The thermodynamic analysis of protein stabilization by sucrose and glycerol against pressure-induced unfolding

We have studied the reaction native left arrow over right arrow 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 (DeltaV). The change in DeltaV was related to the sucrose and glycerol concentrations. The decrease in DeltaV varied with the additive: sucrose caused the largest effect, glycerol the smallest. The theoretical shift of the half-unfolding pressure (P1/2) 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 DeltaV showed a large contribution to the theoretical P1/2 shift, suggesting that the DeltaV change, caused by the sucrose or glycerol was associated with the protein stabilization. The origin of the DeltaV 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.