694 



14. EFFECTS OF pH ON ENZYME INHIBITION 



Table 14-2 

 Values of the Experimentally Determined pK/ for Inhibitors of Fumarase 



AT Different pH's " 



° Values estimated from the curves given by Massey (1953 b). 



tive potencies were estimated from these i:)K/'s, these would vary markedly 

 with the pH chosen, even for the inhibitors which presumably inhibit in 

 the same manner. It is generally true that the highest value of the p/ii / ob- 

 served is the safest to use for comparative purposes since the effect of pH 

 change is always to reduce the apparent pK/ relative to p.K'j. The results 

 on fumarase, however, are not available for pH's lower than 5.7 and hence 

 the maximal pK/'s cannot be determined for all the inhibitors. 



Variation of the True Inhibitor Constant with pH 



The treatment up to this point has usually been based on the assumption 

 that certain ionization states of the enzyme or the inhibitor are active and 

 that the true inhibitor constant, K,, exi)resses the affinity between these 

 active forms. The apparent inhibitor constant, K,', has differed from jK^j 

 because the concentrations of these active forms vary with the pH. In the 

 reaction, HE+ + I~ ^ HEI, it has been assumed that the Kj for this com- 

 plex is independent of the pH and that changes in the inhibition brought 

 about by varying the pH are caused by changes in (I^) or (HE+). The Dix- 

 on treatment is also based primarily on this assumption. Actually, it is 

 quite probable that the affinity of the enzyme for the inhibitor can also alter 

 with pH and that K^ itself will not remain constant. Enzymes presumably 

 possess ionizing groups surrounding the active center. These groups would 

 not particix^ate directly in the binding of the substrate or the inhibitor, 

 nor would they necessarily play any role in the catalysis, but alterations of 

 the charge on these groups might well significantly affect the binding of 



