IUB classification of enzymes MCQs With Answer

IUB classification of enzymes MCQs With Answer is an essential resource for B.Pharm students preparing for pharmacology, biochemistry, and drug metabolism exams. This concise guide covers the IUB/EC (Enzyme Commission) numerical system, enzyme classes (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases), EC numbering rules, nomenclature, cofactors, and clinical relevance in pharmacotherapy. Each question emphasizes conceptual understanding and real-world examples — cytochrome P450, kinases, esterases, proteases — to strengthen application-based learning. Clear explanations and authoritative keywords like IUB classification, EC numbers, enzyme classes, and enzyme nomenclature improve exam readiness and SEO visibility. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What does the ‘EC’ in EC number stand for in enzyme classification?

• Enzyme Category
• Enzyme Code
• Enzyme Commission
• Enzyme Classifier

Correct Answer: Enzyme Commission

Q2. Which organization is responsible for maintaining the modern enzyme nomenclature and classification?

• WHO (World Health Organization)
• IUBMB (International Union of Biochemistry and Molecular Biology)
• FDA (Food and Drug Administration)
• UNESCO

Correct Answer: IUBMB (International Union of Biochemistry and Molecular Biology)

Q3. How many primary classes are there in the IUB/EC enzyme classification?

• 4
• 5
• 6
• 7

Correct Answer: 6

Q4. Which EC class includes oxidoreductases?

• EC 1
• EC 2
• EC 3
• EC 4

Correct Answer: EC 1

Q5. Transferases are responsible for which type of biochemical activity?

• Oxidation-reduction reactions
• Transfer of functional groups between molecules
• Hydrolysis of bonds
• Formation of double bonds

Correct Answer: Transfer of functional groups between molecules

Q6. Hydrolases (EC 3) catalyze which kind of reaction?

• Group transfer without water
• Bond cleavage by addition of water
• Intramolecular rearrangements
• Joining of two molecules with ATP

Correct Answer: Bond cleavage by addition of water

Q7. Lyases (EC 4) typically catalyze which process?

• Oxidation of alcohols
• Addition to or removal from double bonds without hydrolysis
• Transfer of phosphate groups
• Formation of peptide bonds

Correct Answer: Addition to or removal from double bonds without hydrolysis

Q8. Which EC class includes isomerases?

• EC 2
• EC 3
• EC 4
• EC 5

Correct Answer: EC 5

Q9. Ligases (EC 6) generally catalyze:

• Electron transfer reactions
• Bond formation coupled to ATP hydrolysis
• Isomerization within a molecule
• Hydrolytic cleavage of esters

Correct Answer: Bond formation coupled to ATP hydrolysis

Q10. In the EC numbering system (e.g., EC 1.1.1.1), what does the first digit represent?

• The species of origin
• The major enzyme class
• The substrate molecular weight
• The enzyme’s optimum pH

Correct Answer: The major enzyme class

Q11. What information does the second digit of an EC number provide?

• Sub-subclass indicating the type of reaction or group involved
• Serial number of the enzyme
• Organism kingdom
• Type of cofactor required

Correct Answer: Sub-subclass indicating the type of reaction or group involved

Q12. The fourth digit in an EC number (e.g., EC 2.7.1.1) indicates:

• The species where enzyme is found
• The serial identifier of a particular enzyme within its sub-subclass
• The enzyme’s molecular weight
• The enzyme inhibitor class

Correct Answer: The serial identifier of a particular enzyme within its sub-subclass

Q13. Which enzyme class would kinases (that transfer a phosphate group) belong to?

• Oxidoreductases
• Transferases
• Hydrolases
• Lyases

Correct Answer: Transferases

Q14. Alcohol dehydrogenase, which oxidizes alcohols, belongs to which EC class?

• EC 1 (oxidoreductases)
• EC 2 (transferases)
• EC 3 (hydrolases)
• EC 6 (ligases)

Correct Answer: EC 1 (oxidoreductases)

Q15. Proteases that hydrolyze peptide bonds are mainly classified under which EC subclass?

• EC 1.1
• EC 2.7
• EC 3.4
• EC 5.3

Correct Answer: EC 3.4

Q16. Which of the following best describes a holoenzyme?

• A protein without its cofactor
• The active enzyme with its cofactor bound
• An enzyme denatured by heat
• A synthetic enzyme mimic

Correct Answer: The active enzyme with its cofactor bound

Q17. The protein part of an enzyme without its cofactor is called:

• Holoenzyme
• Apoenzyme
• Coenzyme
• Proenzyme

Correct Answer: Apoenzyme

Q18. Which of the following is considered a prosthetic group (a tightly bound cofactor)?

