Coenzymes – biochemical functions MCQs With Answer

Coenzymes – biochemical functions MCQs With Answer

Coenzymes are small organic molecules that work with enzymes to drive essential biochemical reactions in metabolism. This concise guide for B. Pharm students covers coenzyme types, mechanisms, vitamin-derived cofactors (NAD, NADP, FAD, CoA, PLP, TPP, biotin, THF, cobalamin), regeneration pathways, and clinical implications such as deficiency syndromes and drug interactions. Questions emphasize enzyme classes, reaction chemistry, prosthetic groups versus cosubstrates, and diagnostic or therapeutic relevance. Focused MCQs help reinforce core concepts needed for pharmacology, medicinal chemistry, and biochemistry exams. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which coenzyme primarily functions as a hydride (H:-) acceptor/donor in dehydrogenase reactions?

• NAD+
• Coenzyme A
• Biotin
• Thiamine pyrophosphate

Correct Answer: NAD+

Q2. FAD differs from NAD+ mainly because FAD:

• Is a one-electron and two-electron carrier capable of forming a stable semiquinone radical
• Only accepts hydride ions
• Is not derived from a vitamin
• Functions exclusively in the cytosol

Correct Answer: Is a one-electron and two-electron carrier capable of forming a stable semiquinone radical

Q3. Coenzyme A (CoA) carries which functional group during metabolic reactions?

• Acyl group (thioester)
• Methyl group
• Carboxyl group
• Phosphate group

Correct Answer: Acyl group (thioester)

Q4. Pyridoxal phosphate (PLP) is essential for which class of enzymatic reactions?

• Amino group transfer (transamination) and decarboxylation of amino acids
• One-carbon transfers in nucleotide synthesis
• Acyl transfer in fatty acid oxidation
• Oxidative phosphorylation

Correct Answer: Amino group transfer (transamination) and decarboxylation of amino acids

Q5. Thiamine pyrophosphate (TPP) is a coenzyme required for which biochemical process?

• Decarboxylation of alpha-keto acids
• One-carbon methylation
• Carboxylation of pyruvate
• Lipid glycosylation

Correct Answer: Decarboxylation of alpha-keto acids

Q6. Biotin acts as a coenzyme by covalently binding to enzymes and carrying which group?

• Carboxyl (CO2) group for carboxylation reactions
• Methyl group for methylation reactions
• Acyl group for acyl transfer reactions
• Amino group for transamination

Correct Answer: Carboxyl (CO2) group for carboxylation reactions

Q7. Tetrahydrofolate (THF) is most directly involved in:

• One-carbon transfers required for purine and thymidylate synthesis
• Electron transport in mitochondria
• Transport of acyl groups in fatty acid synthesis
• Reducing molecular oxygen

Correct Answer: One-carbon transfers required for purine and thymidylate synthesis

Q8. Cobalamin (vitamin B12) functions in humans mainly as a coenzyme in:

• Methyl group transfer (methionine synthase) and rearrangements (methylmalonyl-CoA mutase)
• Carboxylation of pyruvate
• Oxidative decarboxylation of alpha-ketoglutarate
• Fatty acid beta-oxidation initiation

Correct Answer: Methyl group transfer (methionine synthase) and rearrangements (methylmalonyl-CoA mutase)

Q9. Which coenzyme is regenerated primarily by the pentose phosphate pathway to supply reducing power for biosynthesis?

• NADPH
• NAD+
• FAD
• CoA-SH

Correct Answer: NADPH

Q10. Flavin mononucleotide (FMN) is derived from which vitamin?

• Riboflavin (vitamin B2)
• Niacin (vitamin B3)
• Pantothenic acid (vitamin B5)
• Pyridoxine (vitamin B6)

Correct Answer: Riboflavin (vitamin B2)

Q11. S-adenosylmethionine (SAM) is a universal coenzyme for:

• Methyl group transfers to DNA, proteins, and lipids
• Acylation of proteins
• Decarboxylation reactions
• Carboxylation of biotin-dependent enzymes

Correct Answer: Methyl group transfers to DNA, proteins, and lipids

Q12. Which coenzyme serves as a prosthetic group tightly bound to pyruvate dehydrogenase and participates in acyl transfer via a swinging arm?

