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A radical carboxyl migration
The antibiotic nosiheptide, produced by Streptomyces actuosus, has high activity against multidrug-resistant pathogens. The enzymes involved in its biosynthesis include NosL, which cannibalizes tryptophan and forms a methylindolic acid precursor. Although NosL is included in a growing family of remarkable radical SAM enzymes, the mechanism by which it functions has been dif cult to identify. Using spectroscopic and theoretical approaches, Sicoli et al. show that an unexpected radical intermediate forms during the course of the reaction (see the Perspective by Bridwell-Rabb and Drennan). Unlike similar known enzymes, NosL cleaves a Cα-C bond, which results in the migration of tryptophan's carboxyl group to its side-chain indole.
The radical S-adenosyl-L-methionine tryptophan lyase NosL converts L-tryptophan into 3-methylindolic acid, which is a precursor in the synthesis of the thiopeptide antibiotic nosiheptide. Using electron paramagnetic resonance spectroscopy and multiple L-tryptophan isotopologues, we trapped and characterized radical intermediates that indicate a carboxyl fragment migration mechanism for NosL. This is in contrast to a proposed fragmentation-recombination mechanism that implied Cα–Cβ bond cleavage of L-tryptophan. Although NosL resembles related tyrosine lyases, subtle substrate motions in its active site are responsible for a fine-tuned radical chemistry, which selects the Cα–C bond for disruption. This mechanism highlights evolutionary adaptation to structural constraints in proteins as a route to alternative enzyme function.