Unusual mechanism for an aminomutase rearrangement: Retention of configuration at the migration termini

The phenylalanine aminomutase from Taxus catalyzes the vicinal exchange of the amino group and the pro-3S hydrogen of (2S)-alpha-phenylalanine to make (3R)-beta-phenylalanine. While the migration of the amino group from C2 of the substrate to C3 of the product is already known to proceed intramolecularly with retention of configuration, the stereochemistry of the hydrogen transfer remained unknown, until now. The chemical shifts of the prochiral hydrogens of authentic (3R)-beta-phenylalanine were established by H-1 NMR, and the configuration of each hydrogen was assigned by H-2 NMR analysis of a racemic mixture of [2,3-H-2(2)]-(2S,3R)- and (2R,3S)-beta-phenylalanines synthesized via syn addition of deuterium gas with palladium catalyst to stereospecifically reduce the double bond of an N-acetyl enamine. After the aminomutase was incubated with [3,3-H-2(2)]-(2S)-alpha-phenylalanine, the derived deuterium-labeled beta-diastereoisomer product, derivatized as the N-acetyl methyl ester, was analyzed by H-2 NMR, which revealed that the mutase shuttles the pro-3S hydrogen to C2 of the beta-isomer product (designated 2S,3R) with retention of configuration. Retention of configuration at both reaction termini is unique among all aminomutase mechanisms examined so far. Furthermore, the dynamics of the C-beta-H bond of the substrate were measured in a competitive experiment with deuterium-labeled substrate to calculate a primary kinetic isotope effect on V-max/K-M of 2.0 +/- 0.2, indicating that C-H bond cleavage is likely rate limiting. Isotope exchange data indicate that the migratory deuterium of [H-2(8)]-(2S)-alpha-phenylalanine, at saturation, dynamically exchanges up to 75%, with protons from the solvent during the reaction after the first 10% of product is formed. The calculated equilibrium constant of 1.1 indicates that the beta-isomer was slightly favored relative to the alpha-isomer at 30 degrees C.