Question

Which one of the following statements is incorrect?

• A. The competitive inhibitor does not affect the rate of breakdown of the enzyme-substrate complex
• B. The presence of the competitive inhibitor decreases the K${}_m$ of the enzyme for the substrate
• C. A competitive inhibitor reacts reversibly with the enzyme to form an enzyme-inhibitor complex
• D. In competitive inhibition, the inhibitor molecule is not chemically changed by the enzyme

Answer 1

The correct option is B The presence of the competitive inhibitor decreases the K${}_m$ of the enzyme for the substrate

Competitive inhibitors are structural analogues (having similar structure) of a substrate molecule. They compete for the same site(s) on the enzyme molecule that are normally occupied by the substrate molecules.
In the presence of inhibitors, the substrate molecules are not able to bind at the active sites of the enzymes. The inhibitors, in turn, bind reversibly with those active sites forming an enzyme-inhibitor complex. The complex formed is inactive as it cannot form product(s). If the enzyme-substrate is already formed, the competitive inhibitor cannot affect the rate of breakdown of the enzyme-substrate complex. That is why it is recommended to add more substrate molecules for overcoming competitive inhibition.

K${}_m$or the Michaelis-Menten constant refers to the concentration of substrate at which the reaction reaches half its maximum velocity. Due to the presence of competitive inhibitors, fewer substrate molecules are available to reach half of the maximum velocity of the reaction. In order to reach half the maximum velocity, more substrate molecules will be needed thereby increasing $K_m$ for the reaction.
Thus, the presence of competitive inhibitors will increase the $K_m$ of a reaction. The competitive inhibitors are not changed chemically by the enzyme during reversible interactions.