Phenylpropanoids – lignans represent a diverse group of plant secondary metabolites formed by oxidative dimerization of two phenylpropanoid (C6–C3) units. For B.Pharm students, understanding lignan biosynthesis, stereochemistry, common classes (dibenzylbutane, dibenzylbutyrolactone, aryltetralin, arylnaphthalene), and pharmacological activities (antioxidant, estrogenic modulation, cytotoxicity) is essential for pharmacognosy and drug development. Key concepts include enzymes such as phenylalanine ammonia‑lyase, dirigent proteins, pinoresinol–lariciresinol reductase, metabolic conversion by gut microbiota to enterolignans, and analytical techniques like HPLC, GC‑MS and NMR for structural elucidation. This topic links plant biochemistry to therapeutic leads and quality control. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which structural motif defines a phenylpropanoid unit that forms the building block of lignans?
- C6–C3 (phenylpropane) unit
- C6–C1 (benzyl) unit
- C6–C2 (styrene) unit
- C6–C4 (phenethyl) unit
Correct Answer: C6–C3 (phenylpropane) unit
Q2. Which enzyme catalyzes the first committed step in the general phenylpropanoid pathway?
- Phenylalanine ammonia‑lyase (PAL)
- Cinnamate 4‑hydroxylase (C4H)
- 4‑Coumarate:CoA ligase (4CL)
- Dirigent protein (DIR)
Correct Answer: Phenylalanine ammonia‑lyase (PAL)
Q3. Lignan formation typically involves coupling of two phenylpropanoid radicals. Which protein directs stereoselective coupling without itself performing oxidation?
- Dirigent protein (DIR)
- Peroxidase
- Monooxygenase
- Polyketide synthase
Correct Answer: Dirigent protein (DIR)
Q4. Which pair of lignans are enantiomers commonly found in plants and related by stereochemistry at C8–C8′?
- (+)-Pinoresinol and (-)-pinoresinol
- Podophyllotoxin and etoposide
- Sesamin and sesamolin
- Enterolactone and enterodiol
Correct Answer: (+)-Pinoresinol and (-)-pinoresinol
Q5. Pinoresinol is enzymatically reduced to lariciresinol and then to secoisolariciresinol by which enzyme family?
- Pinoresinol–lariciresinol reductase (PLR)
- Dirigent protein (DIR)
- Phenylalanine ammonia‑lyase (PAL)
- Glucosyltransferase (UGT)
Correct Answer: Pinoresinol–lariciresinol reductase (PLR)
Q6. Which analytical technique is most appropriate for high‑resolution structural elucidation of a new lignan including stereochemistry?
- Nuclear magnetic resonance (NMR) spectroscopy
- Thin‑layer chromatography (TLC)
- UV–Vis spectrophotometry
- Folin–Ciocalteu assay
Correct Answer: Nuclear magnetic resonance (NMR) spectroscopy
Q7. Podophyllotoxin is an important lignan precursor used to derive which anticancer drug class?
- Topoisomerase II inhibitors (e.g., etoposide)
- Taxane microtubule stabilizers
- Platinum alkylating agents
- EGFR tyrosine kinase inhibitors
Correct Answer: Topoisomerase II inhibitors (e.g., etoposide)
Q8. Which plant is the richest natural source of secoisolariciresinol and its glycosides, important dietary lignans?
- Flaxseed (Linum usitatissimum)
- Ginkgo biloba
- Cinchona officinalis
- Piper nigrum
Correct Answer: Flaxseed (Linum usitatissimum)
Q9. Gut microbiota convert plant lignans into mammalian lignans; which are the primary enterolignans produced?
- Enterodiol and enterolactone
- Vanillin and vanillic acid
- Sesamol and sesaminol
- Podophyllotoxin and picropodophyllin
Correct Answer: Enterodiol and enterolactone
Q10. Which coupling mode between monolignol radicals gives rise to classic lignan connectivity such as in pinoresinol?
- β–β’ (beta–beta) coupling
- β–O–4 coupling
- 5–5′ coupling
- β–5′ coupling
Correct Answer: β–β’ (beta–beta) coupling
Q11. Sesamin is a lignan predominantly found in which oilseed and known for antioxidant properties?
