Linkage of catalysis and regulation in enzyme action. Carbon isotope effects, solvent isotope effects, and proton inventories for the unregulated pyruvate decarboxylase of Zymomonas mobilis

Article abstract of DOI:10.1021/ja00133a004

The pyruvate decarboxylase of the bacterium Zymomonas mobilis (ZMPDC), in contrast to that of yeast (SCPDC), is not regulated by substrate and shows simple Michaelis-Menten kinetics with rate constants k/B (equivalent to k_cat/K_m) and k (equivalent to k_cat). C₁-carbon-13 isotope effects obtained by direct measurement with 99% ¹³C-labeled substrate, which permits determination of the isotope effect on both k/B and k, give ¹³(klB) = 1.010 ± 0.008 and ¹³k = 1.019 ± 0.008. These are similar to the effects with SCPDC and show that decarboxylation is about 20% rate-limiting at low pyruvate levels and about 40% rate-limiting at high pyruvate levels. From these values, the rate constants for individual events in the catalytic cycle can be estimated (to within about a factor of 2) for ZMPDC: addition of pyruvate to the enzyme, 8 × 10⁵ M^-1 s^-1; off-reaction of pyruvate, 300 s^-1; decarboxylation, 1200 s^-1; product release, 750 s^-1. Solvent isotope effects are small and normal (k/B[HOH]/k/B[DOD] = 1.25 ± 0.05, k[HOH]/ k[DOD] = 1.30 ± 0.01), in strong contrast to those for SCPDC (e.g., an inverse isotope effect of 2 on k/B), which were considered to arise from sulfhydryl-addition reactions coupled to regulation. The proton inventories for ZMPDC are also quite different from those for SCPDC. The overall picture suggests that ZMPDC possesses a similar chemical mechanism but somewhat greater catalytic power than SCPDC because of both stronger uniform binding of all states and greater specific stabilization of transition states relative to reactant states. Thus introduction of the regulatory features of SCPDC is coincident with a reduction in catalytic power.