Radical SAM tryptophan lyase (NosL): how the protein controls the carboxyl radical •CO2- migration.

The radical S-adenosyl-L-methionine tryptophan lyase uses radical-based chemistry to convert L-tryptophan into 3-methyl-2-indolic acid, a fragment in the biosynthesis of the thiopeptide antibiotic nosiheptide. This complex reaction involves several successive steps corre-sponding to i) the activation by a specific hydrogen-atom abstraction, ii) an unprecedented •CO2- radical migration, iii) a cyanide frag-ment release and iv) the termination of the radical-based reaction. In vitro study of this reaction is made more difficult because the en-zyme produces significant amount of a shunt product instead of the natural product. Here, using a combination of X-ray crystallography, electron paramagnetic resonance spectroscopy and quantum and hybrid quantum mechanical / quantum mechanical calculations, we have deciphered the fine mechanism of the key •CO2- radical migration, highlighting how the pre-organized active site of the protein tightly controls this reaction.