VARIATION OF ENZYME INHIBITION WITH pH 



675 



of HEI it can also be formed from E and I. Of course, when I can com- 

 bine with both E and HE, a scheme such as: 



■^ 



HE 



El 



^ 



HES -> HE + P 



HEI 



(14-71] 



must be set up. If I combines with both forms equally, there is no effect 

 of the pH on the inhibition; if the affinities of E and HE for I differ, the sys- 

 tem will behave in an intermediate fashion between a pH-independent 

 system and the systems discussed above. If the inhibitor is a charged ion, 

 a marked difference in binding to E and HE will be the most likelv. 



0.8- 



0.6 



0.4- 



0.2 - 



01 



S(mM)- 



10 



100 



1000 



Fig. 14-9. Variation of the inhibition with the substrate concentration at diiJerent 

 pH's for case IVc (Eq. 14-61 ). K^ = 1 mJ/, A% = 10 mM, K^ = 10-^ and (I) = 3 ml/. 



Some further discussion must be given to terminology in systems such 

 as 14-71. This reaction scheme is equivalent to: 



E 



A-^ HE ,,- 

 ^EI 



HEI 



(14-72) 



which is usually described by stating tliat the inhibitor can combine with 

 both ionization forms of the enzvme. It is also equivalent to: 



,^ 



(14-73) 



HE 



E 



^ 



HEI 



EI "Ao 



