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Kinetics and Thermodynamics of Lactate Dehydrogenases from Beef Heart, Beef Muscle, and Flounder Muscle

1975; Canadian Science Publishing; Volume: 53; Issue: 11 Linguagem: Inglês

10.1139/o75-164

0008-4018

Uwe Borgmann, Keith J. Laidler, Thomas W. Moon,

Free Radicals and Antioxidants

The eight rate constants for a four-step ordered ternary-complex mechanism have been compared for lactate dehydrogenases (EC 1.1.1.27) from three sources, beef heart, beef muscle, and flounder muscle. The rate constants were determined at temperatures ranging from 5 °C to 50 °C, and the corresponding activation parameters ΔG ‡ , ΔH ‡ , and ΔS ‡ were calculated. Significant differences are noted for the values for the three types of enzyme. The relative heights of the activation barriers are much the same in all three cases, differences in kinetic behavior resulting mainly from differences in the stable binary and ternary enzyme–substrate complexes. These complexes are, in general, at lower free-energy and enthalpy levels for the beef-heart and beef-muscle enzymes than for the flounder-muscle enzyme. A high degree of compensation is found between the enthalpies and entropies of activation, resulting in relatively small differences between the free energies (and rates) for homologous steps with different enzymes. Analysis of the results, on the assumption that the compensation effect is due to weak-bonding effects, suggests that there are fewer weak bonds in the stable complexes of the muscle enzymes.