A radically unexpected mechanism

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Science  18 Mar 2016:
Vol. 351, Issue 6279, pp. 1266-1267
DOI: 10.1126/science.aaf4942

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The S-adenosylmethionine (SAM) radical enzyme superfamily plays a central role in the biosynthesis of many vitamins, cofactors, and antibiotics. Radical SAM enzymes catalyze challenging chemical reactions such as C-H bond activation (1), ring contraction (2), and molecular skeletal rearrangements (3, 4). They overcome the dif culty of these reactions by forming a highly oxidizing radical species, 5′-deoxyadenosyl (5′-dAdo·), from SAM and a reduced iron-sulfur cluster (5). This radical species can selectively abstract a hydrogen atom from a substrate, enabling complex chemical transformations. There are more than 113,000 radical SAM enzymes, but only a small number are both biochemically and structurally characterized, and many of the reaction mechanisms remain enigmatic. On page 1320 of this issue, Sicoli et al. provide evidence for unexpected radical intermediates in the mechanism of the radical SAM enzyme NosL (6).