Inhibitors

Inhibitors MDCAT MCQs with answers

30 past paper MCQs on Inhibitors, from the Enzymes unit of MDCAT Biology. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.Competitive inhibitors compete with the: (MDCAT 2022)

  1. substrate
  2. enzyme
  3. product
  4. coenzyme
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Correct answer: (a) substrate

They resemble the substrate and take its place in the active site.

2.Competitive inhibitors are: (MDCAT 2023)

  1. homologous to the substrate
  2. analogous to the substrate
  3. smaller than the substrate
  4. larger than the substrate
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Correct answer: (b) analogous to the substrate

They resemble the substrate closely enough in shape to occupy its active site.

3.Reversible inhibitors differ from Irreversible Inhibitors because they: (UHS MDCAT 2025)

  1. Binds permanently and cannot be removed
  2. Bind temporarily and can be removed
  3. Permanently inactivate the enzyme
  4. Change enzyme structure permanently
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Correct answer: (b) Bind temporarily and can be removed

A reversible inhibitor is held by weak bonds and lets go, so the enzyme recovers.

4.A substance that binds to an enzyme, but NOT at the active site and reduces the enzyme activity is called: (KMU MDCAT 2025)

  1. Competitive inhibitor
  2. Substrate
  3. Non-Competitive inhibitor
  4. Cofactor
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Correct answer: (c) Non-Competitive inhibitor

A non-competitive inhibitor binds elsewhere and distorts the active site from a distance.

5.Reversible inhibitors affect the enzyme catalyzed reaction by: (NUMS MDCAT 2025)

  1. Forming covalent bonds with active site
  2. Destroying the globular structure
  3. Forming weak linkages with enzymes
  4. Activating the catalytic sites
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Correct answer: (c) Forming weak linkages with enzymes

A reversible inhibitor is held by weak bonds and can be washed out, leaving the enzyme unharmed.

6.Which of the following is a characteristic of a non-competitive enzyme inhibitor? (SIBA MDCAT 2025)

  1. Binds to the enzyme active site
  2. Can be overcome by increasing substrate concentration
  3. Increases the speed of the reaction
  4. Point of action is allosteric site
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Correct answer: (d) Point of action is allosteric site

A non-competitive inhibitor binds at an allosteric site and cannot be displaced by more substrate.

7.Which of the following statement is incorrect with reference to the enzyme action? (SZABMU MDCAT 2025)

  1. Substrate binds with the enzyme at its active site
  2. Non-competitive inhibitor binds the enzyme at a site distinct from that of active site
  3. Addition of a lot of succinate do not reverse the inhibition of succinic dehydrogenase by malonate
  4. malonate is a competitive inhibitor of succinic dehydogenase
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Correct answer: (c) Addition of a lot of succinate do not reverse the inhibition of succinic dehydrogenase by malonate

Malonate is a competitive inhibitor, so flooding the enzyme with succinate does reverse it.

8.Which competitive inhibitor blocks the enzyme responsible for bacterial cell wall synthesis? (SZABMU MDCAT 2025)

  1. Sulphonamide
  2. Streptomycin
  3. Tetracycline
  4. Penicillin
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Correct answer: (d) Penicillin

Penicillin competes with the substrate of the enzyme that cross links the bacterial cell wall.

9.Enzymatic activity can be inhibited by? (KMU MDCAT 2024)

  1. Heavy metal ions
  2. Methane
  3. Mutase
  4. Noble gases
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Correct answer: (a) Heavy metal ions

Heavy metal ions bind the sulphydryl groups of the enzyme and destroy its shape.

10.A competitive inhibitor: (KMU MDCAT 2024)

  1. Accelerates the chemical reaction
  2. Competes with the enzyme
  3. Is irreversible
  4. Is reversible
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Correct answer: (d) Is reversible

A competitive inhibitor merely occupies the active site for a while and can be displaced by more substrate.

11.In competitive inhibition, the inhibitor (KMU MDCAT 2024)

  1. Binds with substrate
  2. Competes with enzyme
  3. Competes with substrate
  4. Irreversibly binds with enzyme
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Correct answer: (c) Competes with substrate

A competitive inhibitor resembles the substrate and takes its place in the active site.

12.Competitive inhibitors compete with? (UHS MDCAT 2023)

  1. Enzyme
  2. Substrate
  3. Product
  4. Coenzyme
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Correct answer: (b) Substrate

It resembles the substrate and takes its place in the active site, so the two compete.

13.Non-competitive inhibitor molecules have: (UHS MDCAT 2023)

  1. A similar structure to the normal substrate molecule
  2. A quite different structure from the substrate molecule
  3. A different conformation but fit into the active site
  4. A similar conformation but does not fit into the active site
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Correct answer: (b) A quite different structure from the substrate molecule

It bears no resemblance to the substrate and binds elsewhere, at an allosteric site.

14._____ reduces the enzyme activity by blocking the substrate from entering the active site due to similar shapes: (SZABMU MDCAT 2023)

  1. Competitive inhibitors
  2. Non-competitive inhibitors
  3. Co-enzymes
  4. Activators
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Correct answer: (a) Competitive inhibitors

A competitive inhibitor resembles the substrate closely enough to sit in the active site.

15.Non-competitive inhibitors react with enzymes at: (NUMS MDCAT 2023)

  1. Allosteric site
  2. Active site
  3. Passive site
  4. Regulatory site
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Correct answer: (a) Allosteric site

A non-competitive inhibitor binds at an allosteric site away from the active site.

