EFFECTS OF PRESSURE 841 



portant to prepare the media at the same ionic strengths to eliminate this 

 factor. Other factors should also be kept constant when possible. The 

 substrate concentration may determine the degree of buffer action; at 

 66.7 niM, phosphate inhibits urease 82% at a urea concentration of 2.2 mM 

 and only 19% when the urea is 33.2 mM (Kistiakowsky et al., 1952). 



EFFECTS OF PRESSURE 



Whenever any process is accompanied by a change in the total volume of 

 the components, pressure wnll exert some effect on the rate and equilibrium 

 of the process. The volume changes may be made up of two general parts: 

 one derived from the reactants proper and the other from the solvent wa- 

 ter. The simple equilibrium reaction: 



A-W„, + B-W„ ^ AB-Wj, + (m + w - p)W (15-104) 



where the subscripts indicate the number of water molecules associated 

 with each species, may involve a volume change which is given by the sum 

 of AV^, the volume change of the reactants (F^^ — Va ~ ^b)' ^-^^l AV^, 

 the volume change of the solvent water. If the total volume increases, high 

 pressures will shift the equilibrium to the left because they will oppose 

 the expansion; if the volume decreases, the equilibrium will be shifted to 

 the right at high pressures. The equilibrium constant will thus depend on 

 the pressure. In a like manner, if the complex AB represents the activated 

 state in the reaction of A and B to form products, the rate of the reaction 

 will be either slowed or accelerated depending on the volume change during 

 the formation of the activated complex. In this case, the rate constant de- 

 pends on the pressure. Studies of the effects of pressure on the rates and equi- 

 libria of reactions may be of value in elucidating the molecular mechanisms 

 and the changes in solvent structure. 



Molecular Basis for Volume Changes in Enzyme Reactions and Inhibition 



Let us first consider the volume changes related only to the reactants. 

 The bond lengths in the activated complex may be different from those in 

 the normal molecules. This is more apt to occur in chemical reactions than 

 in the simple binding or association of two substances, since in the former 

 situation the transition state may be characterized by unique electronic 

 configuration. Also in chemical reactions, the final products will contain 

 in most cases new bonds and new molecular configurations, so that the sum 

 of the product volumes will not necessarily be equal to the sum of the reac- 

 tant volumes. Furthermore, in simple association reactions, the two mole- 

 cules may not fit together accurately and the volume will be increased as 

 a result of the empty space between them. In addition, slight volume 

 changes may arise from the constriction of a reactant in the strong electric 



