Changing Fates of the Substrate Radicals Generated in the Active Sites of the B₁₂-Dependent Radical SAM Enzymes OxsB and AlsB

OxsB is a B12-dependent radical SAM enzyme that catalyzes the oxidative ring contraction of 2′-deoxyadenosine 5′-phosphate to the dehydrogenated, oxetane containing precursor of oxetanocin A phosphate. AlsB is a homologue of OxsB that participates in a similar reaction during the biosynthesis of albucidin. Herein, OxsB and AlsB are shown to also catalyze radical mediated, stereoselective C2′-methylation of 2′-deoxyadenosine monophosphate. This reaction proceeds with inversion of configuration such that the resulting product also possesses a C2′ hydrogen atom available for abstraction. However, in contrast to methylation, subsequent rounds of catalysis result in C–C dehydrogenation of the newly added methyl group to yield a 2′-methylidene followed by radical addition of a 5′-deoxyadenosyl moiety to produce a heterodimer. These observations expand the scope of reactions catalyzed by B12-dependent radical SAM enzymes and emphasize the susceptibility of radical intermediates to bifurcation along different reaction pathways even within the highly organized active site of an enzyme.