Enzyme inhibitors are central to pharmacology, affecting drug action, metabolism and therapeutic outcomes for B. Pharm students. This clear, exam-focused introduction covers major types — competitive, noncompetitive, uncompetitive, mixed, irreversible and mechanism-based (suicide) inhibitors — and links them to clinical and biochemical examples such as statins, methotrexate, aspirin, penicillin, allopurinol and organophosphates. You will learn how inhibitors change enzyme kinetics (Km, Vmax), influence drug interactions, and produce adverse effects relevant to pharmacy practice. These focused insights bridge theory with drug examples and clinical implications, enhancing both exam readiness and practical understanding. You will practice mechanism identification, predict kinetic changes and recognize clinical consequences and adverse effects—essential skills for exams and clinical pharmacy practice. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which type of enzyme inhibitor increases the apparent Km but does not change Vmax?
- Noncompetitive inhibitor
- Competitive inhibitor
- Uncompetitive inhibitor
- Irreversible inhibitor
Correct Answer: Competitive inhibitor
Q2. Which inhibitor decreases Vmax without changing Km in ideal conditions?
- Competitive inhibitor
- Noncompetitive inhibitor
- Substrate analogue
- Allosteric activator
Correct Answer: Noncompetitive inhibitor
Q3. Which inhibition type typically lowers both Km and Vmax and is observed when an inhibitor binds only to the enzyme-substrate complex?
- Competitive inhibition
- Uncompetitive inhibition
- Mixed inhibition
- Irreversible inhibition
Correct Answer: Uncompetitive inhibition
Q4. Aspirin irreversibly inhibits cyclooxygenase (COX) by acetylation. What class of inhibition is this?
- Reversible competitive
- Mechanism-based (irreversible)
- Uncompetitive
- Noncompetitive reversible
Correct Answer: Mechanism-based (irreversible)
Q5. Methotrexate inhibits dihydrofolate reductase (DHFR). It is best classified as which type?
- Noncompetitive inhibitor
- Competitive inhibitor
- Suicide inhibitor
- Allosteric inhibitor
Correct Answer: Competitive inhibitor
Q6. Allopurinol is often described as a mechanism-based inhibitor of xanthine oxidase. Which phrase best describes this?
- Reversible competitive antagonist
- Suicide (irreversible) inhibitor
- Noncompetitive reversible inhibitor
- Allosteric activator
Correct Answer: Suicide (irreversible) inhibitor
Q7. Which of the following drugs is a classic irreversible inhibitor of bacterial transpeptidase (penicillin-binding protein)?
- Methicillin
- Penicillin
- Chloramphenicol
- Tetracycline
Correct Answer: Penicillin
Q8. In Lineweaver–Burk plots, competitive inhibition is characterized by what change?
- Parallel lines with decreased intercepts
- Lines intersecting on the y-axis (same 1/Vmax)
- Lines intersecting on the x-axis (same -1/Km)
- No change in slope or intercepts
Correct Answer: Lines intersecting on the y-axis (same 1/Vmax)
Q9. Which inhibitor type can be overcome by increasing substrate concentration?
- Noncompetitive inhibitor
- Irreversible inhibitor
- Competitive inhibitor
- Uncompetitive inhibitor
Correct Answer: Competitive inhibitor
Q10. Organophosphate insecticides inhibit acetylcholinesterase by phosphorylating the active site. How is this inhibition best described?
- Reversible competitive
- Noncompetitive reversible
- Irreversible covalent inhibition
- Uncompetitive inhibition
Correct Answer: Irreversible covalent inhibition
Q11. Trimethoprim inhibits bacterial DHFR. In combined therapy with sulfonamides, the effect is best described as:
- Synergistic inhibition of folate pathway
- Antagonistic interaction
- No effect
- Competitive displacement
Correct Answer: Synergistic inhibition of folate pathway
Q12. Which parameter directly reflects the catalytic turnover number when enzyme concentration is constant?
- Km
- Vmax
- Km/Vmax ratio
- IC50
Correct Answer: Vmax
Q13. A mixed inhibitor typically has what effect on enzyme kinetics?
- Only increases Km
- Only decreases Vmax
- Decreases Vmax and may increase or decrease Km
- No change in Km or Vmax
Correct Answer: Decreases Vmax and may increase or decrease Km
Q14. Which clinical drug is a competitive inhibitor of HMG-CoA reductase?
- Furosemide
- Atorvastatin
- Metformin
- Digoxin
Correct Answer: Atorvastatin
Q15. Proton pump inhibitors (e.g., omeprazole) inhibit H+/K+ ATPase by forming a disulfide bond with cysteine residues. This is an example of:
- Reversible competitive inhibition
- Irreversible (covalent) inhibition
- Allosteric activation
- Uncompetitive inhibition
Correct Answer: Irreversible (covalent) inhibition
Q16. Which inhibitor type reduces both Vmax and Km but cannot be reversed by adding substrate?
