DSC study of cold and heat denaturation processes of beta-lactoglobulin A with guanidine hydrochloride

The cold and heat denaturations of bovine beta-lactoglobulin A (beta-lg A) has been studied in solutions of guanidine hydrochloride (GuHCl) by differential scanning calorimetry (DSC). The experimental results are presented and discussed. It is shown that the number of protons bound by the monomeric molecules of beta-lg A was unchanged before and after its heat denaturation below pH 3, and that the activation energy of the heat denaturation was depressed owing to the presence of GuHCl. In the solutions with 2.50 and 3.06 mol/L of GuHCl, both the cold and heat denaturations of beta-lg A were observed. In comparison with the heat denaturation, the activation energy of cold denaturation was far lower and the number of GuHCl molecules bound by the unfolded polypeptide chains after cold denaturation increased a lot. The absolute value of the enthalpy of cold denaturation was larger than that of heat denaturation. It was found by the analysis that the contribution to the total denaturational enthalpy of conformational change itself of the monomeric molecules of beta-lg A was the lowest among the globulins, according to the average of the number of heavy atoms.