Asymmetrical Synthesis of l ‐Homophenylalanine Using Engineered Escherichia coli Aspartate Aminotransferase

Site‐directed mutagenesis was performed to change the substrate specificity of Escherichia coli aspartate aminotransferase (AAT). A double mutant, R292E/L18H, with a 12.9‐fold increase in the specific activity toward l ‐lysine and 2‐oxo‐4‐phenylbutanoic acid (OPBA) was identified. E. coli cells expressing this mutant enzyme could convert OPBA to l ‐homophenylalanine ( l ‐HPA) with 97% yield and more than 99.9% ee using l ‐lysine as amino donor. The transamination product of l ‐lysine, 2‐keto‐6‐aminocaproate, was cyclized nonenzymatically to form Δ 1 ‐piperideine 2‐carboxylic acid in the reaction mixture. The low solubility of l ‐HPA and spontaneous cyclization of 2‐keto‐6‐aminocaproate drove the reaction completely toward l ‐HPA production. This is the first aminotransferase process using l ‐lysine as inexpensive amino donor for the l ‐HPA production to be reported.