Tryptophan Synthase Uses an Atypical Mechanism To Achieve Substrate Specificity
Andrew R Buller1, Paul van Roye1, Javier Murciano-Calles1
1Division of Chemistry and Chemical Engineering 210-41, California Institute of Technology , 1200 East California Boulevard, Pasadena, California 91125, United States.
Abstract:
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.
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