Introduction to enzymes MCQs With Answer

Introduction to enzymes MCQs With Answer

Understanding enzymes is essential for B.Pharm students preparing for pharmacology, biochemistry, and therapeutics. This introduction to enzymes MCQs with answer covers core concepts—enzyme classification, active site specificity, Michaelis–Menten kinetics, inhibition types, coenzymes and cofactors, zymogens, and clinical enzyme assays. Questions emphasize practical applications: drug–enzyme interactions, enzyme kinetics calculations (Km, Vmax, kcat), and methods used in enzyme purification and immobilization. Designed to build conceptual clarity and exam readiness, these focused MCQs help consolidate theory and problem-solving skills relevant to pharmacy practice and drug development. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which of the following best describes the active site of an enzyme?

• A rigid pocket that never changes shape
• A region complementary to the substrate that may undergo conformational change
• A random surface area where substrates bind nonspecifically
• The entire enzyme surface

Correct Answer: A region complementary to the substrate that may undergo conformational change

Q2. The Michaelis constant (Km) is defined as:

• The substrate concentration at which reaction rate is half of Vmax
• The maximum velocity of the enzyme
• The turnover number of the enzyme
• The inhibitor concentration that halves enzyme activity

Correct Answer: The substrate concentration at which reaction rate is half of Vmax

Q3. Which plot linearizes Michaelis–Menten kinetics to estimate Km and Vmax?

• Arrhenius plot
• Lineweaver–Burk plot
• Henderson–Hasselbalch plot
• Scatchard plot

Correct Answer: Lineweaver–Burk plot

Q4. A competitive inhibitor affects enzyme kinetics by:

• Decreasing Vmax and decreasing Km
• Decreasing Vmax with no change in Km
• Increasing Km with no change in Vmax
• Increasing both Km and Vmax

Correct Answer: Increasing Km with no change in Vmax

Q5. kcat represents:

• The Michaelis constant expressed in s^-1
• The turnover number: substrate molecules converted per enzyme site per second
• The equilibrium constant for substrate binding
• The inhibitor binding rate

Correct Answer: The turnover number: substrate molecules converted per enzyme site per second

Q6. Which class of enzymes catalyzes oxidation–reduction reactions?

• Transferases
• Oxidoreductases
• Hydrolases
• Isomerases

Correct Answer: Oxidoreductases

Q7. An uncompetitive inhibitor:

• Binds only to the free enzyme
• Binds only to the enzyme–substrate complex
• Competes with substrate for the active site
• Irreversibly modifies the active site

Correct Answer: Binds only to the enzyme–substrate complex

Q8. Which factor commonly alters enzyme activity by changing protein conformation?

• pH and temperature
• Light intensity
• Magnetic field strength
• Atmospheric pressure

Correct Answer: pH and temperature

Q9. A prosthetic group in an enzyme is:

• A loosely bound cofactor that dissociates easily
• A tightly bound non-protein molecule essential for activity
• A peptide segment removed during activation
• An inhibitor bound at allosteric site

Correct Answer: A tightly bound non-protein molecule essential for activity

Q10. The induced-fit model proposes that:

• Substrate and enzyme shapes are always perfectly complementary
• Enzyme active site molds itself around the substrate upon binding
• Enzymes are rigid and never change conformation
• Substrates induce enzyme synthesis

Correct Answer: Enzyme active site molds itself around the substrate upon binding

Q11. Which term describes inactive enzyme precursors activated by proteolytic cleavage?

• Isoenzymes
• Zymogens
• Allosteric enzymes
• Prosthetic enzymes

Correct Answer: Zymogens

Q12. Lineweaver–Burk double reciprocal plot intercepts on the y-axis correspond to:

• 1/Km
• 1/Vmax
• -1/Km
• -1/Vmax

Correct Answer: 1/Vmax

Q13. Which of the following is an example of an isoenzyme used clinically?

• Creatine kinase (CK) isoforms
• Cytochrome c oxidase only
• Pepsin A exclusively
• RNA polymerase II

Correct Answer: Creatine kinase (CK) isoforms

Q14. Allosteric enzymes are typically regulated by:

• Competitive inhibition only
• Binding of effectors at sites other than the active site
• Permanent covalent modification only
• Denaturation by heat

Correct Answer: Binding of effectors at sites other than the active site

Q15. Which kinetic assumption underlies the Michaelis–Menten equation?

