Pressure protein denaturation compared to thermal and chemical unfolding - Analyses with cooperative models

The thermodynamics of pressure induced protein unfolding (denaturation) could so far not be directly compared with protein unfolding induced by temperature or chemical agents. Here we provide a new cooperative model for pressure induced protein denaturation, that allows the quantitative comparison of the 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) a 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 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 not only differ with respect to their cooperativity and the thermodynamic parameters, but also with respect to the volume changes. Whereas thermal and chemical denaturation increase the protein volume, denaturation by pressure reduces the protein volume, suggesting significant structural differences of the denatured proteins. Using cooperative models for protein analyses thus yields significant new insights into the protein unfolding/folding processes.