Dynamic Kinetic Resolution of α-Aminonitriles to Form Chiral α-Amino Acids

We have succeeded in the enzymatic synthesis of (R)-alpha-aminobutyric acid from racemic alpha-aminobutyronitrile. This has been demonstrated by the use of non-stereoselective nitrile hydratase (NHase) from Rhodococcus opacus 71D, D-aminopeptidase from Ochrobactrum anthropi C1-38 and alpha-amino-epsilon-caprolactam (ACL) racemase from Achromobacter obae. Racemic alpha-aminobutyronitrile was completely converted in 6 h at 30 degrees C to (R)-alpha-aminobutyric acid whose optical purity was more than 99%. (S)-alpha-Aminobutyric acid was also synthesized from alpha-aminobutyronitrile by NHase, ACL racemase and L-amino acid amidase from Brevundimonas diminuta TPU 5720. In a similar manner, other (R)- or (S)-alpha-amino acids with more than 97.5% ee could be synthesized from the corresponding alpha-aminonitriles. This is the first report on the dynamic kinetic resolution (DKR) of alpha-aminonitriles to form chiral alpha-amino acids. The key enzyme in this DKR is non-stereoselective NHase, which had been newly screened from soil samples, and its gene cloned.