CHAPTER 14 



EFFECTS OF pH ON ENZYME INHIBITION 



The degree of enzyme inhibition frequently varies with the pH and this 

 would be expected inasmuch as the concentrations and constants in the 

 general inhibition Eq. 3-11 may all be pH-dependent. It might also be in- 

 ferred from the fact that enzymes are proteins and hence possess ionizing 

 groups which determine in part the pattern of electrical charge over the 

 enzyme surface and regulate the extent to which the enzyme interacts 

 with any substance, whether substrate, activator, coenzyme, or inhibitor. 

 The observed variations in enzyme activity with pH, either in the absence 

 or presence of inhibitors, provide data that may be used to demonstrate 

 ionizing groups at the active center, and often allow the acid dissociation 

 constants of these groups to be calculated. Knowledge of these constants 

 makes it possible to speculate on the nature of the ionizing groups and a 

 great deal of what is known about the properties of enzyme active centers 

 has been derived from studies of the effects of pH. Most of the investigations 

 have been concerned with the changes in enzyme activity produced by pH 

 variations and relatively little has been done with inhibitors. However, 

 for the purpose of characterizing ionizing groups at the active center, in- 

 hibition often provides more readily interpretable data than activity stud- 

 ies because the changes in the inhibition constant, K^, are directly re- 

 lated to the binding of the inhibitor to the enzyme, whereas in the case of 

 the substrate one must also consider the possibility that pH affects the rate 

 of breakdown of the ES complex and that the Michaelis constant, K„^, 

 may not be a simple dissociation constant. Just as it is easier to determine 

 directly K^ than Kg, so it is often more convenient to interpret changes 

 in Kj^ than in K^^. It should be realized that in any case the effects of pH 

 may be very complex and that the complexities increase rapidly as one 

 proceeds from single pure enzymes to multienzyme systems and eventually 

 to cells. It is true that many of these effects of pH are at present beyond 

 our powers of analysis. Nevertheless, if one begins with simple enzymes 

 and progresses to more complex systems, one finds that some general prin- 

 ciples appear and that, indeed, pH studies can provide a great deal of in- 

 formation on enzyme and inhibition mechanisms. 



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