N-Acetyl amino acid racemases (NAAARs) have demonstrated their potential in the enzymatic synthesis of chiral amino acids, molecules of significant biotechnology interest. In order to identify novel activities and to improve these enzymes by engineering approaches, suitable screening methods are necessary. Previous engineering of the NAAAR from Amycolatopsis Ts-1-60 was achieved by relying on an in vivo selection system that linked the viability of an E. coli l-methionine auxotroph to the activity of the improved enzyme. However, this assay was only suitable for the screening of N-acetyl-d-methionine, therefore limiting the potential to evolve this enzyme toward other natural or non-natural acetylated amino acids. Here, we report the optimization and application of a spectrophotometric microtiter-plate-based assay for NAAAR. The assay is based on the detection of the amino acid reaction product formed by hydrolysis of the N-acylated substrate by an l-amino acid acylase and its subsequent oxidation by an FAD-dependent l-amino acid oxidase (l-AAO). Cofactor recycling of the l-AAO leads to the formation of hydrogen peroxide which is easily monitored using horseradish peroxidase (HRP) and o-dianisidine. This method allowed for the determination of the kinetic parameters of NAAAR and led to the identification of N-acetyl-d-naphthylalanine as a novel NAAAR substrate. This robust method is also suitable for the high-throughput screening of NAAAR mutant gene libraries directly from cell lysates.
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