Bimolecular Nucleophilic Substitution (S_N2) Reaction Catalyzed by l-Threonine Aldolase

The bimolecular nucleophilic substitution (S_N2) and aldol reactions are cornerstone transformations in organic and biochemical synthesis, yet they operate through fundamentally distinct mechanisms, substrates, and product frameworks. Nature has evolved dedicated enzyme families to catalyze these reactions separately, namely, aldolases for aldol condensations and methyltransferases or similar enzymes for S_N2 pathways. Aldolases have not been reported to catalyze S_N2 reactions. We herein report the unprecedented repurposing of l-threonine aldolase to catalyze an S_N2 reaction. This new activity enables direct asymmetric alkylation of the sp³-hybridized Cα-H bond in glycine using readily accessible α-halide carbonyl compounds. Mechanistic and computational analyses elucidate how the active site of the enzyme precisely aligns substrates into a geometry that facilitates the S_N2 transition state. The developed biocatalytic platform provides efficient access to diverse enantiomerically enriched α-amino acids (29 examples), achieving yields up to 95% and exceptional stereocontrol (e.r. > 99:1). This discovery not only expands the catalytic repertoire of aldolases but also underscores the potential for repurposing aldolases to unlock non-native reactivities in sustainable synthesis.