Enzyme classification MCQs With Answer

Enzyme Classification MCQs With Answer is a focused quiz collection designed for M.Pharm students specializing in Advanced Pharmaceutical Biotechnology. This compilation reinforces understanding of the Enzyme Commission (EC) system, the six major enzyme classes, subclasses, and the logic behind EC numbering. Questions emphasize mechanistic distinctions (oxidation–reduction, group transfer, hydrolysis, elimination, isomerization, and ligation), representative examples, cofactor dependencies, isozymes, and practical implications for drug design and biocatalysis. Each MCQ includes a clear answer to help self-assessment and exam preparation. Use these questions to deepen conceptual knowledge and strengthen application skills required for higher-level pharmaceutical biotechnology coursework and research.

Q1. Which of the following is NOT one of the six major enzyme classes defined by the Enzyme Commission (EC)?

• Oxidoreductases
• Transferases
• Carboxylases
• Hydrolases

Correct Answer: Carboxylases

Q2. The EC number 2.7.1.1 refers to which general class and subclass of enzymes?

• Oxidoreductases acting on CH-OH group
• Transferases transferring phosphorus-containing groups
• Hydrolases acting on ester bonds
• Lyases forming carbon-carbon bonds

Correct Answer: Transferases transferring phosphorus-containing groups

Q3. Alcohol dehydrogenase is classified under which EC major class?

• Hydrolases
• Transferases
• Oxidoreductases
• Ligases

Correct Answer: Oxidoreductases

Q4. Hexokinase, which phosphorylates glucose using ATP, best fits into which enzyme class?

• Lyases
• Transferases
• Isomerases
• Hydrolases

Correct Answer: Transferases

Q5. Which enzyme class primarily catalyzes bond cleavage by the addition of water?

• Hydrolases
• Ligases
• Oxidoreductases
• Isomerases

Correct Answer: Hydrolases

Q6. Pyruvate decarboxylase, which removes CO2 from pyruvate without hydrolysis, belongs to which EC class?

• Lyases
• Hydrolases
• Transferases
• Isomerases

Correct Answer: Lyases

Q7. An epimerase that converts one stereoisomer into another is categorized as which EC class?

• Oxidoreductases
• Isomerases
• Lyases
• Ligases

Correct Answer: Isomerases

Q8. Aminoacyl-tRNA synthetases that catalyze bond formation between amino acids and tRNA at the expense of ATP are classified as:

• Oxidoreductases
• Ligases
• Hydrolases
• Transferases

Correct Answer: Ligases

Q9. In the EC numbering system, what does the second digit indicate?

• The individual enzyme serial number within the subclass
• The species in which the enzyme was first found
• The subclass, defining the type of bond or group acted upon
• The cofactor required by the enzyme

Correct Answer: The subclass, defining the type of bond or group acted upon

Q10. Which of the following statements about EC numbers is TRUE?

• The fourth digit denotes the cofactor requirement
• EC numbers are assigned based on amino acid sequence similarity
• EC numbers classify enzymes by the chemical reaction they catalyze
• EC numbers are unique identifiers for gene products rather than reactions

Correct Answer: EC numbers classify enzymes by the chemical reaction they catalyze

Q11. Alcohol dehydrogenase typically uses which cofactor for catalysis?

• ATP
• NAD+ / NADH
• Biotin
• Heme

Correct Answer: NAD+ / NADH

Q12. Isozymes (isoenzymes) are best described as:

• Different enzymes that catalyze different reactions
• Same polypeptide chain performing different functions
• Different molecular forms that catalyze the same reaction
• Enzymes inactive until covalently modified

Correct Answer: Different molecular forms that catalyze the same reaction

Q13. Which EC primary class number corresponds to lyases?

• 1
• 3
• 4
• 6

Correct Answer: 4

Q14. The reaction type “ATP:glutamate ligase (glutamine-hydrolyzing)” is an example of which systematic naming principle used in EC nomenclature?

• Naming by gene locus
• Naming by substrate and reaction catalyzed
• Naming by inhibitor sensitivity
• Naming by cellular location

Correct Answer: Naming by substrate and reaction catalyzed

Q15. Which EC subclass (third digit) would you expect for enzymes that form C–N bonds using ATP (e.g., peptide bond forming enzymes)?

• 6.1 (forming carbon-oxygen bonds)
• 6.2 (forming carbon-sulfur bonds)
• 6.3 (forming carbon-nitrogen bonds)
• 6.4 (forming carbon-carbon bonds)

Correct Answer: 6.3 (forming carbon-nitrogen bonds)

Q16. Which enzyme example best represents a hydrolase acting on peptide bonds?

• Trypsin
• DNA polymerase
• Pyruvate kinase
• Isocitrate dehydrogenase

Correct Answer: Trypsin

Q17. Aldolase catalyzes cleavage of fructose-1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate without water; this mechanism places it in which EC class?

• Hydrolases
• Lyases
• Isomerases
• Oxidoreductases

Correct Answer: Lyases

Q18. Which of the following is the most accurate description of what the fourth digit in an EC number signifies?

• The type of cofactor used
• The specific substrate molecule size
• The individual enzyme’s serial identifier within the sub-subclass
• The pH optimum of the enzyme

Correct Answer: The individual enzyme’s serial identifier within the sub-subclass

Q19. Does the EC classification explicitly encode information about enzyme cofactors or prosthetic groups?

• Yes — the third digit lists required cofactors
• No — EC classification focuses on the chemical reaction, not cofactors
• Yes — the EC number’s first digit indicates cofactor family
• Only for oxidoreductases is cofactor information encoded

Correct Answer: No — EC classification focuses on the chemical reaction, not cofactors

Q20. Which EC major class catalyzes intramolecular rearrangements including racemizations and epimerizations?

• Isomerases
• Ligases
• Hydrolases
• Transferases

Correct Answer: Isomerases

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