beta-methyltryptophan and indole

Tryptophan synthase (TrpS) catalyzes the final steps in the biosynthesis of l-tryptophan from l-serine (Ser) and indole-3-glycerol phosphate (IGP). We report that native TrpS can also catalyze a productive reaction with l-threonine (Thr), leading to (2S,3S)-β-methyltryptophan. Surprisingly, β-substitution occurs in vitro with a 3.4-fold higher catalytic efficiency for Ser over Thr using saturating indole, despite a >82000-fold preference for Ser in direct competition using IGP. Structural data identify a novel product binding site, and kinetic experiments clarify the atypical mechanism of specificity: Thr binds efficiently but decreases the affinity for indole and disrupts the allosteric signaling that regulates the catalytic cycle.

Topics: Allosteric Regulation; Archaeal Proteins; Binding Sites; Biocatalysis; Biosynthetic Pathways; Crystallography, X-Ray; Glycerophosphates; Indoles; Kinetics; Models, Molecular; Molecular Structure; Protein Binding; Protein Domains; Protein Subunits; Pyrococcus furiosus; Serine; Spectrophotometry; Substrate Specificity; Threonine; Tryptophan; Tryptophan Synthase