Direct observation of the enthalpy change accompanying the native to molten-globule transition of cytochrome c by using isothermal acid-titration calorimetry

The enthalpy change accompanying the reversible acid-induced transition from the native (N) to the molten-globule (MG) state of bovine cytochrome c was directly evaluated by isothermal acid-titration calorimetry (IATC), a new method for evaluating the pH dependence of protein enthalpy. The enthalpy change was 30 kJ/mol at 30degreesC, pH 3.54, with 500 MM KCl. The results of the global analysis of the temperature dependence of the excess enthalpy from 20 to 35 degreesC demonstrated that the N to MG transition is a two-state transition with a small heat capacity change of 1.1 kJK(-1) mol(-1) The present findings were also indicative of the pH dependence of the enthalpy and the heat capacity of the MG state, - 13 kJ mol(-1) pH(-1) and - 1.0 U K- 1 mol(-1) pH- 1, respectively, at 30degreesC within a pH range from 2 to 3. (C) 2004 Elsevier B.V. All rights reserved.