Abstract
The binding of α-amylase inhibitor isolated from white kidney bean Phaseolus vulgayis (abbreviated to PHA) to two active components of porcine pancreatic α-amylase [EC 3.2.1.1] (abbreviated to PPA I and II) were studied by isothermal calorimetry at pH 6.9 and at various temperatures ranging from 20 to 40°C. The net enthalpy changes of binding were determined after correction for the buffer ionization heat. Thermodynamic quantities for the binding of PHA to the enzymes were derived as fuctions of temperature from inhibitor constants and isothermal calorimetric experiments. At 25°C, the thermodynamic quantities were ΔG=-51.8 kJ moL-1, ΔH=5.2 kJ mol-1, ΔS=0.19 kJ K-1 mol-1 and ΔCp=-1.18 kJ K-1 mol-1 for PPA land ΔG=-53.9 kJ mol-1, ΔH=1.9 kJ mol-1, ΔS=0.19 kJ K-1 mol-1 and ΔCp= -1.25 kJ K-1 mol-1 for PPA II. From these results, it was concluded that, like many other protein-protein interactions studied calorimetrically so far, the binding of PHA to PPA I and II are also characterized by a predominant contribution from hydrophobic effects.
