Free Enzyme Inhibitors MCQs with Answers

1. A competitive inhibitor slows an enzyme catalysed reaction by

• A. binding the active site because it resembles the substrate
• B. binding a site away from the active site and distorting it
• C. permanently destroying the enzyme
• D. lowering the pH of the surroundings

Explanation: A competitive inhibitor is structurally similar to the substrate and occupies the active site, so substrate and inhibitor compete for the same place. Because the competition depends on relative concentrations, adding more substrate overcomes the inhibition and Vmax is unchanged. The description in the second option is non-competitive inhibition, which binds an allosteric site instead.

2. Increasing the substrate concentration will NOT relieve the effect of

• A. a competitive inhibitor
• B. a non-competitive inhibitor
• C. a low enzyme concentration
• D. a substrate below the saturation point

Explanation: A non-competitive inhibitor binds at a site other than the active site and changes the enzyme's shape, so the substrate cannot compete it away no matter how much is added and Vmax is genuinely reduced. Competitive inhibition, by contrast, is overcome by flooding the system with substrate. This difference is the standard way of telling the two apart experimentally.

3. Malonate inhibits succinate dehydrogenase because it resembles succinate. This is an example of

• A. non-competitive inhibition
• B. competitive inhibition
• C. feedback inhibition
• D. denaturation

Explanation: Malonate is close enough in structure to succinate to occupy the same active site without being converted, which is the definition of competitive inhibition, and it is the textbook example. The inhibition can be reversed by raising the succinate concentration. Feedback inhibition is a regulatory mechanism in which the end product of a pathway inhibits an earlier enzyme, usually allosterically.

4. Heavy metal ions such as mercury and silver inhibit many enzymes irreversibly because they

• A. compete with the substrate for the active site
• B. raise the pH of the solution
• C. supply the enzyme with excess energy
• D. bind to the sulphydryl groups of the protein and destroy its shape

Explanation: These ions attack the sulphur containing side chains, breaking disulphide bridges and permanently distorting the tertiary structure, so the damage cannot be reversed by removing the inhibitor or adding more substrate. This is essentially poisoning rather than regulation, and it is why mercury contamination is so dangerous. A competitive inhibitor, by contrast, binds reversibly at the active site.

5. In feedback inhibition of a metabolic pathway

• A. the final product inhibits an enzyme acting early in the pathway
• B. the first substrate inhibits the last enzyme
• C. the enzyme is destroyed after each reaction
• D. the pathway runs faster as product accumulates

Explanation: When enough product has been made it binds an allosteric site on an early enzyme and switches it off, so the cell stops spending resources on something it already has, and production restarts when the product is used up. This is negative feedback applied to metabolism and is the standard way biosynthetic pathways are regulated. Inhibiting an early step also prevents intermediates from piling up.

6. An allosteric site on an enzyme is

• A. a second active site that binds the same substrate
• B. a site away from the active site where a regulatory molecule binds and changes the enzyme's shape
• C. the region that is denatured first by heat
• D. the point where the enzyme attaches to the cell membrane

Explanation: A molecule binding at an allosteric site alters the conformation of the whole enzyme, and therefore the shape of the active site, either activating or inhibiting it. Non competitive inhibition and feedback inhibition both work this way, which is why adding more substrate cannot overcome them. The site is chemically distinct from the active site and binds a different molecule.

7. Compared with the uninhibited enzyme, a non competitive inhibitor

• A. raises Vmax and leaves Km unchanged
• B. raises both Vmax and Km
• C. lowers Km and leaves Vmax unchanged
• D. lowers Vmax and leaves Km unchanged

Explanation: Because the inhibitor binds elsewhere and cannot be displaced by substrate, a fraction of the enzyme is permanently out of action, so the maximum achievable rate falls while the affinity of the remaining active sites for the substrate is unaltered. A competitive inhibitor shows the opposite pattern, an unchanged Vmax with an apparently raised Km. This pair of effects is the standard way of telling the two kinds apart in the laboratory.

8. Competitive inhibitors

• A. Bind to allosteric site
• B. Cannot be overcome by increasing substrate
• C. Bind to active site
• D. Change Vmax but not Km

Explanation: Competitive inhibitors bind to the active site, competing with substrate. They can be overcome by increasing substrate concentration.

9. The Michaelis constant (Km) represents

• A. Maximum reaction velocity
• B. Substrate concentration at half Vmax
• C. Enzyme concentration
• D. Inhibitor concentration

Explanation: Km is the substrate concentration at which the reaction velocity is half of Vmax. It indicates enzyme-substrate affinity.

10. Allosteric enzymes

• A. Follow Michaelis-Menten kinetics strictly
• B. Have multiple subunits and regulatory sites
• C. Are not affected by feedback inhibition
• D. Always show competitive inhibition

Explanation: Allosteric enzymes have multiple subunits with regulatory (allosteric) sites where activators or inhibitors bind, affecting enzyme conformation.