Radical SAM enzymes catalyze a remarkable diversity of biological radical reactions using a common radical initiation mechanism involving a [4Fe-4S] cluster and SAM. The initial reductive cleavage of SAM leads to the formation of a catalytically central organometallic species (Ω) in which a SAM-derived 5-deoxyadenosyl radical (5-dAdo•) is covalently bound through the 5-C to an iron of the [4Fe-4S] cluster. Homolytic Fe-C5 bond cleavage then forms the 5-dAdo• radical intermediate, which abstracts a hydrogen atom from substrate. Efforts directed towards generating, trapping, and characterizing key radical intermediates during this radical initiation process will be presented and discussed. Further, recently-discovered photochemistry in radical SAM enzymes, which can promote radical initiation in the absence of substrate, will be presented. This photochemistry has led to fundamental new understanding of the regioselectivity of reductive S-C bond cleavage that is central to radical SAM mechanisms.
