Highly atom economic synthesis of D-2-aminobutyric acid through an in vitro tri-enzymatic catalytic system

Abstract

D-2-Aminobutyric acid is an unnatural amino acid serving as an important intermediate in pharmaceutical production. Developing a synthetic method that uses cheaper starting materials and produces less by-product is a pressing demand. A tri-enzymatic catalytic system, which is composed of L-threonine ammonia lyase (L-TAL), D-amino acid dehydrogenase (D-AADH), and formate dehydrogenase (FDH), has thus been developed for the synthesis of D-2-aminobutyric acid with high optical purity. In this cascade reaction, the readily available L-threonine serves as the starting material, carbon dioxide and water are the by-products. D-2-Aminobutyric acid was obtained with >90% yield and >99% enantioselective excess, even without adding external ammonia, demonstrating that the ammonia from the first reaction can serve as the amino donor for the reductive amination step. This multi-enzymatic system provides an attractive method with high atomic economy for the synthesis of D-α-amino acids from the corresponding L-α-amino acids, which are readily produced by fermentation.