Asymmetric Alkylation of Ketones Catalyzed by Engineered TrpB

The β-subunit of tryptophan synthase (TrpB) catalyzes a PLP-mediated β-substitution reaction between indole and serine to form L-Trp. A succession TrpB protein engineering campaigns expand the enzyme's nucleophile substrate range has enabled biocatalytic production diverse non-canonical amino acids (ncAAs). Here, we show that ketone-derived enolates can serve as nucleophiles in achieve asymmetric alkylation ketones, an outstanding challenge synthetic chemistry. We engineered by directed evolution catalyze propiophenone 2-fluoroacetophenone with high degree selectivity. In reactions propiophenone, preference for opposite product diastereomer emerges over course evolution, demonstrating full control stereochemistry at new chiral center be achieved. addition this platform is crucial first step toward development efficient methods synthesize prolines other chirally dense nitrogen heterocycles.