Putting the Piezolyte Hypothesis under Pressure.

A group of small molecules that stabilize proteins against high hydrostatic pressure has been classified as piezolytes, a subset of stabilizing cosolutes. This distinction would imply that piezolytes counteract the effects of high hydrostatic pressure through effects on the volumetric properties of the protein. The purpose of this study was to determine if cosolutes proposed to be piezolytes have an effect on the volumetric properties of proteins through direct experimental measurements of volume changes upon unfolding of model proteins lysozyme and ribonuclease A, in solutions containing varying cosolute concentrations. Solutions containing the proposed piezolytes glutamate, sarcosine, and betaine were used, as well as solutions containing the denaturants guanidinium hydrochloride and urea. Changes in thermostability were monitored using differential scanning calorimetry whereas changes in volume were monitored using pressure perturbation calorimetry. Our findings indicate that increasing stabilizing cosolute concentration increases the stability and transition temperature of the protein, but does not change the temperature dependence of volume changes upon unfolding. The results suggest that the pressure stability of a protein in solution is not directly affected by the presence of these proposed piezolytes, and so they cannot be granted this distinction.