Role of cations in the catalysis of thermostable pectinmethylesterase extracted from marsh grapefruit pulp

The de-esterification of high methoxyl pectin by thermostable pectinmethyle sterase (TS-PME) from Marsh grapefruit was studied at pH 7.5, in a temperat ure range between 25 degrees and 50 degrees C and in the presence of variou s cations. Arrhenius plots were constructed for CaCl2 (5 to 20 mM), SrCl2 ( 5 to 20 mM), and spermidine (2.5 to 10 mM) added reactions. Enthalpy (Delta H double dagger) and entropy (Delta S double dagger) of activation changed with cation type and concentration. The presence of cations modified the f ree energy of the resulting enzyme/substrate complex. The entropy of activa tion was positive at all concentrations of CaCl2 studied, and negative in t he same concentration range with SrCl2. Reactions with spermidine showed ne gative entropy of activation at concentrations <5 mM and positive values of entropy at higher concentrations.