| Abstract: | Deuterium and 13C isotope effects for the enzymic decarboxylation of oxalacetate showed that both deuterium- and 13C-sensitive steps in the reaction are partially rate limiting. A normal alpha-secondary effect of 1.2 per deuterium was calculated for the reaction in which pyruvate-d3 was the substrate, suggesting that the enolate of pyruvate was an intermediate in the reaction. The large normal alpha-secondary deuterium isotope effect of 1.7 when oxalacetate-d2 was the substrate suggests that the motions of the secondary hydrogens are coupled to that of the primary hydrogen during the protonation of the enolate of pyruvate. The reduction in the magnitude of the 13C isotope effect for the oxamate-dependent decarboxylation of oxalacetate from 1.0238 to 1.0155 when the reaction was performed in D2O (primary deuterum isotope effect = 2.1) clearly indicates that the transfer of the proton and carboxyl group between biotin and pyruvate does not occur via a single concerted reaction. Mechanisms in which biotin is activated to react with CO2 (prior to transfer of the proton on N-1) by bond formation between the sulfur and the ureido carbon, or in which the sequence of events is decarboxylation of oxalacetate, proton transfer from biotin to enolpyruvate, and carboxylation of enolbiotin, predict that the 13C isotope effect in D2O should be substantially lower than the observed value. A stepwise mechanism that does fit the data is one in which a proton is removed from biotin by a sulfhydryl group on the enzyme prior to carboxyl transfer, as long as the sulfhydryl group has an abnormally low pK.(ABSTRACT TRUNCATED AT 250 WORDS) |
