VARIATION OF ENZYME INHIBITION WITH pH 667 



VARIATION OF ENZYME INHIBITION WITH pH 



The mechanisms responsible for the alterations in enzyme inhibition 

 when the pH is changed are generally similar to those outlined with re- 

 spect to enzyme activity (page 653). The fundamental effects are almost all 

 mediated through the modification in the binding of the inhibitor to the 

 enzyme (or to any other components of the system to which it may be 

 bound) and these effects arise from changes in the ionizations of groups on 

 any of the interacting molecules. In one respect, the interpretation of the 

 results with inhibitors is more straightforward than in studies of enzyme 

 activity: the experimentally determined or apparent inhibition constant 

 {K,') is either the true dissociation constant (Ki) for the EI complex or 

 is related to it in a simple manner by the proper pH functions. It is also 

 true that certain factors that alter the enzyme activity when the pH is 

 changed will not necessarily alter the degree of inhibition if the activity is 

 changed equally in the absence and presence of the inhibitor. A typical 

 curve for the variation of inhibition with pH may be obtained from the 

 data of Stockell and Smith (1957) for the inhibition of papain by carbo- 

 benzoxy-L-glutamate (Fig. 14-6). The decrease in the inhibition with rising 

 pH was attributed to the ionization of the y-carboxyl group, since its 

 pK^ is about 4.4. 



Expressions for the pH Dependence of Enzyme Inhibition 



Inasmuch as a complete and rigorous treatment of enzyme activity as 

 modified by pH is too elaborate to be practical, it is obvious that such a 

 treatment of inhibition, which is yet more complex, would be futile. Fur- 

 thermore, it is likely that most real inhibitions fall into rather restricted 

 categories and that a general treatment would seldom be applicable. Thus 

 we shall consider various simple systems, from which principles that may 

 be applied to more complex inhibitions will emerge. In any inhibited en- 

 zyme system, there will be at least three substances that may ionize — the 

 enzyme (active center), the substrate, and the inhibitor. In addition, the 

 ionization of the various complexes with the enzyme must be taken into 

 account when they occur. 



Case I: only the inhibitor ionizes. When the only component of the system 

 ionizing within the pH range studied is the inhibitor, and if mutual deple- 

 tion of the enzyme and inhibitor does not occur, the treatment is very sim- 

 ple. In every case, the experimentally determined or apparent inhibitor 

 constant, K,', must be replaced with f^K^, where /, is the appropriate pH 

 function for the form of the inhibitor that is involved and K, is the true 

 inhibitor constant. Thus for the competitive inhibition where only HI 

 binds to the enzyme: 