• Rapid equilibrium between enzyme and inhibitor
• Steady-state approximation where ES concentration is constant
• Complete substrate depletion during reaction
• Irreversible substrate binding

Correct Answer: Steady-state approximation where ES concentration is constant

Q16. Enzyme specificity that recognizes a group of similar substrates is called:

• Absolute specificity
• Group specificity
• Stereospecificity
• Broad specificity without preference

Correct Answer: Group specificity

Q17. Which coenzyme is derived from niacin and participates in redox reactions?

• NAD+ (nicotinamide adenine dinucleotide)
• FAD (flavin adenine dinucleotide)
• CoA (coenzyme A)
• Biotin

Correct Answer: NAD+ (nicotinamide adenine dinucleotide)

Q18. Noncompetitive inhibition is characterized by:

• Increase in Km with no change in Vmax
• Decrease in Vmax with no change in Km
• Increase in both Km and Vmax
• No change in enzyme kinetics

Correct Answer: Decrease in Vmax with no change in Km

Q19. The value of Km gives information about:

• Enzyme catalytic rate only
• Affinity of enzyme for substrate
• Amount of enzyme present in solution
• Temperature stability

Correct Answer: Affinity of enzyme for substrate

Q20. Which method separates proteins based on size for enzyme purification?

• Ion-exchange chromatography
• Gel filtration (size-exclusion) chromatography
• Affinity chromatography
• Isoelectric focusing

Correct Answer: Gel filtration (size-exclusion) chromatography

Q21. A suicide inhibitor inactivates an enzyme by:

• Reversibly blocking the active site
• Being converted by the enzyme into a reactive species that covalently modifies the enzyme
• Competing with substrate at high concentrations
• Changing pH of the medium

Correct Answer: Being converted by the enzyme into a reactive species that covalently modifies the enzyme

Q22. Which parameter is obtained directly from Michaelis–Menten plot by extrapolation?

• kcat
• Vmax
• Turnover ratio
• IC50

Correct Answer: Vmax

Q23. Enzyme immobilization is advantageous because it:

• Makes enzymes less specific
• Facilitates enzyme reuse and stability in industrial processes
• Always increases enzyme turnover number
• Prevents substrate access completely

Correct Answer: Facilitates enzyme reuse and stability in industrial processes

Q24. Which amino acid commonly participates in acid–base catalysis at enzyme active sites?

• Glycine
• Histidine
• Leucine
• Valine

Correct Answer: Histidine

Q25. An enzyme that requires a metal ion for activity is called:

• Apoenzyme
• Metalloenzyme
• Isoenzyme
• Allosteric enzyme

Correct Answer: Metalloenzyme

Q26. Which assay method measures initial reaction velocity under steady-state conditions?

• End-point assay after substrate exhaustion
• Initial rate assay measuring product formation over short time
• Equilibrium dialysis
• Native gel electrophoresis

Correct Answer: Initial rate assay measuring product formation over short time

Q27. Competitive inhibition can be overcome by:

• Increasing inhibitor concentration only
• Increasing substrate concentration
• Decreasing enzyme concentration
• Lowering temperature

Correct Answer: Increasing substrate concentration

Q28. The catalytic triad in serine proteases typically includes which three residues?

• Serine, cysteine, and histidine
• Serine, histidine, and aspartate
• Lysine, arginine, and histidine
• Aspartate, glutamate, and serine

Correct Answer: Serine, histidine, and aspartate

Q29. Which term describes reversible covalent modification that regulates enzyme activity in signal transduction?

• Proteolysis only
• Phosphorylation
• Denaturation
• Methylation of DNA

Correct Answer: Phosphorylation

Q30. In a two-substrate (bisubstrate) enzyme mechanism, sequential mechanisms require:

• One substrate binds and product released before second binds
• Both substrates bind to form ternary complex before any product is released
• No substrate binding order
• Substrates react without enzyme involvement

Correct Answer: Both substrates bind to form ternary complex before any product is released

Q31. Which parameter describes the strength of binding between an inhibitor and enzyme?

• Km
• Ki (inhibition constant)
• Vmax
• kcat

Correct Answer: Ki (inhibition constant)

Q32. Which enzyme is commonly used as a marker of myocardial infarction due to isoenzyme patterns?

• Lactate dehydrogenase (LDH)
• DNA polymerase
• Hexokinase only
• ATP synthase

Correct Answer: Lactate dehydrogenase (LDH)

Q33. A decrease in enzyme activity due to proteolytic degradation is an example of:

• Allosteric activation
• Irreversible inactivation
• Competitive inhibition
• Reversible covalent modification

Correct Answer: Irreversible inactivation

Q34. Which kinetic plot reduces weighting error at low substrate concentrations compared to Lineweaver–Burk?

