VARIATION OF ENZYME INHIBITION WITH pH 



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is a supply of E, the inhibition will not be as great as would be expected 

 from typical competitive inhibition or as great as it is at a lower pH. 

 The other question that might reasonably be asked is: why does the sub- 

 strate concentration affect the noncompetitive inhibitions in (c)-(f)? The 

 answer lies again in the coupled equilibria. As (S) increases, it tends to 

 deplete the inactive form of the enzyme and it is this that modifies the 

 response to the inhibitor, by a mechanism similar to that described above. 

 It will be worthwhile to illustrate some of the asjoects of these systems 

 by plotting the variation of the inhibition with pH or (S). All six types 

 of inhibition are compared in Fig. l-t-T for constant (I) and (S) and the 



Fig. 14-7. Variation of the inhibition with the pH for the six possible 

 situations in which only the enzyme active center ionizes (case IV). The 

 letters on the curves correspond to those in the text. A', = 1 mil/, K^ = 10 



marked pH dependence may be seen in the region extending about 2 pH 

 units on either side of the i>K^ for the enzyme. Figure 14-8 shows the pH 

 effects on the inhibition at different substrate concentrations for system 

 (a), i.e., competitive inhibition with HE, the active from of the enzyme. 

 Here it may be observed that the pH region over which the inhibition is 

 independent of pH varies with (S). This means that the greater the enzyme 



