Carbon isotope effect on the enzymatic decarboxylation of pyruvic acid

The decarboxylation of pyruvic acid by the thiamine pyrophosphate dependent pyruvate decarboxylase from brewer's yeast is accompanied by a carboxyl carbon isotope effect $\text{k}{}^{12}\text{k}{}^{13}\text{= 1.0083±0.0003}$ at 25°, pH 6.8. The small size of the isotope effect indicates that decarboxylation is not rate-determining in the overall reaction. The rate constant for decarboxylation of the enzyme-bound pyruvate-thiamine pyrophosphate complex is greater by about a factor of five than the rate constant for dissociation of this complex to form free pyruvate and the enzyme-thiamine pyrophosphate complex.