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

Abstract

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 Δ G( T) shows a strong temperature dependence, then the protein becomes subject to both high- and low-temperature destabilization. In the simplest case Δ G( T) is parabolic, therefore the high temperature T H, where Δ( G( T H) = 0, is complemented by a low temperature T L, where Δ G( T L) = 0. It is generally stated that the partial molal heat capacity change Δ C accompanying the heat denaturation is positive and independent of the temperature. This implies that heating the protein through T L results in a negative Δ C which seems physically unsatisfactory. The constant Δ C model is explored and a physically more realistic model is advanced which allows for a temperature-dependent Δ C which changes sign at some temperature within the range of stability of the native protein; Δ 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.