The thermodynamics of protein stability. Cold destabilization as a general phenomenon.

A theoretical analysis of the temperature/stability profiles of proteins shows that, where a two-state model represents the denaturation, and where the free energy of denaturation delta G(T) shows a strong temperature dependence, then the protein becomes subject to both high- and low-temperature destabilization. In the simplest case delta G(T) is parabolic, therefore the high temperature TH, where delta (G(TH) = 0, is complemented by a low temperature TL, where delta G(TL) = 0. It is generally stated that the partial molal heat capacity change delta C accompanying the heat denaturation is positive and independent of the temperature. This implies that heating the protein through TL results in a negative delta C which seems physically unsatisfactory. The constant delta C model is explored and a physically more realistic model is advanced which allows for a temperature-dependent delta C which changes sign at some temperature within the range of stability of the native protein; delta G(T) then has the form of a skewed parabola. Experimental heat capacity data for native lysozyme and for a flexible polymer lend support to this model. The molecular basis of cold inactivation of proteins is discussed in the light of the thermodynamic analysis.