Tryptophan synthase uses an atypical mechanism to achieve substrate specificity

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 >82,000-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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