Pressure denaturation of proteins

The study of pressure effects on protein stability has occupied a relatively marginal position in the field of protein folding, with very few thorough thermodynamic, structural and kinetic studies of this phenomenon. Moreover, theoretical treatment of the issue with a few recent exceptions, has been limited to declarations of its complexity and lack of concordance with the results from other approaches. This paucity of data and theory notwithstanding, understanding the fundamental physical basis for pressure effects on proteins is essential to progress in the field of protein folding. Moreover, pressure presents certain advantages as a perturbation methodology that render it an important, useful and complementary approach. In the present review, the issue of the fundamental basis for the effects of pressure is discussed. Reference is made to studies in the literature, but I have concentrated the detailed presentation on the body of work in pressure-induced protein unfolding carried out by my research group and collaborators on staphylococcal nuclease (Snase) over the past 5 years. The origins of the value of the change in volume upon unfolding must be understood prior to any thorough theoretical analysis of pressure effects. The various arguments for the multiple contributing factors are discussed and then recent studies from my research group designed to probe this question are presented, the overall conclusion being that the existence of packing defects in the folded structure represents the most likely candidate for the negative change in volume upon unfolding. Moreover, the results of the temperature dependence of the volume change for unfolding of Snase implicate the difference in thermal expansivity in the temperature dependence of the value of the volume change of unfolding. Next I present results of a characterization of the physical properties of the pressure denatured state of Snase, and compare these to studies on a number of other pressure denatured proteins. Finally, the results of a series of pressure-jump kinetic studies on the folding/unfolding reactions of this protein are discussed. It is too early to conclude whether the results from these pressure studies on Snase stability and their interpretations are general. For this, many more studies on a number of small, reversibly folding proteins will be required.