Beyond native reactivity: radical SAM enzymes as platforms for new-to-nature chemistry

Radical S-adenosylmethionine (rSAM) enzymes constitute one of the largest and most versatile enzyme superfamilies in biology, catalyzing diverse radical-mediated reactions essential for metabolism, cofactor biosynthesis, nucleic acid modification, and natural product formation. Central to their chemistry is the reductive cleavage of S-adenosylmethionine (SAM) by an iron-sulfur cluster, generating the highly reactive 5'-deoxyadenosyl radical (5'-dAdo˙), which initiates a broad range of challenging transformations. Recent advances have expanded the reactivity of rSAM enzymes beyond their natural roles, revealing unprecedented mechanistic flexibility and synthetic potential. Photochemical activation strategies now enable light-driven reduction of [4Fe-4S] clusters in the presence of biological or chemical photosensitizers, thereby initiating radical formation. In parallel, cobalamin-dependent radical SAM methyltransferases have opened new avenues for ethyl and fluoromethyl transfer chemistry using corresponding SAM analogues. Notably, the use of a strategically engineered, stable analogue, such as 7-deazaadenine-tetrazole-substituted F-SAM (F-7dz-tSAM), has successfully overcome inherent cofactor degradation. Noncanonical radical reactivity, exemplified by ArsL-catalyzed C-As bond formation and NosL-mediated photoinduced trifluoromethylation, demonstrates the capacity of rSAM enzymes to perform new-to-nature transformations. Lastly, the repurposing of rSAM enzymes for formylglycine generation has enabled orthogonal aldehyde-tag formation for bioorthogonal protein labeling, further expanding their utility in chemical biology. Collectively, these advances establish rSAM enzymes as versatile platforms for radical biocatalysis, chemical biology, and new-to-nature chemistry.