Crystallographic and thermodynamic evidence of negative cooperativity of flavin and tryptophan binding in the flavin-dependent halogenases AbeH and BorH.

The flavin-dependent halogenase AbeH produces 5-chlorotryptophan in the biosynthetic pathway of the chlorinated bisindole alkaloid BE-54017. We report that , AbeH (assisted by the flavin reductase AbeF) can chlorinate and brominate tryptophan as well as other indole derivatives and substrates with phenyl and quinoline groups. We solved the X-ray crystal structures of AbeH alone and complexed with FAD, as well as crystal structures of the tryptophan-6-halogenase BorH alone, in complex with 6-chlorotryptophan, and in complex with FAD and tryptophan.

Structure and Activity of the Thermophilic Tryptophan-6 Halogenase BorH.

Flavin-dependent halogenases carry out regioselective aryl halide synthesis in aqueous solution at ambient temperature and neutral pH using benign halide salts, making them attractive catalysts for green chemistry. BorH and BorF, two proteins encoded by the biosynthetic gene cluster for the chlorinated bisindole alkaloid borregomycin A, are the halogenase and flavin reductase subunits of a tryptophan-6-halogenase. Quantitative conversion of l-tryptophan (Trp) to 6-chlorotryptophan could be achieved using 1.