportunity for Mössbauer analysis of NifB‐co. The results indicate that NifB‐co is a diamagnetic (S=0) 8‐Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the ⁵⁷Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB‐co is either a 4Fe²⁺–4Fe³⁺ or 6Fe²⁺–2Fe³⁺ cluster having valence‐delocalized states.