Enzymatic Conversion of Unnatural Amino Acids by Yeast D-Amino Acid Oxidase

Unnatural amino acids, particularly synthetic a-amino acids, are becoming crucial tools for modern drug discovery research. In particular, this application requires enantiomerically pure isomers. In this work we report on the resolution of racemic mixtures of the amino acids D,L-naphthylalanine and D,L-naphthylglycine by using a natural enzyme, D-amino acid oxidase from the yeast Rhodotorula gracilis. A significant improvement of the bioconversion is obtained using a single-point mutant enzyme designed by a rational approach. With this D-amino acid oxidase variant the complete resolution of all the unnatural amino acids tested was obtained: in this case, the bioconversion requires a shorter time and a lower amount of biocatalyst compared to the wild-type enzyme. The simultaneous production of the corresponding a-keto acid, a possible precursor of the amino acid in the L-form, improves the significance of the procedure.