EFFECTS OF PRESSURE 845 



Effects of Pressure on Enzyme Rates 



Some of the factors that may be involved in bringing about volume changes 

 during enzyme reactions have been discussed. There are a few additional 

 pressure effects that could be important in certain situations. The acidic 

 ionization constants for groups on either the enzyme or the substrate will 

 be altered by pressure increase. The dissociation of a proton from an 

 acidic group will lead to the appearance of two ionic charges which will 

 decrease the solvent volume. A rise in the pressure will thus favor ionization 

 and lead to a decrease in the pK^. On the other hand, the dissociation of a 

 basic group, such as — NH3+-> — NHg + H+, will not so significantly 

 alter the water structure and the \)KJs of such groups will not be changed 

 very much by increases in the pressure. It is evident that any changes in 

 the P-Sl^'s of groups at or near the active center may modify the binding of 

 the substrate or the rate of decomposition of the ES complex. 



At very high pressures, perhaps over 5000 atm, enzymes may be irrever- 

 sibly inactivated, and it is possible that even at lower pressures there 

 may be a shift in the equilibrium between native and denatured forms of 

 the enzyme. It is known that in moderate pressure ranges the rate and 

 extent of protein denaturation are reduced, indicating an increase in vol- 

 ume for the denaturation process, but that at higher pressures the dena- 

 turatio"n may be accelerated. It would be well in all pressure work to test 

 the reversibility of the effects to distinguish between the direct effects on 

 rate and equilibrium constants and the denaturation-type changes in the 

 enzyme structure. Actually, the water structure itself is changed to some 

 extent at high pressures, as shown in viscosity studies; for example, a rise 

 in the pressure to 1000 atm increases the viscosity about 5% (Moelwyn- 

 Hughes. 1957, p. 1234). It is not certain if such changes could exert sig- 

 nificant effects on enzyme reactions but in diffusion-limited systems it is 

 likely they could play a minor role. 



Examples of the effects of high pressures on enzyme rates are summar- 

 ized by Laidler (1958) and calculations of the volume changes for the 

 various steps are presented. In most cases it is not quite certain as to the 

 interpretation of the pressure data because the experiments were not done 

 at widely different substrate concentrations. All enzyme reactions studied 

 are sensitive to pressure changes and in some there are apparently very 

 large volume changes. In general, JT^,* is negative (— 5 to — 30 ml/mole), 

 indicating that the formation of the activated complex in the breakdown 

 of the ES complex to products involves a decrease in volume. High pres- 

 sures, therefore, usually increase k^ and accelerate the enzyme reaction, 

 unless the effects on /il,„ are such as to oppose this. The formation of the 

 ES complex can involve either increases or decreases in the volume, as 

 might be expected from the different factors that may be involved in the 

 binding. The most thorough study is probably that of muscle ATPase 



