Claude Balny

The pressure behavior of proteins may be summarized as a the pressure-induced disordering of their structures. This thermodynamic parameter has effects on proteins that are similar but not identical to those induced by temperature, the other thermodynamic parameter. Of particular importance are the intermolecular interactions that follow partial protein unfolding and that give rise to the formation of fibrils. Because some proteins do not form fibrils under pressure, these observations can be related to the shape of the stability diagram. Weak interactions which are differently affected by hydrostatic pressure or temperature play a determinant role in protein stability. Pressure acts on the 2 degrees, 3 degrees and 4 degrees structures of proteins which are maintained by electrostatic and hydrophobic interactions and by hydrogen bonds. We present some typical examples of how pressure affects the tertiary structure of proteins (the case of prion proteins), induces unfolding (ataxin), is a convenient tool to study enzyme dissociation (enolase), and provides arguments to understand the role of the partial volume of an enzyme (butyrylcholinesterase). This approach may have important implications for the understanding of the basic mechanism of protein diseases and for the development of preventive and therapeutic measures.

We have studied the reaction native ⇆ denatured for the 33‐kDa protein isolated from photosystem II. Sucrose and glycerol have profound effects on pressure‐induced unfolding. The additives shift the equilibrium to the left; they also cause a significant decrease in the standard volume change (ΔV). The change in ΔV was related to the sucrose and glycerol concentrations. The decrease in ΔV varied with the additive: sucrose caused the largest effect, glycerol the smallest. The theoretical shift of the half‐unfolding pressure (P_1/2) calculated from the net increase in free energy by addition of sucrose and glycerol was lower than that obtained from experimental mea‐ surements. This indicates that the free energy change caused by preferential hydration of the protein is not the unique factor involved in the protein stabilization. The reduction in ΔV showed a large contribution to the theoretical P_1/2 shift, suggesting that the ΔV change, caused by the sucrose or glycerol was associated with the protein stabilization. The origin of the ΔV change is discussed. The rate of pressure‐induced unfolding in the presence of sucrose or glycerol was slower than the refolding rate although both were significantly slower than that observed without any stabilizers.