Fig. 2: NifB-co binding to NifEN.
From: Trafficking of a nitrogenase FeMo-cofactor assembly intermediate
a, Residues C15E and C25E act as apical ligands to bound NifB-co, structuring the otherwise disordered N terminus of NifE. b, Coulomb map for the N terminus of NifE with bound NifB-co. c, X-band EPR spectra of reduced NifEN and NifE*N show the presence of the [4Fe:4S] cluster in both samples. d, In contrast, only NifEN binds NifB-co, as evidenced by the absence of the EPR signature of the cluster in NifE*N. e, Diazotrophic growth of A. vinelandii and variants. The NifE* variant shows reduced growth, while ΔnifE does not growth diazotrophically. Deletion of NafV, the electron donor for NifB, substantially slows diazotrophic growth and renders the N terminus of NifE essential. f, Deleting NifX does not affect diazotrophic growth, likely because of the compensatory effect of other NifX-like proteins. In contrast, residue C250E is essential. g, Comparison of cofactor binding in NifEN (blue) and the mature MoFe protein, NifDK (yellow). In NifEN, NifB-co is bound by the N terminus of NifE, residing on the surface of the maturase. In NifDK, FeMo-co is inserted deeply into the protein and cradled between the three Rossmann fold domains of NifD. The corresponding cofactor cavity is present in NifEN, including a putative cysteine ligand to Fe1 (C250E) but remains unoccupied in the structure obtained immediately following cluster transfer. h, Wall-eyed stereo representation of the tight interaction of the C terminus of NifX with NifE. Several salt bridges provide notable binding strength.
