ENTHALPY OF CAPTOPRIL ANGIOTENSIN-I-CONVERTING-ENZYME BINDING

High-sensitivity titration calorimetry is used to measure changes in e nthalpy, heat capacity and protonation for the binding of captopril to the angiotensin I-converting enzyme (ACE; EC 3.4.15.1). The affinity of ACE to captopril is high and changes slightly with the pH, because the number of protons linked to binding is low, The determination of t he enthalpy change at different pH values suggests that the protonated group in the captopril-ACE complex exhibits a heat protonation of app roximately -30 kJ/mol, This value agrees with the protonation of an im idazole group. The residues which may become protonated in the complex could be two histidines existing in two active sites, which are joine d to the amino acids coordinated to Zn2+. Calorimetric measurements in dicate that captopril binds to two sites in the monomer of ACE, this b inding being enthalpically unfavorable and being dominated by a large positive entropy change. Thus, binding is favored by both electrostati c and hydrophobic interactions. The temperature dependence of the free energy of binding Delta G(o) is weak because of the enthalpy-entropy compensation caused by a large heat capacity change, Delta C-p=-4.3+/- 0.1 kJ/K/mol of monomeric ACE. The strong favorable binding entropy an d the negative Delta C-p indicate both a large contribution to binding due to hydrophobic effects, which seem to originate from dehydration of the ligand-protein interface, and slight conformational changes in the vicinity of the active sites. (C) 1998 Federation of European Bioc hemical Societies.