Coenzymes – structure MCQs With Answer

Coenzymes – structure MCQs With Answer

Understanding coenzymes and their structures is essential for B.Pharm students studying biochemistry and pharmacology. This concise, keyword-rich introduction covers coenzyme classes, vitamin-derived cofactors, prosthetic groups, co-substrates, and structural motifs like adenine nucleotides, isoalloxazine, thiazolium and pyridoxal rings. You will learn how coenzymes such as NAD, FAD, FMN, CoA, PLP, TPP, biotin, lipoic acid, SAM, THF and cobalamin support redox, transfer, decarboxylation and methylation reactions. Clear focus on molecular features, reactive centers and enzyme binding enhances exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which coenzyme contains an isoalloxazine ring responsible for flavin-mediated redox chemistry?

• Coenzyme A (CoA)
• Flavin adenine dinucleotide (FAD)
• Nicotinamide adenine dinucleotide (NAD)
• Pyridoxal phosphate (PLP)

Correct Answer: Flavin adenine dinucleotide (FAD)

Q2. NAD+ is biosynthetically derived from which vitamin?

• Vitamin B1 (Thiamine)
• Vitamin B2 (Riboflavin)
• Vitamin B3 (Niacin / Nicotinic acid)
• Vitamin B6 (Pyridoxine)

Correct Answer: Vitamin B3 (Niacin / Nicotinic acid)

Q3. Which coenzyme contains pantothenic acid as part of its structure?

• Biotin
• Coenzyme A (CoA)
• Tetrahydrofolate (THF)
• S-adenosyl methionine (SAM)

Correct Answer: Coenzyme A (CoA)

Q4. The active form of vitamin B6 that forms Schiff base intermediates with amino acids is:

• Pyridoxal phosphate (PLP)
• Thiamine pyrophosphate (TPP)
• Flavin mononucleotide (FMN)
• Lipoic acid

Correct Answer: Pyridoxal phosphate (PLP)

Q5. Biotin serves as a coenzyme for carboxylation reactions and is covalently attached to enzymes via which amino acid residue?

• Serine
• Lysine
• Cysteine
• Histidine

Correct Answer: Lysine

Q6. Which structural feature is characteristic of thiamine pyrophosphate (TPP) and essential for its catalytic role?

• Isoalloxazine ring
• Thiazolium ring with a reactive C2 carbon
• Pyrimidine-adenine dinucleotide
• Corrin ring with central cobalt

Correct Answer: Thiazolium ring with a reactive C2 carbon

Q7. S-adenosyl methionine (SAM) primarily functions in cells as a:

• Electron carrier for oxidative phosphorylation
• Methyl group donor for methylation reactions
• Carboxyl group donor in biotin-dependent enzymes
• Thiol-based acyl carrier

Correct Answer: Methyl group donor for methylation reactions

Q8. Lipoic acid participates in acyl transfer by forming which type of reactive bond when attached to enzyme E2 subunits?

• Phosphate ester
• Disulfide/dithiol redox-active bond
• Thioester
• Aldimine

Correct Answer: Disulfide/dithiol redox-active bond

Q9. The difference between NAD+ and NADP+ is the presence of an additional phosphate group located at which position?

• 2′-hydroxyl of the adenine ribose (2′ phosphate)
• 5′ position of the nicotinamide ribose
• On the nicotinamide ring nitrogen
• At the adenine N9

Correct Answer: 2′-hydroxyl of the adenine ribose (2′ phosphate)

Q10. Flavin mononucleotide (FMN) differs from FAD by lacking which structural moiety?

• Nicotinamide ring
• Adenine nucleotide (adenosine monophosphate)
• Phosphate group
• Ribitol side chain

Correct Answer: Adenine nucleotide (adenosine monophosphate)

Q11. Which coenzyme contains an adenine dinucleotide linked by pyrophosphate and includes a nicotinamide moiety?

• NAD (Nicotinamide adenine dinucleotide)
• Coenzyme A
• Thiamine pyrophosphate (TPP)
• Biotin

Correct Answer: NAD (Nicotinamide adenine dinucleotide)

Q12. Coenzymes that remain tightly bound to the enzyme as permanent groups are referred to as:

• Co-substrates
• Prosthetic groups
• Allosteric effectors
• Transition state analogs

Correct Answer: Prosthetic groups

Q13. Which coenzyme forms a high-energy thioester bond to carry acyl groups in metabolic reactions?

• Coenzyme A (CoA)
• S-adenosyl methionine (SAM)
• Tetrahydrofolate (THF)
• FAD

Correct Answer: Coenzyme A (CoA)

Q14. PLP-dependent enzymes catalyze transamination by forming which intermediate type between PLP and the substrate amino group?

• Thioester
• Schiff base (aldimine)
• Disulfide bond
• Radical intermediate

Correct Answer: Schiff base (aldimine)

Q15. Riboflavin (vitamin B2) is the precursor for which coenzymes?

