17 : 4/ Enz/me Kinetics of Hydrolytic Reactions 



329 



The action of heavy metallic ions on many enzyme systems is an example 

 of noncompetitive inhibition. This reaction scheme is presented 

 diagrammatically in Figure 9. It is pictorially clear that noncompeti- 

 tive inhibitions are more complex than competitive ones. 



Accordingly, only the competitive inhibitors will be analyzed here. 

 Either the normal substrate S or the inhibitor S' may react with the 

 enzyme E, but not both of them. If x, the concentration of S, is much 



Figure 9. Noncompetitive inhibition of the hydrolysis of the 

 normal substrate S by the inhibitor S'. The shapes chosen 

 have no physical significance. 



greater than x', the concentration of S', the reaction must proceed as 

 if S' were not there. Thus, no matter how large the value of x', by 

 choosing a sufficiently large value of x it is possible to obtain the un- 

 inhibited maximum velocity. 



Stoichiometrically, a competitively inhibited Michaelis-Menten 

 reaction can be represented by 



e-p-p' x' *,' p' 



E +S'^±E-S' 



e-p-p' x Aj p 



E +S^±E-S 



E-SJ-E+ Products 



