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In addition to the well-known two-electron chemistry of the thiolate side chain of cysteine and the methyl transfer capacity of S-adenosylmethionine, both of these amino acids also engage in extensive one-electron chemical enzymatic transformations. Cysteinyl radicals feature prominently in ribonucleotide reductases and also in penicillin and cephalosporin synthases. S-Adenosylmethionine, coordinated to one of the iron atoms in 4Fe–4S clusters, is the source of 5′-deoxyadenosyl radicals as the initiator of radical reaction manifolds in a superfamily of radical SAM enzymes. Three fates of SAM across the radical SAM family reflect distinct mechanisms. In some cases SAM acts catalytically. In other cases SAM is cleaved stoichiometrically to methionine and 5′-deoxyadenosine products. In other enzymatic transformations two SAM molecules participate to give two sets of mechanistically diagnostic products, one as a radical generator, the other as a [CH3]+ donor.

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