• NAD and NADP
• FAD and FMN
• CoA and pantetheine
• PLP and pyridoxine

Correct Answer: FAD and FMN

Q16. Which coenzyme is essential for transketolase and other one-carbon transfer reactions involving thiamine?

• Thiamine pyrophosphate (TPP)
• Tetrahydrobiopterin
• Biotin
• Flavin mononucleotide (FMN)

Correct Answer: Thiamine pyrophosphate (TPP)

Q17. The complete structure of NAD+ includes which of the following components?

• Nicotine free base and single ribose
• Nicotinamide, two ribose units, two phosphates and adenine
• Corrin ring and cobalt ion
• Pantothenic acid and cysteamine

Correct Answer: Nicotinamide, two ribose units, two phosphates and adenine

Q18. Tetrahydrofolate (THF) is the active coenzyme form of which vitamin that carries one-carbon units?

• Vitamin B12 (Cobalamin)
• Folic acid (Vitamin B9)
• Biotin (Vitamin B7)
• Riboflavin (Vitamin B2)

Correct Answer: Folic acid (Vitamin B9)

Q19. Biotin-dependent carboxylases use biotin to transfer CO2 to substrates; biotin’s reactive site is located on which part of its structure?

• Thiophene ring sulfur
• Valeric acid side chain linked to ureido ring
• Pyridine nitrogen
• Phosphate group

Correct Answer: Valeric acid side chain linked to ureido ring

Q20. Which coenzyme contains a corrin ring with a central cobalt atom and participates in radical-based rearrangements?

• Coenzyme Q (ubiquinone)
• Coenzyme B12 (cobalamin)
• FAD
• PLP

Correct Answer: Coenzyme B12 (cobalamin)

Q21. In dehydrogenase reactions, which coenzyme commonly accepts a hydride ion (H-) from substrates?

• Biotin
• NAD+
• Thiamine pyrophosphate (TPP)
• Coenzyme A

Correct Answer: NAD+

Q22. Which structural feature enables coenzyme A to form thioesters with acyl groups?

• Terminal primary amine
• Sulfhydryl (-SH) group of the pantetheine moiety
• Adenine nucleotide
• Isoalloxazine ring

Correct Answer: Sulfhydryl (-SH) group of the pantetheine moiety

Q23. Flavin coenzymes often participate in single-electron transfers because the flavin ring can stabilize which intermediate?

• Carbanion
• Semiquinone radical
• Schiff base
• Thioester

Correct Answer: Semiquinone radical

Q24. Which coenzyme is the predominant cellular methyl donor involved in DNA, protein and lipid methylation?

• S-adenosyl methionine (SAM)
• Tetrahydrofolate (THF)
• Coenzyme Q
• Pyridoxal phosphate (PLP)

Correct Answer: S-adenosyl methionine (SAM)

Q25. The adenosyl moiety present in coenzyme B12 is linked to cobalt via which bond type in 5′-deoxyadenosylcobalamin?

• Carbon-cobalt (Co–C) bond
• Phosphodiester bond
• Peptide bond
• Thioether bond

Correct Answer: Carbon-cobalt (Co–C) bond

Q26. Which coenzyme is required for pyruvate dehydrogenase E1-catalyzed decarboxylation of pyruvate?

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

Correct Answer: Thiamine pyrophosphate (TPP)

Q27. Folate coenzymes carry one-carbon groups in various oxidation states; which of these is not a typical one-carbon form carried by THF?

• Formyl
• Methyl
• Methylene
• Acyl

Correct Answer: Acyl

Q28. Which coenzyme contains a nicotinamide ring that undergoes reversible reduction to accept a hydride?

• FAD
• NAD+
• Biotin
• Coenzyme A

Correct Answer: NAD+

Q29. Which vitamin deficiency would directly impair synthesis of coenzyme A in the body?

• Vitamin B3 (Niacin)
• Vitamin B5 (Pantothenic acid)
• Vitamin B6 (Pyridoxine)
• Vitamin B12 (Cobalamin)

Correct Answer: Vitamin B5 (Pantothenic acid)

Q30. Which coenzyme is commonly bound tightly (as prosthetic group) to flavoproteins and contains an isoalloxazine nucleus linked to a ribityl phosphate?

• FMN (Flavin mononucleotide)
• NADP+
• Coenzyme A
• THF

Correct Answer: FMN (Flavin mononucleotide)

Q31. During aminotransferase reactions, PLP forms an internal aldimine with an enzyme lysine; this intermediate is termed:

• External aldimine
• Internal aldimine
• Enamine
• Imine conjugate

Correct Answer: Internal aldimine

Q32. Which coenzyme is necessary for biotin-dependent acetyl-CoA carboxylase activity in fatty acid biosynthesis?

• Thiamine pyrophosphate (TPP)
• Biotin
• NADP+
• S-adenosyl methionine (SAM)

Correct Answer: Biotin

Q33. Which structural motif is common to many nucleotide-derived coenzymes like NAD, FAD, CoA and SAM?

