Thermodynamics of the Temperature-dependent Denaturation ofα-Lactalbumin

Abstract

The proposed similarity of conformation between .alpha.-lactalbumin (.alpha.-LA) and hen egg-white lysozyme was tested by the comparison of the thermodynamic parameters obtained from the temperature dependence of denaturation. For the denaturing reaction by guanidine hydrochloride, the value of .DELTA.Cp [molar heat capacity at constant pressure] for .alpha.-LA is almost identical with that for lysozyme, which suggests that the amount of the hydrophobic side chains buried in the interior of the molecule is the same in the native state; the value of .DELTA.H.degree. [standard enthalpy] and .DELTA.S.degree. [standard entropy] for .alpha.-LA are also close to those for lysozyme, and the small differences are explicable by the proposed molecular model of .alpha.-LA, which implies that the somewhat large difference in .DELTA.G.degree. [standard Gibb''s free energy] observed previously between the 2 proteins does not originate from large conformational differences. These results support the conformational similarity between .alpha.-LA and lysozyme as represented by the molecular model. The heat-denatured state of .alpha.-LA is also characterized by the parameters and discussed.