Active site model of (R)-selective ω-transaminase and its application to the production of D-amino acids

omega-Transaminase (omega-TA) is one of the important biocatalytic toolkits owing to its unique enzyme property which enables the transfer of an amino group between primary amines and carbonyl compounds. In addition to preparation of chiral amines, omega-TA reactions have been exploited for the asymmetric synthesis of l-amino acids using (S)-selective omega-TAs. However, despite the availability of (R)-selective omega-TAs, catalytic utility of the omega-TAs has not been explored for the production of d-amino acids. Here, we investigated the substrate specificity of (R)-selective omega-TAs from Aspergillus terreus and Aspergillus fumigatus and demonstrated the asymmetric synthesis of d-amino acids from alpha-keto acids. Substrate specificity toward d-amino acids and alpha-keto acids revealed that the two (R)-selective omega-TAs possess strict steric constraints in the small binding pocket that precludes the entry of a substituent larger than an ethyl group, which is reminiscent of (S)-selective omega-TAs. Molecular models of the active site bound to an external aldimine were constructed and used to explain the observed substrate specificity and stereoselectivity. alpha-Methylbenzylamine (alpha-MBA) showed the highest amino donor reactivity among five primary amines (benzylamine, alpha-MBA, alpha-ethylbenzylamine, 1-aminoindan, and isopropylamine), leading us to employ alpha-MBA as an amino donor for the amination of 5 reactive alpha-keto acids (pyruvate, 2-oxobutyrate, fluoropyruvate, hydroxypyruvate, and 2-oxopentanoate) among 17 ones tested. Unlike the previously characterized (S)-selective omega-TAs, the enzyme activity of the (R)-selective omega-TAs was not inhibited by acetophenone (i.e., a deamination product of alpha-MBA). Using racemic alpha-MBA as an amino donor, five d-amino acids (d-alanine, d-homoalanine, d-fluoroalanine, d-serine, and d-norvaline) were synthesized with excellent product enantiopurity (enantiomeric excess > 99.7 %).