An Engineered Tryptophan Synthase Opens New Enzymatic Pathways to β-Methyltryptophan and Derivatives

beta-Methyltryptophans (beta-mTrp) are precursors in the biosynthesis of bioactive natural products and are used in the synthesis of peptidomimetic-based therapeutics. Currently beta-mTrp is produced by inefficient multistep synthetic methods. Here we demonstrate how an engineered variant of tryptophan synthase from Salmonella (StTrpS) can catalyse the efficient condensation of l-threonine and various indoles to generate beta-mTrp and derivatives in a single step. Although l-serine is the natural substrate for TrpS, targeted mutagenesis of the StTrpS active site provided a variant (beta L166V) that can better accommodate L-Thr as a substrate. The condensation of L-Thr and indole proceeds with retention of configuration at both a-and beta-positions to give (2S, 3S)-beta-mTrp. The integration of StTrpS (beta L166V) with l-amino acid oxidase, halogenase enzymes and palladium chemocatalysts provides access to further D-configured and regioselectively halogenated or arylated beta-mTrp derivatives.