Proteins volume changes under pressure

Frye KJ, Royer CA. Probing the contribution of internal cavities to the volume change of protein unfolding under pressure. Protein Sci. 1998 7 2217-2222. 

The partial molar volume is a thermodynamic quantity that plays an essential role in the analysis of pressure effects on chemical reactions, reaction rate as well as chemical equilibrium in solution. In the field of biophysics, the pressure-induced denaturation of protein molecules has continuously been investigated since an egg white gel was observed under the pressure of 7000 atmospheres [60]. The partial molar volume is a key quantity in analyzing such pressure effects on protein conformations When the pressure in increased, a change of the protein conformation is promoted in the direction that the partial molar volume reduces. A considerable amount of experimental work has been devoted to measuring the partial molar volume of a variety of solutes in many different solvents. However, analysis and interpretation of the experimental data are in many cases based on drastically simplified models of solution or on speculations without physical ground, even for the simplest solutes such as alkali-halide ions in aqueous solution. Matters become more serious when protein molecules featuring complicated conformations are considered. 

The few investigations reported on proteins under pressure (cf, e.g., the values for ribonuclease. Table 11) showed that the volume effects were insignificant or rather small, at least for pressures below 50 MPa. Recently, volume changes associated with temperature and pressure unfolding has been observed in the case of staphylococcal nuclease [2001S1]. 

Osmotic stress measures forces corresponding to pressures over six decades, from 0.01 to 10,000 atm. With X-ray diffraction by ordered arrays, as shown in Figure 1, molecular dimensions and intermolecular spacings can be obtained with accuracies often better than 0.2 A reproducibility. Alternatively, with probes of ionic channel conductance or of protein activity, the behavior of single molecules can be observed and the osmotically sensitive part of the underlying structural transformation can be extracted. Consequently, the change in the volume of associated water as the system goes between active and inactive forms can also be extracted. 

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