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Angew Chem Int Ed Engl. 2014 Oct 27;53(44):11840-4. doi: 10.1002/anie.201407320. Epub 2014 Sep 5.

Crystal structure of tryptophan lyase (NosL): evidence for radical formation at the amino group of tryptophan.

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Metalloproteins Unit, Institut de Biologie Structurale, UMR5075, CEA, CNRS, Université Grenoble-Alpes, 71, avenue des Martyrs, CS 10090, 38044 Grenoble cedex 9 (France).


Streptomyces actuosus tryptophan lyase (NosL) is a radical SAM enzyme which catalyzes the synthesis of 3-methyl-2-indolic acid, a precursor in the synthesis of the promising antibiotic nosiheptide. The reaction involves cleavage of the tryptophan Cα-Cβ bond and recombination of the amino-acid-derived -COOH fragment at the indole ring. Reported herein is the 1.8 Å resolution crystal structure of NosL complexed with its substrate. Unexpectedly, only one of the tryptophan amino hydrogen atoms is optimally placed for H abstraction by the SAM-derived 5'-deoxyadenosyl radical. This orientation, in turn, rules out the previously proposed delocalized indole radical as the species which undergoes Cα-Cβ bond cleavage. Instead, stereochemical considerations indicate that the reactive intermediate is a (·)NH tryptophanyl radical. A structure-based amino acid sequence comparison of NosL with the tyrosine lyases ThiH and HydG strongly suggests that an equivalent (·)NH radical operates in the latter enzymes.


biotransformations; density functional calculations; enzymes; radicals; reaction mechanisms

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