• Lipoic acid (lipoamide)
• Coenzyme Q
• FAD
• Thiamine pyrophosphate

Correct Answer: Lipoic acid (lipoamide)

Q13. Which vitamin deficiency would most likely impair pyruvate dehydrogenase activity?

• Thiamine (B1)
• Vitamin C
• Vitamin D
• Vitamin K

Correct Answer: Thiamine (B1)

Q14. The active coenzyme form of niacin functions primarily as:

• NAD+ and NADP+, hydride carriers in redox reactions
• A carrier of one-carbon units
• An acyl carrier
• A carboxylation catalyst

Correct Answer: NAD+ and NADP+, hydride carriers in redox reactions

Q15. Which statement distinguishes a prosthetic group from a cosubstrate?

• A prosthetic group is tightly bound; a cosubstrate binds transiently and is regenerated
• A prosthetic group is always a metal ion and cosubstrate is organic
• Cosubstrates are permanently bound to enzymes
• Prosthetic groups are consumed stoichiometrically in reactions

Correct Answer: A prosthetic group is tightly bound; a cosubstrate binds transiently and is regenerated

Q16. Which coenzyme is directly required for carboxylation reactions in fatty acid synthesis?

• Biotin
• FAD
• NAD+
• Thiamine pyrophosphate

Correct Answer: Biotin

Q17. Which coenzyme is critical for glutathione reduction and detoxification reactions?

• NADPH
• NAD+
• FAD
• CoA

Correct Answer: NADPH

Q18. Folate deficiency in pregnant women primarily disrupts which coenzyme-dependent process?

• Thymidylate (dTMP) synthesis and one-carbon transfers needed for nucleotide synthesis
• Beta-oxidation of fatty acids
• Oxidative phosphorylation
• Protein acylation

Correct Answer: Thymidylate (dTMP) synthesis and one-carbon transfers needed for nucleotide synthesis

Q19. Which coenzyme contains a reactive aldehyde that forms Schiff base intermediates with amino acids?

• Pyridoxal phosphate (PLP)
• Biotin
• NAD+
• Thiamine pyrophosphate

Correct Answer: Pyridoxal phosphate (PLP)

Q20. The antimicrobial drug isoniazid interferes with which vitamin-derived coenzyme pathway, causing peripheral neuropathy if not supplemented?

• Vitamin B6 (pyridoxine) / PLP pathway
• Niacin / NAD+ pathway
• Riboflavin / FMN pathway
• Biotin pathway

Correct Answer: Vitamin B6 (pyridoxine) / PLP pathway

Q21. Which enzyme class predominantly requires NAD+ or NADP+ as cofactors?

• Oxidoreductases
• Hydrolases
• Lyases
• Ligases

Correct Answer: Oxidoreductases

Q22. Which coenzyme plays a central role in fatty acid synthesis as a carrier of two-carbon units during elongation?

• Coenzyme A (acetyl-CoA)
• S-adenosylmethionine (SAM)
• FADH2
• Biotin

Correct Answer: Coenzyme A (acetyl-CoA)

Q23. Lipoamide functions in the pyruvate dehydrogenase complex primarily by:

• Shuttling acyl groups between active sites via a flexible arm
• Carrying hydride ions
• Donating single-carbon units
• Binding ATP

Correct Answer: Shuttling acyl groups between active sites via a flexible arm

Q24. Which coenzyme derivative is required for carboxylation of glutamic acid residues in clotting factors?

• Reduced vitamin K hydroquinone (vitamin K-cycle cofactor)
• Thiamine pyrophosphate
• Biotin
• Pyridoxal phosphate

Correct Answer: Reduced vitamin K hydroquinone (vitamin K-cycle cofactor)

Q25. A drug that inhibits dihydrofolate reductase (DHFR) will directly reduce levels of which coenzyme?

