Engineering of Hydroxymandelate Oxidase and Cascade Reactions for High-Yielding Conversion of Racemic Mandelic Acids to Phenylglyoxylic Acids and (R)- and (S)-Phenylglycines

Abstract

An alcohol oxidase (AOx) for the (S)-enantioselective oxidation of 4-hydroxymandelic acid (4-HMA) to 4-hydroxyphenylglyoxylic acid (4-HPGA) with enhanced activity was developed by directed evolution of hydroxymandelate oxidase (HMO) through three rounds of iterative saturation mutagenesis. The engineered HMO mutant A80G-T159S-T162Q (HMOTM) catalyzed the oxidation of (S)-4-HMA to 4-HPGA with a 23-fold enhancement in catalytic efficiency (kcat/KM). Substrate docking simulation on HMOTM suggested that A80G reoriented FMN, while T159S and T162Q formed hydrogen bonds with the carboxylic group of the substrate, thus facilitating substrate binding and catalysis. (S)-Enantioselective HMOTM was used together with mandelic acid racemase (MR) and catalase (KatE) to achieve high-yielding oxidation of rac-4-HMA to 4-HPGA with either purified enzymes (up to 93% yield and 426 mM) or Escherichia coli (HMC) cells expressing the three enzymes (up to 93% yield and 140 mM). The HMOTM-MR-KatE cascades were applied for the oxidation of seven other substituted MAs, producing the corresponding phenylglyoxylic acids with 90–99% conversions using purified enzymes or whole cells. Efficient conversion of racemic α-hydroxy acids to (S)- or (R)-α-amino acids was achieved by combining HMOTM-MR-KatE with (S)-enantioselective transaminase (EcαTA) or (R)-enantioselective transaminase (DpgAT), together with glutamate dehydrogenase (GluDH). Coupling of E. coli (HMC) with E. coli (E-G) expressing EcαTA and GluDH for one-pot biotransformation of five rac-MAs produced the corresponding (S)-phenylglycines (PGs) in 91–99% ee with 73–98% conversions. Using E. coli (HMC) and E. coli (D-G) expressing DpgAT and GluDH enabled the production of five (R)-PGs with 93–99% ee and 73–98% conversions from the corresponding rac-MAs. The engineered AOx, AOx-MR-KatE cascades, and AOx-MR-Kat-GluDH-(S)- or (R)-TA cascades provide useful tools for highly active and enantioselective oxidation of racemic hydroxyacids and high-yielding conversion of racemic hydroxyacids to ketoacids and enantiopure (S)- or (R)-aminoacids, respectively, which are challenging and useful chemical reactions.