- Competitive inhibitor
- Uncompetitive inhibitor
- Irreversible inhibitor
- Noncompetitive inhibitor
Correct Answer: Uncompetitive inhibitor
Q17. Neostigmine is used to treat myasthenia gravis by inhibiting acetylcholinesterase. How is neostigmine classified?
- Reversible carbamate inhibitor
- Irreversible organophosphate
- Noncompetitive inhibitor
- Suicide inhibitor
Correct Answer: Reversible carbamate inhibitor
Q18. Which term describes a molecule that binds to the active site and competes with the substrate?
- Allosteric modulator
- Competitive inhibitor
- Irreversible blocker
- Uncompetitive inhibitor
Correct Answer: Competitive inhibitor
Q19. In drug assays, a reversible inhibitor’s effect can be distinguished from irreversible inhibitor by which method?
- Increasing enzyme concentration
- Dialysis or dilution to remove inhibitor
- Increasing temperature only
- Measuring pH changes
Correct Answer: Dialysis or dilution to remove inhibitor
Q20. Which of the following best describes a suicide inhibitor?
- A reversible competitive inhibitor
- An inactive prodrug activated by the enzyme to form an irreversible inhibitor
- A noncompetitive reversible ligand
- An allosteric activator
Correct Answer: An inactive prodrug activated by the enzyme to form an irreversible inhibitor
Q21. Phenylmethylsulfonyl fluoride (PMSF) irreversibly inhibits serine proteases by reacting with the active-site serine. This is an example of:
- Competitive inhibition
- Reversible noncovalent inhibition
- Irreversible covalent inhibition
- Uncompetitive inhibition
Correct Answer: Irreversible covalent inhibition
Q22. Which kinetic parameter represents substrate concentration at half-maximal velocity?
- Vmax
- Km
- kcat
- IC50
Correct Answer: Km
Q23. Which drug is a reversible inhibitor of angiotensin-converting enzyme (ACE)?
- Captopril
- Penicillin
- Aspirin
- Allopurinol
Correct Answer: Captopril
Q24. A patient is given a high substrate concentration in the presence of a noncompetitive inhibitor. What happens to reaction rate?
- Rate returns to uninhibited Vmax
- Rate increases beyond Vmax
- Rate is still reduced compared to uninhibited Vmax
- Rate becomes zero
Correct Answer: Rate is still reduced compared to uninhibited Vmax
Q25. Which technique is commonly used to determine mechanism of inhibition experimentally?
- Western blotting
- Lineweaver–Burk double reciprocal plots
- PCR amplification
- Mass spectrometry only
Correct Answer: Lineweaver–Burk double reciprocal plots
Q26. Which inhibitor class would most likely show parallel Lineweaver–Burk lines for inhibited and uninhibited reactions?
- Competitive inhibitor
- Uncompetitive inhibitor
- Noncompetitive inhibitor
- Mixed inhibitor
Correct Answer: Uncompetitive inhibitor
Q27. Imatinib is an example of which type of pharmacological inhibitor?
- Protease inhibitor
- Tyrosine kinase inhibitor (ATP-competitive)
- Ion channel blocker
- Uncompetitive enzyme inhibitor
Correct Answer: Tyrosine kinase inhibitor (ATP-competitive)
Q28. Which antibiotic acts by inhibiting the 50S ribosomal peptidyl transferase and is not typically classified as an enzyme inhibitor of metabolic enzymes?
- Chloramphenicol
- Sulfonamide
- Penicillin
- Atorvastatin
Correct Answer: Chloramphenicol
Q29. Sulfonamides inhibit dihydropteroate synthase by mimicking which substrate?
- Folate
- PABA (para-aminobenzoic acid)
- Thymidine
- Glucose
Correct Answer: PABA (para-aminobenzoic acid)
Q30. A reversible inhibitor with a very low Ki value indicates what about binding affinity?
- Low affinity for the enzyme
- High affinity for the enzyme
- No binding occurs
- Non-specific binding only
Correct Answer: High affinity for the enzyme
Q31. Which inhibitor type often binds to an allosteric site rather than the active site?
- Competitive inhibitor
- Noncompetitive inhibitor
- Substrate analogue
- Suicide inhibitor
Correct Answer: Noncompetitive inhibitor
Q32. Which drug inhibits monoamine oxidase irreversibly and can cause hypertensive crisis with tyramine intake?
- Phenelzine
- Fluoxetine
- Propranolol
- Ranitidine
Correct Answer: Phenelzine
Q33. In enzyme inhibition studies, IC50 refers to:
- The inhibitor concentration reducing enzyme activity by 50%
- The substrate concentration at half Vmax
- The maximum velocity of enzyme reaction
- The enzyme concentration in the assay
Correct Answer: The inhibitor concentration reducing enzyme activity by 50%
Q34. Which statement is true for irreversible inhibitors?
