Switching Enantioselectivity in Phenylalanine Ammonia Lyase for the Synthesis of Electron‐Deficient Aromatic <scp>d</scp>‐Amino Acids

Aromatic d-amino acids (d-AAs) are valuable building blocks in drug discovery and peptide therapeutics, yet their direct and efficient biocatalytic synthesis remains a challenge. Here, we report the rational engineering of phenylalanine ammonia lyase from Planctomyces brasiliensis (PbPAL) to enable asymmetric hydroamination for the enantioselective synthesis of d-aromatic amino acids. By targeting active-site residue L205, we identified variants capable of highly d-enantioselective hydroamination, with L205F enabling the transformation of electron-deficient aryl acrylates with >99% enantiomeric excess (ee). The synthetic utility of this platform was demonstrated by gram-scale synthesis of d-benzoxazole and substituted 2-pyridylalanines. Structural and mutational studies revealed distinct roles for the 4-methylideneimidazole-5-one (MIO) prosthetic group and active-site residues L205, Y64, and K397 in modulating enantioselectivity. These results enabled the identification of PbPAL variants with the opposite selectivity, such as L205V-K397A, which preferentially produce l-amino acids. This work broadens the utility of PALs as programmable biocatalysts for asymmetric synthesis.