An Aminocaprolactam Racemase from Ochrobactrum anthropi with Promiscuous Amino Acid Ester Racemase Activity

The kinetic resolution of amino acid esters (AAEs) is a useful synthetic strategy for the preparation of single enantiomer amino acids. The development of an enzymatic dynamic kinetic resolution (DKR) process for AAEs, which would give a theoretical yield of 100% of the enantiopure product, would require an amino acid ester racemase (AAER), however, no such enzyme has been described. We have identified low AAER activity of 15 U mg-1 in a homolog of a PLP-dependent α-amino ε-caprolactam racemase (ACLR) from Ochrobactrum anthropi. We have determined the structure of this enzyme, OaACLR, to a resolution of 1.87 Å and using structure-guided saturation mutagenesis, in combination with a colorimetric screen for AAER activity, we have identified a mutant, L293C, in which the promiscuous AAER activity of this enzyme towards L-phenylalanine methyl ester is improved 3.7 fold.