Review

This article summarizes all experimental facts concerning the cold denaturation of single-domain, multi-domain, and multimeric globular proteins in aqueous solutions with and without urea and guanidine hydrochloride. The facts obtained by various experimental techniques are analyzed thermodynamically and it is shown that the cold denaturation is a general phenomenon caused by the very specific and strongly temperature-dependent interaction of protein nonpolar groups with water. Hydration of these groups, in contrast to expectations, is favorable thermodynamically, i.e., the Gibbs energy of hydration is negative and increases in magnitude at a temperature decrease. As a result, the polypeptide chain, tightly packed in a compact native structure, unfolds at a sufficiently low temperature, exposing internal nonpolar groups to water. The reevaluation of the hydration effect on the base of direct calorimetric studies of protein denaturation and of transfer of non-polar compounds into water leads to revision of the conventional conception on the mechanism of hydrophobic interaction. The last appears to be a complex effect in which the positive contributor is van der Waals interactions between the nonpolar groups and not the hydration of these groups as it was usually supposed.

The fluorescence method of Sklar et al. (Sklar, L.A., Hudson, B.S. and Simoni, R.D. (1977) Biochemistry 16, 5100-5108) using cis-parinaric acid as a probe was applied to determine the effective hydrophobicity of proteins. The initial slope (S0) of fluorescence intensity vs. protein concentration plot was used as an index of the protein hydrophobicity. A good correlation was observed for S0 of native proteins, denatured proteins and surfactant-bound proteins with an effective hydrophobicity determined by the hydrophobic partition method. The effective hydrophobicity determined fluorometrically showed significant correlations with interfacial tension and emulsifying activity of the proteins studied. The fluorescence technique using cis-parinaric acid is useful for determination of the effective hydrophobicity, as the procedure is much simpler and quicker than hydrophobic chromatography and hydrophobic partition.