Evidence for a difference in mechanism between the reactions with carboxypeptidase A of the (-) and (+) - enantiomers of a thiolester substrate.

Cinnamoyl-carboxypeptidase A_γ has been isolated from the reaction of the enzyme with (+)-$\text{S-(trans}$-cinnamoyl)-α-mercapto-β-phenylpropionate. This is the first instance in which an acyl-enzyme has been detected directly during the esterase action of carboxypeptidase A. Also, the failure to detect such a species in the carboxypeptidase A-catalyzed hydrolysis of the (?)-enantiomer of the thiolester under similar conditions provides the first experimental demonstration that an enzyme can act on different enantiomers of a substrate through different mechanisms.