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

The de-esterification of high methoxyl pectin by thermostable pectinmethylesterase (TS-PME) from Marsh grapefruit was studied at pH 7.5, in a temperature range between 25 degrees and 50 degrees C and in the presence of various cations. Arrhenius plots were constructed for CaCl(2) (5 to 20 mM), SrCl(2) (5 to 20 mM), and spermidine (2.5 to 10 mM) added reactions. Enthalpy (DeltaH()) and entropy (DeltaS()) of activation changed with cation type and concentration. The presence of cations modified the free energy of the resulting enzyme/substrate complex. The entropy of activation was positive at all concentrations of CaCl(2) studied, and negative in the same concentration range with SrCl(2). Reactions with spermidine showed negative entropy of activation at concentrations <5 mM and positive values of entropy at higher concentrations.