Thermodynamics of thermal unfolding of bovine apo‐α‐lactalbumin

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Abstract
Thermal unfolding of bovine α‐lactalbumin in 10 mM borate buffer at pH 8.0 in the presence of 0.01–1.0 M NaCl was studied in terms of CD ellipticity. The apoprotein changes the conformation from a native‐like (N) to an unfolded (U) form, which has an appreciable amount of the secondary structure but no tertiary structure, in the two‐state type. Various thermodynamic parameters of the transition were analyzed. The differences in enthalpy and heat capacity between the N and U states are similar to the corresponding differences of the holoprotein obtained with the calorimetric method by Pfeil. It is shown that one Na+ binds with a binding constant larger than 102– 103 M‐1 to a specific site (probably to the Ca2+‐binding site) in the molecule and the bound Na+ stabilizes the N form of the apoprotein.