Enhanced conversion of racemic alpha-arylalanines to (R)-beta-arylalanines by coupled racemase/aminomutase catalysis

The Taxus phenylalanine aminomutase (PAM) enzyme converts several (S)-alpha-arylalanines to their corresponding (R)-beta-arylalanines. After incubating various racemic substrates with 100 microg of PAM for 20 h at 31 degrees C, each (S)-alpha-arylalanine was enantioselectively isomerized to its corresponding (R)-beta-product. With racemic starting materials, the ratio of (R)-beta-arylalanine product to the (S)-alpha-substrate ranged between 0.4 and 1.8, and the remaining nonproductive (R)-alpha-arylalanine became enriched. To utilize the (R)-alpha-isomer, the catalysis of a promiscuous alanine racemase from Pseudomonas putida (KT2440) was coupled with that of PAM to increase the production of enantiopure (R)-beta-arylalanines from racemic alpha-arylalanine substrates. The inclusion of a biocatalytic racemization along with the PAM-catalyzed reaction moderately increased the overall reaction yield of enantiopure beta-arylalanines between 4% and 19% (depending on the arylalanine), which corresponded to as much as a 63% increase compared to the turnover with the aminomutase reaction alone. The use of these biocatalysts, in tandem, could potentially find application in the production of chiral beta-arylalanine building blocks, particularly, as refinements to the process are made that increase reaction flux, such as by selectively removing the desired (R)-beta-arylalanine product from the reaction mixture.