Structural Insights into the Mechanism of the Radical SAM Carbide Synthase NifB, a Key Nitrogenase Cofactor Maturating Enzyme.

Nitrogenase is a key player in the global nitrogen cycle, as it catalyzes the reduction of dinitrogen into ammonia. The active site of the nitrogenase MoFe protein corresponds to a [MoFeSC-()-homocitrate] species designated FeMo-cofactor, whose biosynthesis and insertion requires the action of over a dozen maturation proteins provided by the NIF (for trogen ixation) assembly machinery. Among them, the radical SAM protein NifB plays an essential role, concomitantly inserting a carbide ion and coupling two [FeS] clusters to form a [FeSC] precursor called NifB-co.