Pressure Protein Denaturation Compared to Thermal and Chemical Unfolding: Analyses with Cooperative Models

The thermodynamics of pressure-induced protein denaturation could so far not be directly compared with protein denaturation induced by temperature or chemical agents. Here, we provide a new cooperative model for pressure-induced protein denaturation that allows the quantitative comparison of all three denaturing processes based on their free energy, enthalpy, entropy, and cooperativity. As model proteins, we use apolipoprotein A-1 and lysozyme. The comparison shows that heat-induced unfolding is the most cooperative process. It is characterized by large positive enthalpies and entropies and (due to enthalpy-entropy compensation) small negative free energies. Pressure denaturation is less cooperative. The entropies and enthalpies are less positive, and the resulting free energies are more negative. Chemically induced unfolding is the least cooperative and shows the most negative free energies, in particular, if guanidinium hydrochloride (exhibiting a high binding affinity to certain proteins) is used as a denaturant. The three unfolding processes differ not only with respect to their cooperativity and the thermodynamic parameters but also with respect to the volume changes, suggesting structural differences of the denatured proteins. Using cooperative models thus yields significant new insights into the protein unfolding/folding processes.