• Tetrahydrofolate (THF)
• NADH
• FAD
• CoA

Correct Answer: Tetrahydrofolate (THF)

Q26. Which coenzyme is essential for the transfer of formyl and methylene groups in de novo purine synthesis?

• Tetrahydrofolate (THF)
• S-adenosylmethionine (SAM)
• NAD+
• Biotin

Correct Answer: Tetrahydrofolate (THF)

Q27. In which cellular compartment is NADPH primarily produced by the pentose phosphate pathway?

• Cytosol
• Mitochondrial matrix
• Endoplasmic reticulum lumen
• Peroxisome

Correct Answer: Cytosol

Q28. Which coenzyme participates directly in electron transport as a lipid-soluble carrier within the inner mitochondrial membrane?

• Coenzyme Q (ubiquinone)
• FAD
• NAD+
• FMN

Correct Answer: Coenzyme Q (ubiquinone)

Q29. Which vitamin is a precursor for Coenzyme A?

• Pantothenic acid (vitamin B5)
• Niacin (vitamin B3)
• Riboflavin (vitamin B2)
• Biotin (vitamin B7)

Correct Answer: Pantothenic acid (vitamin B5)

Q30. Which coenzyme is commonly involved in the transfer of amino groups during nucleotide metabolism?

• Pyridoxal phosphate (PLP)
• Coenzyme A
• FAD
• Biotin

Correct Answer: Pyridoxal phosphate (PLP)

Q31. The active site lysine of biotin-dependent carboxylases forms a covalent bond with biotin via which linkage?

• Amide linkage to the valeric acid side chain of biotin
• Disulfide bond
• Phosphoester bond
• Glycosidic bond

Correct Answer: Amide linkage to the valeric acid side chain of biotin

Q32. Which coenzyme’s reduced form donates electrons to cytochrome P450 monooxygenases in microsomal drug metabolism?

• NADPH (via cytochrome P450 reductase)
• NADH
• FADH2 directly
• CoA

Correct Answer: NADPH (via cytochrome P450 reductase)

Q33. Which coenzyme is indispensable for transketolase reactions in the pentose phosphate pathway?

• Thiamine pyrophosphate (TPP)
• Pyridoxal phosphate (PLP)
• CoA
• Biotin

Correct Answer: Thiamine pyrophosphate (TPP)

Q34. Which coenzyme directly participates in formation of S-adenosylmethionine (SAM)?

• Methionine and ATP (forming SAM)
• Folate
• Biotin
• Riboflavin

Correct Answer: Methionine and ATP (forming SAM)

Q35. Which coenzyme is required for the conversion of methylmalonyl-CoA to succinyl-CoA?

• Vitamin B12 (adenosylcobalamin)
• THF
• Biotin
• PLP

Correct Answer: Vitamin B12 (adenosylcobalamin)

Q36. Which coenzyme is most directly involved in reductive biosynthesis such as fatty acid synthesis?

• NADPH
• NADH
• FAD
• CoQ

Correct Answer: NADPH

Q37. Which coenzyme acts as a carrier for sugar moieties in glycosyltransferase reactions?

• UDP-glucose (nucleotide sugar carrier)
• Pyridoxal phosphate
• Biotin
• Coenzyme A

Correct Answer: UDP-glucose (nucleotide sugar carrier)

Q38. Which coenzyme deficiency is classically associated with pellagra (dermatitis, diarrhea, dementia)?

• Niacin (leading to impaired NAD+/NADP+ synthesis)
• Riboflavin
• Biotin
• Folate

Correct Answer: Niacin (leading to impaired NAD+/NADP+ synthesis)

Q39. How does acetyl-CoA participate in regulation of metabolism besides being a substrate?

