Nitrogenase, a key player in the global nitrogen cycle, catalyzes the reduction of atmospheric N2 to 2 NH3 at ambient temperature and normal pressure. Its active site, also designated FeMo-cofactor, corresponds to a [MoFe7S9C-(R)-homocitrate] species, whose biosynthesis and insertion requires the action of over a dozen accessory proteins provided by the NIF (for NItrogen Fixation) assembly machinery. Among them, the radical S-adenosyl-L-methionine protein NifB plays an essential role, concomitantly inserting a carbide ion and fusing two [Fe4S4] clusters to form a [Fe8S9C] precursor called NifB-co.
Researchers from the Matalloproteins group determined the long-sought X-ray structure of NifB from Methanotrix thermoacetophila at 1.95 Å resolution in a state pending the binding of one [Fe4S4] cluster substrate. The structure reveals a unique ligand binding mode for the K1 cluster, involving two cysteine residues in addition to a histidine and glutamate. A loop holding two conserved residues is inserted in the active site, likely protecting the already present [Fe4S4] clusters and regulating the sequence of events, controlling SAM dual reactivity and preventing unwanted radical-based chemistry before the K2 [Fe4S4] cluster substrate is loaded into the protein.
Structural insights into the mechanism of the radical SAM carbide synthase NifB, a key nitrogenase cofactor maturating enzyme. Sosa Fajardo A, Legrand P, Payá-Tormo L, Martin L, Pellicer Martinez MT, Echavarri-Erasun C, Vernède X, Rubio LM, Nicolet Y. J Am Chem Soc. 2020 May 30 ;142(25):11006-11012.
doi : 10.1021/jacs.0c02243.
Contact : Yvain Nicolet