• Peptide backbone
• Adenine nucleotide (adenosine moiety)
• Polyisoprenoid tail
• Corrin ring

Correct Answer: Adenine nucleotide (adenosine moiety)

Q34. The reactive sulfur in lipoic acid is involved in which type of enzymatic reaction within the pyruvate dehydrogenase complex?

• Methyl transfer
• Acyl transfer and redox (disulfide interchange)
• Hydride transfer
• Carboxylation

Correct Answer: Acyl transfer and redox (disulfide interchange)

Q35. Which coenzyme can act as both a one-electron and two-electron carrier due to its flexible redox states?

• Nicotinamide adenine dinucleotide (NAD)
• Biotin
• Flavin coenzymes (FMN/FAD)
• Coenzyme A

Correct Answer: Flavin coenzymes (FMN/FAD)

Q36. Which coenzyme participates directly in carboxylation by transiently carrying CO2 as carboxybiotin?

• PLP
• Biotin
• NADPH
• Coenzyme Q

Correct Answer: Biotin

Q37. Which vitamin is the precursor of S-adenosyl methionine (SAM) via methionine activation?

• Vitamin B12
• Folate
• Methionine (amino acid) derived; indirectly requires folate and B12
• Riboflavin

Correct Answer: Methionine (amino acid) derived; indirectly requires folate and B12

Q38. The adenylate moiety in coenzyme A is linked to pantetheine via which kind of bond?

• Glycosidic bond
• Phosphodiester bond
• Peptide bond
• Ether bond

Correct Answer: Phosphodiester bond

Q39. Which coenzyme derived from folate provides methyl groups for methionine synthesis?

• 5-methyltetrahydrofolate (5-methyl-THF)
• Formyl-THF
• Methylene-THF
• Biotinyl-THF

Correct Answer: 5-methyltetrahydrofolate (5-methyl-THF)

Q40. Which of the following coenzymes contains a reactive thioester when acylated and is central to fatty acid metabolism?

• S-adenosyl methionine (SAM)
• Coenzyme A (acyl-CoA)
• Biotin-carboxylate
• FMN

Correct Answer: Coenzyme A (acyl-CoA)

Q41. Which coenzyme’s structure includes a nicotinamide ring linked to an ADP-ribose unit?

• FAD
• NAD
• Tetrahydrofolate (THF)
• PLP

Correct Answer: NAD

Q42. In flavoproteins, covalent attachment of FAD to a protein often occurs via which amino acid residue?

• Lysine
• Tyrosine or histidine (depending on enzyme; commonly histidine or cysteine linkage)
• Glycine
• Proline

Correct Answer: Tyrosine or histidine (depending on enzyme; commonly histidine or cysteine linkage)

Q43. Which coenzyme is crucial for branched-chain alpha-ketoacid dehydrogenase complex and contains lipoamide and TPP components?

• Coenzyme A alone
• Multiple coenzymes including TPP, lipoamide, FAD and CoA
• Biotin only
• S-adenosyl methionine only

Correct Answer: Multiple coenzymes including TPP, lipoamide, FAD and CoA

Q44. Which coenzyme is most directly involved in reductive biosynthesis (anabolic reactions) as a reducing equivalent?

• NADH
• NADPH
• FADH2
• CoA-SH

Correct Answer: NADPH

Q45. The vitamin precursor for coenzyme B12 is which element-containing nutrient essential for cobalamin synthesis in bacteria?

• Nickel
• Cobalt
• Iron
• Zinc

Correct Answer: Cobalt

Q46. Which coenzyme acts as a mobile carrier of reducing equivalents within the electron transport chain and is lipid-soluble?

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

Correct Answer: Coenzyme Q (ubiquinone)

Q47. The reactive center of pyridoxal phosphate (PLP) that forms the Schiff base is which functional group on the PLP molecule?

• Aldehyde group at the 4′ position
• Phosphate ester
• Thiol group
• Carboxylate

Correct Answer: Aldehyde group at the 4′ position

Q48. Which coenzyme contains a glutamate tail in its structure when bound to enzymes, enhancing affinity and functional flexibility?

• Tetrahydrofolate (polyglutamylated folate)
• NAD+
• FAD
• Coenzyme A

Correct Answer: Tetrahydrofolate (polyglutamylated folate)

Q49. Which coenzyme’s structure is characterized by a pantetheine arm ending in a reactive thiol that swings between enzyme active sites?

• Coenzyme A (CoA)
• FMN
• PLP
• S-adenosyl methionine (SAM)

Correct Answer: Coenzyme A (CoA)

Q50. Which coenzyme is directly regenerated by NADH oxidase or by electron transport to maintain glycolysis and other dehydrogenase reactions?

• NAD+
• Biotin
• PLP
• Coenzyme A

Correct Answer: NAD+

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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