LINKAGE OF CATALYSIS AND REGULATION IN ENZYME ACTION - CARBON-ISOTOPEEFFECTS, SOLVENT ISOTOPE EFFECTS, AND PROTON INVENTORIES FOR THE UNREGULATED PYRUVATE DECARBOXYLASE OF ZYMOMONAS-MOBILIS

The pyruvate decarboxylase of the bacterium Zymomonas mobilis (ZMPDC), in contrast to that of yeast (SCPDC), is not regulated by substrate a nd shows simple Michaelis-Menten kinetics with rate constants k/B (equ ivalent to k(cat)/K-m) and k (equivalent to k(cat)). C-1-carbon-13 iso tope effects obtained by direct measurement with 99% C-13-labeled subs trate, which permits determination of the isotope effect on both k/B a nd k, give (13)(k/B) 1.010+/-0.008 and (13)k = 1.019+/-0.008. These ar e similar to the effects with SCPDC and show that decarboxylation is a bout 20% rate-limiting at low pyruvate levels and about 40% rate-limit ing 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 x 10(5) M(-1) s(-1); off-reaction of pyruvate, 300 s(-1); decarboxylat ion, 1200 s(-1); product release, 750 s(-1). Solvent isotope effects a re 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 sulfh ydryl-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 tr ansition states relative to reactant states. Thus introduction of the regulatory features of SCPDC is coincident with a reduction in catalyt ic power.