• Allosteric regulator and acetyl donor for protein acetylation (epigenetic and metabolic regulation)
• Direct electron carrier to the ETC
• Forms hydrogen bonds with DNA to regulate transcription
• Functions as a hormone in the bloodstream

Correct Answer: Allosteric regulator and acetyl donor for protein acetylation (epigenetic and metabolic regulation)

Q40. Which analytical assay would best measure NADH production in enzyme kinetics for dehydrogenases?

• Spectrophotometric assay at 340 nm (NADH absorbance)
• ELISA for NAD+
• Mass spectrometry for CoA
• Fluorescent probe for biotin

Correct Answer: Spectrophotometric assay at 340 nm (NADH absorbance)

Q41. Which coenzyme’s chemical reactivity involves stabilization of carbanion intermediates via an electron sink?

• Pyridoxal phosphate (PLP)
• Coenzyme A
• Biotin
• NAD+

Correct Answer: Pyridoxal phosphate (PLP)

Q42. In which metabolic pathway is lipoamide absolutely essential as a cofactor?

• Pyruvate dehydrogenase complex linking glycolysis to the TCA cycle
• Fatty acid beta-oxidation in peroxisomes
• Glycogen synthesis
• Urea cycle

Correct Answer: Pyruvate dehydrogenase complex linking glycolysis to the TCA cycle

Q43. Which coenzyme participates in the regeneration of methionine from homocysteine?

• Methylcobalamin (vitamin B12) and methyl-THF (folate cycle)
• Biotin and ATP
• Thiamine and Mg2+
• FMN and FAD

Correct Answer: Methylcobalamin (vitamin B12) and methyl-THF (folate cycle)

Q44. Which coenzyme is required for oxidative decarboxylation reactions that generate FADH2 and NADH in the TCA cycle?

• FAD as prosthetic group for succinate dehydrogenase and NAD+ for other dehydrogenases
• CoA for hydride transfer
• Biotin for decarboxylation
• Pyridoxal phosphate for oxidation

Correct Answer: FAD as prosthetic group for succinate dehydrogenase and NAD+ for other dehydrogenases

Q45. Which coenzyme is involved in fatty acid chain desaturation by donating electrons to desaturases?

• NADH and NADPH via cytochrome b5 reductase system
• CoA directly
• Biotin as a cofactor
• Thiamine pyrophosphate

Correct Answer: NADH and NADPH via cytochrome b5 reductase system

Q46. Which coenzyme is essential for branched-chain alpha-keto acid dehydrogenase complex activity?

• Thiamine pyrophosphate (TPP) along with lipoamide, FAD, NAD+, and CoA
• Pyridoxal phosphate alone
• Biotin only
• Folate only

Correct Answer: Thiamine pyrophosphate (TPP) along with lipoamide, FAD, NAD+, and CoA

Q47. In clinical biochemistry, a marked increase in methylmalonic acid indicates deficiency of which coenzyme?

• Vitamin B12 (cobalamin)
• Folate
• Niacin
• Pantothenic acid

Correct Answer: Vitamin B12 (cobalamin)

Q48. Sulfonamide antibiotics inhibit folate synthesis in bacteria by competing with which substrate to block coenzyme formation?

• PABA (para-aminobenzoic acid), preventing folate coenzyme synthesis
• Niacin precursor synthesis
• Pantothenate uptake
• Riboflavin binding

Correct Answer: PABA (para-aminobenzoic acid), preventing folate coenzyme synthesis

Q49. Which coenzyme is most directly involved in histone methylation affecting gene expression?

• S-adenosylmethionine (SAM)
• Tetrahydrofolate (THF)
• NADH
• CoA

Correct Answer: S-adenosylmethionine (SAM)

Q50. Which of the following best describes the biochemical classification of coenzymes?

• Organic non-protein molecules, often vitamin-derived, that assist enzyme catalysis as prosthetic groups or cosubstrates
• Inorganic ions that serve as primary energy currency
• Structural proteins that stabilize enzymes
• Polysaccharides used as enzyme substrates

Correct Answer: Organic non-protein molecules, often vitamin-derived, that assist enzyme catalysis as prosthetic groups or cosubstrates

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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