674 



14. EFFECTS OF pH ON ENZYME INHIBITION 



fraction in tlie HES form, the less able is a rise in idH by dissociating HE 

 to form the inactive E with which the inhibitor does not cond^ine. Finally, 

 it is interesting to see how the noncompetitive inhibitions (c)-(f) vary with 

 (S) and this is shown in Fig. 14-9 for type (c). Type (d) would have the pH 

 values on the curves reversed; type (e) would differ from this in that the 

 inhibition would decrease with rising (S); and type (f) would be like type 

 (e) except that the pH values would be as in the figure. The figure shows 

 very well how a rise in (S) actually increases the inhibition when there is 

 noncompetitive inhibition on the active form of the enzyme. 



0.8- 



0.6 



0.4- 



iO.2- 



pH- 



8 



Fig. 14-8. Variation of the inhibition witli the pH at different 



concentrations of substrate for case IVa (Eq. 14:-55). K^ = 1 mM, 



K, = 10 mM, K^ = 10-^ and (I) = 3 miH. 



Some problems are presented by these systems in the determination of 

 the inhiljitor constant by the usual graphical methods and these will be 

 considered after several more inhibition systems have been i^resented. 



A question may be raised as to wdiy in the above formulations no consid- 

 eration was taken of the ionization of the complexes HES or HEI. It was 

 assumed that S or I combined only wnth HE in these cases and not with E. 

 Thus the dissociation of a xH'oton from the HES or HEI complexes would 

 produce ES or EI, which cannot exist, since if EI can arise by dissociation 



