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Solution


Q.

Which one of the following statements is incorrect   [2015]

 
1 The competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex  
2 The presence of the competitive inhibitor decreases the Km of the enzyme for the substrate.  
3 A competitive inhibitor reacts reversibly with the enzyme to form an enzyme-inhibitor complex.  
4 In competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme.  

Ans.

(2)

Competitive inhibition is a reversible inhibition where the inhibitor competes with the normal substrate for the active site of the enzyme. A competitive inhibitor is usually chemically similar to the normal substrate and therefore fits into the active site of an enzyme and binds with it. The inhibition is thus due to a substrate analogue. The enzyme now cannot act upon the substrate, and reaction products are not formed. E.g., the activity of succinate dehydrogenase is inhibited by malonate. Km value or Michaelis constant is defined as the substrate concentration at which half of the enzyme molecules are forming enzyme-substrate (ES) complex or concentration of the substrate when the velocity of the enzyme reaction is half the maximal possible. A smaller Km value indicates greater affinity of the enzyme for its substrate; hence, shows a quicker reaction. The competitive inhibitor decreases the affinity of the enzyme for the substrate, thus increases the Km value.