Enthalpy Changes Accompanying Hydrolysis of 3-(2-Furyl)acryloylimidazole by α-Chymotrypsin

Article abstract of DOI:10.1021/ja00540a025

A calorimetric analysis of the α-chymotrypsin-mediated hydrolysis of 3-(2-furyl)acryloylimidazole at pH 7.85 is presented and compared with the nonenzymatic hydrolysis.The enzyme-mediated hydrolysis is characterized by an apparent pK_α for acylation of 6.60 and a deacylation pK_α of 7.55.Activation enthalpy changes of 7.8+/-1.3 and 15.0+/-0.4 kcal/mol were determined for acylation (k₂) and deacylation (k₃), respectively.An enthalpy change of 0.04+/-0.85 kcal/mol for Michaelis complex formation was also determined from the temperature dependence of K_s.Heats of acylation (-10.1+/-0.2 kcal/mol) and deacylation (-1.2+/-0.55 kcal/mol) corrected for buffer ionization and product ionization heats were separately determined by making use of the large rate difference between acylation and deacylation (k₂=1000k₃) which exists at pH 7.85.The sum of these corrected enthalpy changes (-11.3+/-0.6 kcal/mol) agrees very well with the enthalpy change observed for nonenzymatic hydrolysis of furylacryloylimidazole (-11.3+/-0.5 kcal/mol) and validates the approach used.