Kinetic compensation and the role of cations in pectinesterase catalysis

The catalytic rate constant of thermostable pectinesterase (TS-PE) from Mar sh grapefruit pulp was determined at pH 7 at temperatures between 25 and 60 degrees C. TS-PE activity was measured at NaCl concentrations of 0.05, 0.1 0, 0.15, and 0.20 M and at CaCl2 concentrations of 0.005, 0.010, 0.015, and 0.020 M. For sodium- and calcium-added conditions, the kinetic functions o f this reaction agreed with kinetic compensation relations. The isokinetic temperatures for sodium- and calcium-added conditions were 327.8 and 312.4 K, respectively. Different isokinetic temperatures and compensation paramet ers suggest that sodium and calcium uniquely affect TS-PE catalysis. This i s the first demonstration of kinetic compensation in an enzyme-catalyzed re action.