Elucidation of the Mechanism of Fom3, a Cobalamin Dependent Radical SAM Enzyme

Recent studies have revealed that Fom3, a cobalamin-dependent radical S-adenosylmethionine enzyme, catalyzes the methylation of carbon 2 of cytidylyl-2-hydroxyethylphosphonate during the biosynthesis of fosfomycin, a broad spectrum antibacterial agent; however, the mechanism by which Fom3 catalyzes this reaction remains elusive. Herein, we demonstrate the first successful isolation of Fom3 with both its iron-sulfur and cobalamin cofactors bound. Furthermore, our studies provide key mechanistic insight for a number of steps in the Fom3 reaction.