• Eadie–Hofstee plot
• Smith plot
• Scatchard plot
• Henderson plot

Correct Answer: Eadie–Hofstee plot

Q35. Transition state analogs are effective enzyme inhibitors because they:

• Mimic the substrate’s ground state only
• Bind more tightly than substrates by resembling the transition state
• Never bind to the active site
• Only act as irreversible inhibitors

Correct Answer: Bind more tightly than substrates by resembling the transition state

Q36. Which statement about coenzymes is true?

• Coenzymes are proteins that catalyze reactions
• Coenzymes are small organic molecules that assist enzyme catalysis and may be recycled
• Coenzymes are always permanently bound prosthetic groups
• Coenzymes cannot accept or donate chemical groups

Correct Answer: Coenzymes are small organic molecules that assist enzyme catalysis and may be recycled

Q37. Enzyme activity units (U) are defined as:

• Amount of enzyme that converts 1 mole substrate per minute
• Amount of enzyme that converts 1 micromole substrate per minute under specified conditions
• Number of active sites per enzyme molecule
• Number of substrate molecules bound per second

Correct Answer: Amount of enzyme that converts 1 micromole substrate per minute under specified conditions

Q38. Which of the following destabilizes enzymes by disrupting hydrogen bonds and hydrophobic interactions?

• Denaturants like urea and guanidine
• Physiological salt concentrations
• Optimal substrate concentrations
• Binding of required cofactors

Correct Answer: Denaturants like urea and guanidine

Q39. In enzyme kinetics, Vmax is directly proportional to:

• Substrate concentration only
• Enzyme concentration
• Km value
• Inhibitor concentration only

Correct Answer: Enzyme concentration

Q40. Which mechanism describes covalent catalysis by an enzyme?

• Transient covalent bond formation between enzyme and substrate
• Permanent attachment of substrate to enzyme
• No bond formation, only proximity effects
• Electron transfer without bond formation

Correct Answer: Transient covalent bond formation between enzyme and substrate

Q41. The effect of pH on enzyme activity is primarily due to:

• Changes in temperature sensitivity
• Ionization states of amino acid side chains at active site
• Substrate solubility only
• Changes in atmospheric CO2

Correct Answer: Ionization states of amino acid side chains at active site

Q42. Which of the following is a commonly used spectrophotometric method to follow enzyme reactions?

• Monitoring change in absorbance of NADH at 340 nm
• Measuring radioactivity at 10 keV
• Visual inspection of solution color only
• Observing precipitate formation without instrumentation

Correct Answer: Monitoring change in absorbance of NADH at 340 nm

Q43. Enzyme specificity for stereoisomers is termed:

• Absolute specificity
• Stereospecificity
• Promiscuity
• Allosteric specificity

Correct Answer: Stereospecificity

Q44. The Arrhenius equation relates reaction rate to:

• Substrate concentration only
• Activation energy and temperature
• Enzyme concentration only
• pH and ionic strength

Correct Answer: Activation energy and temperature

Q45. Which drug class often uses enzyme inhibition as primary mechanism of action?

• Beta-blockers only
• ACE inhibitors and protease inhibitors
• Bulking agents
• Ranitidine-like antacids only

Correct Answer: ACE inhibitors and protease inhibitors

Q46. An enzyme with high catalytic efficiency has which relationship between kcat and Km?

• Low kcat/Km
• High kcat and low Km (high kcat/Km)
• Low kcat and high Km
• kcat equals Km numerically

Correct Answer: High kcat and low Km (high kcat/Km)

Q47. Which laboratory technique identifies enzymes based on activity after electrophoresis?

• Western blot using antibodies only
• Zymography
• DNA sequencing
• Mass spectrometry without activity assay

Correct Answer: Zymography

Q48. A noncompetitive inhibitor that binds irreversibly is classified as:

• An uncompetitive reversible inhibitor
• An irreversible allosteric inhibitor
• A competitive reversible inhibitor
• A coenzyme analog

Correct Answer: An irreversible allosteric inhibitor

Q49. Which of the following increases catalytic efficiency by stabilizing the transition state?

• Active site residues providing electrostatic and hydrogen bond interactions
• Large excess of substrate only
• High ionic strength exclusively
• Random mutations in surface residues

Correct Answer: Active site residues providing electrostatic and hydrogen bond interactions

Q50. IC50 value for an inhibitor indicates:

• The substrate concentration giving half-maximal velocity
• The inhibitor concentration that reduces enzyme activity by 50% under specified conditions
• The dissociation constant of enzyme–substrate complex
• The maximum inhibition achievable by any inhibitor

Correct Answer: The inhibitor concentration that reduces enzyme activity by 50% under specified conditions

G S Sachin

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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