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Enzyme Kinetics of Hydrolytic Reactions / 1 7 : 4 



Many inhibitors react with the enzyme in such a manner that the 

 intermediate complex ES cannot be formed. An inhibitor of this type 

 is called a competitive inhibitor. Often, their structure is similar to that of 

 the normal substrate. For example, the enzyme succinic dehydrogenase 

 catalyzes the removal of hydrogen from succinic acid. The enzyme is 

 competitively inhibited by malonic acid. The structural formulas in 

 Figure 8a show the similarities of the normal substrate and its inhibitor. 



HO-C — O 



I 

 H-C— H 

 Succinic 



Acid H-C-H 



I 

 HO— C=0 



Normal 

 Substrate 



HO— C=0 



I 

 H-C— H 



HO-C=0 



Inhibitor 



Malonic 

 Acid 



(a) 



Products 0OO + E 



(b) 



Figure 8. (a) Similarity between structures of enzyme sub- 

 strate and competitive inhibitor, (b) Competitive inhibition 

 of the hydrolysis of normal substrate S by inhibitor S' . The 

 shapes chosen have no physical significance. 



The general scheme is presented in a very symbolic form in Figure 8b. 



A second group of inhibitors does not interfere with the formation of 

 the intermediate complex but blocks its hydrolysis or further reaction. 



