On the Estimation of Stability Parameters from Heat-Induced Conformational Transition Curves of Proteins

F. Ahmad

Department of Biosciences, Jamia Millia Islamia, Jamia Nagar, New Delhi -110 025, India

  (Received 9 September 2003, Accepted 26 September 2003)

     A method is suggested to determine valid and authentic values of thermodynamic stability parameters of proteins from their heat-induced conformational transition curves. We show  (a) that the estimate of DH_m^van, the enthalpy change on denaturation at T_m, the midpoint of denaturation, is significantly less than DH_m^cal, the value obtained by the calorimetric measurements, if the analysis of the conformational transition curve uses the conventional method which assumes a linear temperature-dependence of the pre- and post-transition baselines; and (b) that there exists an excellent agreement between DH_m^van and DH_m^cal values of proteins, if the analysis of thermal denaturation curves assumes that the temperature-dependence of pre- and post-transition baselines is described by a parabolic function. The latter analysis is supported by our observations that the temperature-dependencies of the absorption and circular dichroism properties of protein groups are indeed nonlinear. It is observed that the estimate of DC_p, the constant-pressure heat capacity change is independent of the model used to describe the temperature-dependence of the pre- and post-transition baselines. An important conclusion is that for proteins which exhibit a two-state character, all stability parameters are measured with the same error as that observed with a calorimeter.

 Keywords: Protein stability, Thermal denaturation, Van’t Hoff analysis, Enthalpy change, Heat capacity change, Gibbs energy change, Ribonuclease, Lysozyme