ENZYMES 143 



of maximum rate. A shift of pH away from the optimum in either 

 direction produces a reduction in rate. The effects due to changes in 

 pH can be explained in particular cases but are usually too complex 

 for complete description. Hence, it is more appropriate to consider 

 generally the factors that can play a part and definitely do in particular 

 instances. 



In the first place, changes in pH modify the structure of the enzyme 

 by adding protons to or removing them from the polar groups of the 

 enzyme molecule. A change in a polar group affects the entire inter- 

 dependent electronic structure of the molecule and alters the catalytic 

 activity. If the group titrated is not directly involved in the reaction, 

 the change in rate will be relatively small. However, when the group 

 titrated is fundamental to the reaction, perhaps as the site of the 

 linkage of substrate to enzyme, the effect of change in pH is marked. 

 It is believed that there is a preferred form for any polar group 

 directly involved. For example, when a histidine unit is the site of 

 activity, pH changes around neutrality should produce dramatic 

 changes in enzymatic activity, since the imidazole group of histidine 

 is half charged, half uncharged at about pH 6. 



A still more general effect of pH on enzyme activity occurs at more 

 extreme values of pH. Most enzymes are inherently unstable in 

 strongly acidic or alkaline solutions and denature. Since reaction rate 

 depends on enzyme concentration, denaturation of enzyme auto- 

 matically affects the rate. At the point where the effect of denatura- 

 tion overcomes any existing compensating gains in rate arising from 

 substrate effects and the like, the overall reaction begins to fall. 



There are a few exceptional cases of enzyme stability at extremes 

 of pH. Pepsin, for instance, has an optimum activity in the range 

 of 1.5 to 2.0 and is rather stable in this region at ordinary tempera- 

 tures. On the other hand, pepsin is unusually sensitive to alkali and 

 is almost completely inactivated at pH 8.0. It may be argued that 

 pepsin is fitted for activity in the very acid environment of the 

 stomach and that evolution has selected a molecular type suitable for 

 the situation. 



In addition to changes of the enzyme, a shift in pH may also modify 

 the substrate. A large majority of the substrates for enzymes possess 

 one or more ionizable groups. Any changes in the ionic states of these 

 groups affect not only the basic stability of the substrate molecule 

 but also the interaction of substrate and enzyme to form the reactive 

 complex. In the known cases only one form of substrate shows much 

 tendency to combine with the enzyme and undergo reaction. Thus 

 the equation of page 138 cannot be used for rate comparisons at 