• NAD+ (loosely bound coenzyme)
• Metal ions like Fe2+ in hemoproteins
• Substrate molecules
• Competitive inhibitors

Correct Answer: Metal ions like Fe2+ in hemoproteins

Q19. Which class includes enzymes that catalyze oxidation-reduction with oxygen as acceptor (monooxygenases)?

• EC 1 (oxidoreductases)
• EC 2 (transferases)
• EC 3 (hydrolases)
• EC 4 (lyases)

Correct Answer: EC 1 (oxidoreductases)

Q20. Cytochrome P450 enzymes involved in drug metabolism are primarily classified as:

• Hydrolases
• Transferases
• Oxidoreductases
• Ligases

Correct Answer: Oxidoreductases

Q21. Which subclass of EC 2 contains enzymes transferring phosphorus-containing groups?

• EC 2.1
• EC 2.7
• EC 2.3
• EC 2.5

Correct Answer: EC 2.7

Q22. Which enzyme class would lyases such as decarboxylases belong to?

• EC 1
• EC 2
• EC 4
• EC 6

Correct Answer: EC 4

Q23. Enzyme Commission numbers are especially useful because they:

• Indicate where the enzyme is located in the cell
• Provide a standardized functional classification based on reaction chemistry
• Give the exact amino acid sequence
• Describe an enzyme’s 3D structure

Correct Answer: Provide a standardized functional classification based on reaction chemistry

Q24. Which of the following statements about enzyme common names vs. systematic IUB names is true?

• Common names always indicate reaction mechanism precisely
• Systematic IUB names reflect the reaction and substrates more precisely than common names
• Common names are required by IUBMB
• Systematic names are shorter and easier to use in exams

Correct Answer: Systematic IUB names reflect the reaction and substrates more precisely than common names

Q25. Which enzyme classification includes ligases often called ‘synthetases’ in biochemical literature?

• EC 3
• EC 4
• EC 5
• EC 6

Correct Answer: EC 6

Q26. Esterases that hydrolyze ester bonds (important for prodrug activation) belong predominantly to which EC class?

• EC 1
• EC 2
• EC 3
• EC 5

Correct Answer: EC 3

Q27. In IUB classification, which enzyme would most likely have an EC number starting with ‘3.1’?

• Proteases
• Hydrolases acting on ester bonds
• Oxidases
• Isomerases

Correct Answer: Hydrolases acting on ester bonds

Q28. Which of the following best defines absolute specificity of an enzyme?

• Acts on a broad range of substrates
• Acts on only one specific substrate
• Acts on a specific bond type regardless of substrate
• Acts only under acidic pH

Correct Answer: Acts on only one specific substrate

Q29. Which term describes enzymes existing in multiple molecular forms with similar activity (important clinically as biomarkers)?

• Isoenzymes (isozymes)
• Holoenzymes
• Proenzymes
• Apoproteins

Correct Answer: Isoenzymes (isozymes)

Q30. Which unit is the SI unit of catalytic activity often used in enzyme assays?

• International Unit (IU)
• Katal
• Urea
• Molality

Correct Answer: Katal

Q31. The commonly used activity unit in clinical labs, one International Unit (IU), is defined as:

• The amount of enzyme converting 1 mole of substrate per second
• The amount of enzyme catalyzing 1 micromole of substrate per minute
• The enzyme mass in grams
• One katal divided by 60

Correct Answer: The amount of enzyme catalyzing 1 micromole of substrate per minute

Q32. The IUB classification is primarily based on which criterion?

• The enzyme’s amino acid sequence
• The chemical reaction catalyzed
• The cellular location
• The organism in which it is found

Correct Answer: The chemical reaction catalyzed

Q33. Which class would epimerases, which invert stereochemistry at a single center, belong to?

• EC 3 (hydrolases)
• EC 4 (lyases)
• EC 5 (isomerases)
• EC 6 (ligases)

Correct Answer: EC 5 (isomerases)

Q34. Which class contains enzymes that join two molecules with concomitant hydrolysis of a nucleotide triphosphate?

• Oxidoreductases
• Transferases
• Hydrolases
• Ligases

Correct Answer: Ligases

Q35. Which option correctly identifies the relationship between EC numbers and enzyme names?

