Intermolecular Amine Transfer to Enantioenriched trans-3-Phenylglycidates by an α/β-Aminomutase to Access Both anti-Phenylserine Isomers

beta-Hydroxy-alpha-amino acids are noncanonical amino acids with two stereocenters and with useful applications in the pharmaceutical and agrochemical sectors. Here, a 5-methylidene-3,5-dihydro-4H-imidazol-4-one- dependent aminomutase from Taxus canadensis (TcPAM) was repurposed to transfer the amino group irreversibly from ( 2S)-styryl-alpha-alanine to exogenously supplied trans-3-phenylglycidate enantiomers, producing anti-phenylserines stereoselectively. TcPAM catalysis inverted the intrinsic regioselective chemistry from amination at C-beta to C-alpha of enantioenriched trans-3-phenylglycidates to make phenylserine predominantly (97%) over phenylisoserine (similar to 3% relative abundance). Gas chromatography-mass spectrometry analysis of the chiral auxiliary derivatives of the biocatalyzed products confirmed that the amine transfer was stereoselective for each glycidate enantiomer. TcPAM converted (2S,3R)-3-phenylglycidate to (2S)-anti-phenylserine predominantly (89%) and (2R,3S)-3-phenylglycidate to (2R)-anti-phenylserine (88%) over their antipodes, with inversion of the configuration at C-alpha in each case. Both glycidate enantiomers formed a small amount (similar to 10%) of syn-phenylserine by retaining the configuration at C-alpha. The minor syn-isomer likely came from a beta-hydroxy oxiranone intermediate formed by intramolecular ring opening of the oxirane ring by the carboxylate before amine transfer. TcPAM had a slight preference toward (2S,3R)-3-phenylglycidate, which was turned over (k(cat) = 0.3 min(-1)) 1.5 times faster than the (2R,3S)-glycidate (k(cat) = 0.2 min(-1)). The catalytic efficiencies (k(cat)(app)/K-M(app) approximate to 20 M(-1)s(-1)) of TcPAM for the antipodes were similar. The kinetic data supported a two-substrate ping-pong mechanism for the amination of the phenylglycidates, with competitive inhibition at higher glycidate substrate concentrations.