Mechanism of .alpha.-amino-.epsilon.-caprolactam racemase reaction

Authors:
  • SASyed Ashrafuddin Ahmed
    Syed Ashrafuddin Ahmed
  • NENobuyoshi Esaki
    Nobuyoshi Esaki
  • HTHidehiko Tanaka
    Hidehiko Tanaka
  • KSKenji Soda
    Kenji Soda
Publisher website
Google Scholar
Abstract
.alpha.-Amino-.epsilon.-caprolactam racemase catalyzes the exchange of the .alpha.-hydrogen of the substrate with deuterium during racemization of deuterium oxide. The rate of the hydrogen exchange measured by 1H NMR is lower than that of racemization in deuterium oxide for both the enantiomers. Both the enantiomers of .alpha.-amino-.epsilon.-caprolactam show an overshoot of the optimal rotation during the enzymatic racemization in deuterium oxide (but not in water). This phenomenon may be attributable to a primary deuterium isotope effect at the .alpha.-position: .alpha.-deuterium isotope effects of 3.6 and 2.0 were observed for the racemization of the D and L enantiomers of .alpha.-amino-.epsilon.-caprolactam, respectively. Results of tritium-labeling experiments showed that the enzyme catalyzes both retention and inversion of configuration of the substrate with a similar probability in each turnover. Conversion of [.alpha.-2H]-D-.alpha.-amino-.epsilon.-caprolactam in water and unlabeled D-.alpha.-amino-.epsilon.-caprolactam in deuterium oxide into the L isomer under nearly single turnover conditions with the enzyme showed significant internal return of the .alpha.-hydrogen. These results support a single base mechanism for the racemization reaction catalyzed by the enzyme.