• Every EC number refers to a single, unchanging common name
• A single enzyme may have multiple names but one EC number for the same reaction
• EC numbers change daily and are unreliable
• Common names are unique identifiers worldwide

Correct Answer: A single enzyme may have multiple names but one EC number for the same reaction

Q36. Which of the following is a correct characteristic of IUB systematic enzyme names?

• They are always shorter than common names
• They describe the substrate and type of reaction
• They never include the term ‘ase’
• They are only used for ligases

Correct Answer: They describe the substrate and type of reaction

Q37. Which class is most commonly associated with drug biotransformation reactions (phase I metabolism)?

• EC 3 hydrolases
• EC 1 oxidoreductases
• EC 2 transferases
• EC 6 ligases

Correct Answer: EC 1 oxidoreductases

Q38. Conjugation reactions in phase II drug metabolism (e.g., glucuronidation) are primarily catalyzed by which class?

• Oxidoreductases
• Transferases
• Hydrolases
• Isomerases

Correct Answer: Transferases

Q39. Which IUB class would enolase, which catalyzes interconversion of 2-phosphoglycerate and phosphoenolpyruvate, belong to?

• Oxidoreductases
• Lyases
• Isomerases
• Hydrolases

Correct Answer: Lyases

Q40. Which of the following best describes a dehydrogenase enzyme’s general activity?

• Adding water across a bond
• Removing hydrogen (oxidation) often using NAD+/NADP+
• Transferring phosphate to ADP
• Joining two substrates with ATP

Correct Answer: Removing hydrogen (oxidation) often using NAD+/NADP+

Q41. Which enzyme classification term indicates it acts on peptide bonds?

• Peptidase (a type of hydrolase)
• Kinase (a transferase)
• Oxidase (an oxidoreductase)
• Ligase

Correct Answer: Peptidase (a type of hydrolase)

Q42. The EC subclass ‘3.4’ specifically refers to which group?

• Hydrolases acting on ester bonds
• Hydrolases acting on peptide bonds (peptidases)
• Transferases transferring methyl groups
• Oxidoreductases acting on CH-OH group

Correct Answer: Hydrolases acting on peptide bonds (peptidases)

Q43. Which of the following is true about enzyme inhibitors used as drugs?

• Only hydrolases can be inhibited by drugs
• Inhibitors may be reversible or irreversible and can target any enzyme class
• All inhibitors are competitive
• Inhibitors always increase enzyme activity

Correct Answer: Inhibitors may be reversible or irreversible and can target any enzyme class

Q44. Which EC class would racemases (converting enantiomers) belong to?

• EC 1 oxidoreductases
• EC 3 hydrolases
• EC 5 isomerases
• EC 6 ligases

Correct Answer: EC 5 isomerases

Q45. Which of the following is an example of a transferase important in drug activation or metabolism?

• Cytochrome P450 monooxygenase
• UDP-glucuronosyltransferase (UGT)
• Protease
• DNA ligase

Correct Answer: UDP-glucuronosyltransferase (UGT)

Q46. Which description matches EC 2.7.1.x enzymes?

• Transferases transferring phosphorus-containing groups to an alcohol group
• Oxidoreductases acting on CH-OH group
• Hydrolases acting on peptide bonds
• Ligases forming C-O bonds

Correct Answer: Transferases transferring phosphorus-containing groups to an alcohol group

Q47. The IUBMB periodically updates enzyme classifications primarily because:

• Enzyme structures frequently change
• New biochemical reactions and more precise data become available
• They rename enzymes every year for clarity
• To increase the number of enzyme classes

Correct Answer: New biochemical reactions and more precise data become available

Q48. Which enzyme classification term best fits transaminases involved in amino acid metabolism?

• Hydrolases
• Transferases
• Oxidoreductases
• Isomerases

Correct Answer: Transferases

Q49. In IUB classification, an enzyme labeled EC 1.1.1.1 indicates:

• It is a transferase acting on nitrogen groups
• It is an oxidoreductase acting on CH-OH group with NAD+ or NADP+ as acceptor
• It is a hydrolase acting on glycosidic bonds
• It is a ligase joining two molecules

Correct Answer: It is an oxidoreductase acting on CH-OH group with NAD+ or NADP+ as acceptor

Q50. For B.Pharm students, why is understanding IUB/EC classification clinically important?

• It predicts drug toxicology based solely on structure
• It helps link enzyme reaction types to drug metabolism, design of inhibitors, and therapeutic targets
• It replaces the need for pharmacokinetic studies
• It only matters for academic taxonomy and has no clinical relevance

Correct Answer: It helps link enzyme reaction types to drug metabolism, design of inhibitors, and therapeutic targets

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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