HEAT DENATURATION OF PEPSINOGEN IN A WATER-ETHANOL MIXTURE

The effect of ethanol and pH on thermodynamic parameters and cooperati vity of pepsinogen heat denaturation was studied by scanning microcalo rimetry. Addition of 20% ethanol decreases the protein denaturation te mperature by 10.7 degrees C at pH 6.4 and 15.8 degrees C at pH 8.0. It also decreases the denaturation heat capacity increment from 5.8 to 4 .2 kcal/K(.)mol. The dependences of calorimetric denaturation enthalpy on denaturation temperature both in water and 20% ethanol are linear and intersect at about 95 degrees C. In 20% ethanol the pH shift from 5.9 to 8.0 results in a decreased number of cooperative domains in pep sinogen. This process causes no changes either in the secondary struct ure or in the local surroundings of aromatic amino acids, It is conclu ded that ethanol addition does not affect the cooperativity of pepsino gen denaturation substantially until the pH change provokes redistribu tion of charges in the protein molecule.