16._______ reduce the enzymes productivity by blocking the substrate from entering into the active site due to similar shapes: (NUMS MDCAT 2023)

  1. Competitive inhibitors
  2. Non-competitive inhibitors
  3. Co-enzymes
  4. Activators
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Correct answer: (a) Competitive inhibitors

A competitive inhibitor resembles the substrate and occupies the active site.

17.Competitive inhibitors stop an enzyme from working by: (UHS MDCAT 2023)

  1. Changing the shape of the enzyme
  2. Merging with the substrate instead
  3. Blocking the active site of the enzyme
  4. Combining with the product of the reaction
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Correct answer: (c) Blocking the active site of the enzyme

A competitive inhibitor takes the place of the substrate in the active site.

18.Competitive inhibhnors are ______________? (UHS MDCAT 2023)

  1. Homologous to substrate
  2. Analogous to substrate
  3. Smaller than substrate
  4. Larger than substrate
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Correct answer: (a) Homologous to substrate

A competitive inhibitor is close enough in shape to the substrate to sit in the active site.

19.Inhibitors which block the enzyme by forming weak bonds are called? (PMC MDCAT 2022)

  1. Competitive inhibitors
  2. Non-competitive inhibitors
  3. Irreversible Inhibitors
  4. Reversible inhibitors
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Correct answer: (d) Reversible inhibitors

Weak bonds let go again, so the inhibition is reversible.

20.lnhibiting enzymatic activity by blocking its active site is: (PMC MDCAT 2022)

  1. Competitive inhibition
  2. Non-competitive inhibition
  3. Feedback inhibition
  4. Allosteric inhibition
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Correct answer: (a) Competitive inhibition

A competitive inhibitor takes the place of the substrate in the active site.

21.Which of the following react with enzyme at other than active site? (PMC MDCAT 2022)

  1. Activator
  2. Coenzymes
  3. Non-Competitive Inhibitor
  4. End-Product Inhibitor
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Correct answer: (c) Non-Competitive Inhibitor

A non-competitive inhibitor binds at an allosteric site elsewhere on the molecule.

22.Which of the following may attach on active site of an enzyme? (PMC MDCAT 2022)

  1. Products
  2. Coenzyme
  3. Activator
  4. Inhibitor
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Correct answer: (d) Inhibitor

A competitive inhibitor is what fits the active site itself.

23.In non-competitive inhibition, the extent of inhibition depends only on? (PMC MDCAT 2022)

  1. Concentration of enzyme
  2. Concentration of substrate
  3. Concentration of ES complex
  4. Concentration of Inhibitor
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Correct answer: (d) Concentration of Inhibitor

More substrate cannot displace it, so only how much inhibitor there is matters.

24.Which option is correct about non-competitive inhibition: (NUMS MDCAT 2022)

  1. Malonate act as inhibitor
  2. Enzyme shape distorted
  3. Competition for active site
  4. Inactive enzyme getting reactivated
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Correct answer: (b) Enzyme shape distorted

A non-competitive inhibitor binds elsewhere and distorts the shape of the active site.

25.Which of the following statements is incorrect? (PMC Practice 2021)

  1. The competitive inhibitor affects 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
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Correct answer: (b) The presence of the competitive inhibitor decreases the Km of the enzyme for the substrate

A competitive inhibitor raises the apparent $K_m$, since more substrate is needed to reach half the maximum rate.

26.An example of competitive inhibition of an enzyme is the inhibition of; (UHS MDCAT 2019)

  1. Succinic dehydrogenase by malonic acid
  2. Cytochrome oxidase by cyanide
  3. Hexokinase by glucose
  4. Carbonic anhydrase by carbon dioxide
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Correct answer: (a) Succinic dehydrogenase by malonic acid

Malonic acid resembles succinic acid closely enough to occupy the active site of the dehydrogenase.

27.A student of chemical engineering mistakenly engulfed the toxic compound “A” which was a potent inhibitor of a certain enzyme. He was immediately brought to a hospital where the doctor injected, intravenously, substrate “B” to minimize the toxic effect of Compound A. His life was saved from serious damage. The treatment method shows that compound A was a ______ inhibitor. (UHS MDCAT 2019)

  1. Non competitive
  2. Irreversible
  3. Temperature sensitive
  4. Competitive reversible
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Correct answer: (d) Competitive reversible

Flooding the enzyme with substrate displaced the inhibitor, which is the mark of competitive reversible inhibition.

28.If the molecule can bind to another site of the enzyme rather than the true active site, it is referred to as ______ (UHS MDCAT 2018)

  1. Competitive inhibitors
  2. Allosteric inhibition
  3. Noncompetitive inhibitors
  4. Irreversible inhibition
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Correct answer: (c) Noncompetitive inhibitors

A molecule binding away from the active site is a non-competitive inhibitor.

29.Name the inhibitors which can be neutralized by adding more substrate: (UHS MDCAT 2017)

  1. Competitive Inhibitors
  2. Permanent inhibitors
  3. Non-competitive inhibitors
  4. Irreversible inhibitors
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Correct answer: (a) Competitive Inhibitors

A competitive inhibitor is displaced from the active site when the substrate is in excess.

30.Which one of the following is an example of competitive inhibitor? (UHS MDCAT 2015)

  1. Glucose
  2. Fumerate
  3. Succinic Acid
  4. Melonate
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Correct answer: (d) Melonate

Malonate resembles succinate closely enough to occupy the active site of the dehydrogenase.

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