Tandem Action of the O<sub>2</sub>- and FADH<sub>2</sub>-Dependent Halogenases KtzQ and KtzR Produce 6,7-Dichlorotryptophan for Kutzneride Assembly
作者:John R. Heemstra、Christopher T. Walsh
DOI:10.1021/ja806467a
日期:2008.10.29
Kutznerides are actinomycete-derived antifungal nonribosomal hexadepsipeptides which are assembled from five unsual nonproteinogenic amino acids and one hydroxy acid. Conserved in all structurally characterized kutznerides is a dichlorinated tricyclic hexahydropyrroloindole postulated to be derived from 6,7-dichlorotryptophan. In this Communication, we identify KtzQ and KtzR as tandem acting FADH(2)-dependent halogenases that work sequentially on free L-tryptophan to generate 6,7-dichloro-L-tryptophan. Kinetic characterization of these two enzymes has shown that KtzQ (along with the flavin reductase KtzS) acts first to chlorinate at the 7-position of L-tryptophan. KtZR, with a similar to 120 fold preference for 7-chloro-L-tryptophan over L-tryptophan, then installs the second chlorine at the 6-position of 7-chloro-L-tryptophan to generate 6,7-dichloro-L-tryptophan. These findings provide further insights into the enzymatic logic of carbon-chloride bond formation during the biosynthesis of halogenated secondary metabolites.