- Their effects are easily reversed by dilution
- They covalently modify the enzyme, reducing active enzyme concentration
- They always increase Km only
- They act only at allosteric sites
Correct Answer: They covalently modify the enzyme, reducing active enzyme concentration
Q35. Warfarin acts by inhibiting vitamin K epoxide reductase. This inhibition affects which process?
- Protein glycosylation
- Gamma-carboxylation of clotting factors
- DNA replication
- Renal excretion of drugs
Correct Answer: Gamma-carboxylation of clotting factors
Q36. Which of the following is TRUE about competitive inhibitors in vivo?
- Their effect cannot be overcome by increasing substrate
- They alter Vmax but not Km
- They may be displaced by high substrate or cofactor concentrations
- They permanently inactivate the enzyme
Correct Answer: They may be displaced by high substrate or cofactor concentrations
Q37. Which laboratory approach can identify whether an inhibitor forms a covalent bond with an enzyme?
- Time-dependent loss of activity and mass spectrometry of enzyme
- Measuring only Km values
- Visual color change of solution
- Counting cell numbers
Correct Answer: Time-dependent loss of activity and mass spectrometry of enzyme
Q38. Clavulanic acid is often combined with beta-lactam antibiotics because it:
- Is a competitive inhibitor of transpeptidase
- Inhibits beta-lactamase, protecting the antibiotic
- Acts as a diuretic
- Enhances renal excretion of the antibiotic
Correct Answer: Inhibits beta-lactamase, protecting the antibiotic
Q39. In the presence of an uncompetitive inhibitor, how does increasing substrate concentration affect inhibition?
- Inhibition becomes less effective
- Inhibition becomes more effective (greater inhibition)
- No change in inhibition
- Inhibition is completely reversed
Correct Answer: Inhibition becomes more effective (greater inhibition)
Q40. Which drug class includes agents that block the ATP binding site of kinases and act as competitive inhibitors?
- Statins
- Kinase inhibitors (e.g., imatinib)
- Proton pump inhibitors
- Beta-lactams
Correct Answer: Kinase inhibitors (e.g., imatinib)
Q41. Which of the following is an example of an enzyme inhibited by a transition-state analogue?
- HMG-CoA reductase inhibited by substrate analogue
- Enzyme inhibited by a molecule mimicking the transition state
- Allosteric inhibition of hemoglobin
- Non-enzymatic binding proteins
Correct Answer: Enzyme inhibited by a molecule mimicking the transition state
Q42. Which analgesic irreversibly inhibits platelet COX and thereby impairs platelet aggregation?
- Paracetamol (acetaminophen)
- Aspirin
- Ibuprofen
- Naproxen
Correct Answer: Aspirin
Q43. Which factor makes a reversible inhibitor appear irreversible in a short assay?
- Very slow dissociation rate (long residence time)
- Immediate covalent bonding
- Complete absence of enzyme
- Presence of activator
Correct Answer: Very slow dissociation rate (long residence time)
Q44. Which laboratory value would you examine to assess potent irreversible inhibition of a metabolic enzyme in vivo?
- Immediate Km only
- Enzyme activity over time and recovery after removal of inhibitor
- Color of urine only
- Heart rate
Correct Answer: Enzyme activity over time and recovery after removal of inhibitor
Q45. Which statement best describes a pharmacologic antagonist that binds to the same receptor site as an agonist but without intrinsic activity?
- Allosteric activator
- Competitive antagonist (receptor inhibitor)
- Irreversible enzyme inhibitor
- Uncompetitive enzyme inhibitor
Correct Answer: Competitive antagonist (receptor inhibitor)
Q46. A drug that reduces enzyme concentration by covalent modification will primarily affect which kinetic parameter?
- Km only
- Vmax (apparent) by reducing available active enzyme
- Substrate specificity only
- Buffer pH
Correct Answer: Vmax (apparent) by reducing available active enzyme
Q47. Which class of inhibitors is most useful for designing time-dependent cancer therapies that permanently inactivate oncogenic enzymes?
- Rapid reversible inhibitors
- Suicide (mechanism-based) irreversible inhibitors
- Nonselective vitamins
- Basic salts
Correct Answer: Suicide (mechanism-based) irreversible inhibitors
Q48. Which bacterial enzyme is inhibited by sulfonamides, leading to folate synthesis blockade?
- Dihydropteroate synthase
- Dihydrofolate reductase
- DNA gyrase
- RNA polymerase
Correct Answer: Dihydropteroate synthase
Q49. In drug discovery, what advantage do transition-state analog inhibitors have?
- Poor specificity compared to substrates
- Often high affinity and specificity by mimicking the transition state
- Always reversible with high Ki
- Always non-selective
Correct Answer: Often high affinity and specificity by mimicking the transition state
Q50. Which clinical consequence is commonly associated with irreversible inhibition of detoxifying enzymes (e.g., CYPs) in the liver?
- Reduced drug half-life
- Increased risk of drug accumulation and toxicity
- Immediate excretion of co-administered drugs
- Enhanced metabolic activation of all drugs
Correct Answer: Increased risk of drug accumulation and toxicity
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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