Figure 19.

Model of the mechanism of NifB-co formation by NifB. (A) Proposed steps based on biochemical evidence: the K1 cluster presents mixed Cys-His coordination; SAM-dependent methyl group transfer to the K2 cluster; reductive cleavage of a second SAM molecule and formation of a 5′-dA radical at the active site; methyl group H atom abstraction by the 5′-dA radical; oxidation/deprotonation events to remove to H atoms; core rearrangement and insertion of a S atom at the belt of the precursor. (B) Hypothesized role of FdxN in reduction of the K2 cluster to prepare it for methyl group acceptance.