- Sesame (Sesamum indicum)
- Soybean (Glycine max)
- Sunflower (Helianthus annuus)
- Coconut (Cocos nucifera)
Correct Answer: Sesame (Sesamum indicum)
Q12. Which class of lignans contains a lactone ring formed by intramolecular oxidation, common in anti‑tumor prototypes?
- Dibenzylbutyrolactone lignans
- Dibenzylbutane lignans
- Arylnaphthalene lignans
- Neolignans
Correct Answer: Dibenzylbutyrolactone lignans
Q13. Which spectroscopic feature in 1H NMR would help identify an aryltetralin lignan like podophyllotoxin?
- Characteristic aromatic proton patterns and methine/methylene signals in aliphatic region
- Strong absorbance at 280 nm only
- Single broad peak at 7.0 ppm only
- No observable signals due to polymerization
Correct Answer: Characteristic aromatic proton patterns and methine/methylene signals in aliphatic region
Q14. A key step in lignan biosynthesis is formation of coniferyl alcohol radical. Which enzyme class oxidizes monolignols to radicals?
- Peroxidases and laccases (oxidative enzymes)
- Reductases
- Hydrolases
- Transferases
Correct Answer: Peroxidases and laccases (oxidative enzymes)
Q15. Which reagent/technique is most suitable for quantifying lignan glycosides in herbal extracts in a quality‑control lab?
- HPLC with UV or MS detection
- Gravimetric analysis
- Simple acid titration
- Paper chromatography
Correct Answer: HPLC with UV or MS detection
Q16. Neolignans differ from classical lignans primarily by what characteristic?
- Non‑β–β’ linkages forming C–C or C–O bonds at different positions
- Presence of sugar moieties only
- Being monomeric phenylpropanoids
- Formation exclusively in animals
Correct Answer: Non‑β–β’ linkages forming C–C or C–O bonds at different positions
Q17. Which physiological activity is commonly attributed to dietary lignans in epidemiological studies?
- Potential modulation of hormone‑dependent cancers via weak estrogenic/antiestrogenic effects
- Direct antimicrobial action like antibiotics
- Rapid hypoglycemic effect
- Immediate analgesic action
Correct Answer: Potential modulation of hormone‑dependent cancers via weak estrogenic/antiestrogenic effects
Q18. Which enzyme family glycosylates lignans to form lignan glycosides, increasing water solubility and affecting bioavailability?
- UDP‑glycosyltransferases (UGTs)
- Cytochrome P450 monooxygenases
- Carboxylesterases
- Proteases
Correct Answer: UDP‑glycosyltransferases (UGTs)
Q19. Which structural descriptor distinguishes arylnaphthalene lignans from aryltetralin lignans?
- Presence of a naphthalene core versus a tetralin (hydrogenated naphthalene) core
- Number of sugar units attached
- Type of ester linkages only
- Exclusive occurrence in marine organisms
Correct Answer: Presence of a naphthalene core versus a tetralin (hydrogenated naphthalene) core
Q20. Which lignan is a pharmacologically active compound derived from Podophyllum species and serves as a cytotoxic lead?
- Podophyllotoxin
- Sesamin
- Pinoresinol
- Magnolol
Correct Answer: Podophyllotoxin
Q21. Which metabolic transformation in humans reduces dietary secoisolariciresinol to enterodiol?
- Reductive dehydroxylation by gut microbiota
- Direct hepatic glucuronidation only
- Oxidative decarboxylation
- Nitration by microbial enzymes
Correct Answer: Reductive dehydroxylation by gut microbiota
Q22. In stereospecific biosynthesis of lignans, what role does the dirigent protein play with radical intermediates?
- Orients radicals to give enantiomerically enriched products
- Directly oxidizes phenylpropanoids to radicals
- Glycosylates lignans to increase solubility
- Degrades lignans to simple phenolics
Correct Answer: Orients radicals to give enantiomerically enriched products
Q23. Which chromatographic technique coupled with mass spectrometry is most useful for profiling a complex lignan mixture including aglycones and glycosides?
- LC–MS/MS (liquid chromatography–tandem mass spectrometry)
- Paper chromatography with iodine visualization
- Simple column chromatography with kerosene
- Gel electrophoresis
Correct Answer: LC–MS/MS (liquid chromatography–tandem mass spectrometry)
Q24. Which chemical feature often increases the antioxidant capacity of lignans?
- Presence of phenolic hydroxyl groups and conjugated aromatic systems
- Higher molecular weight alone
- Presence of aliphatic chains with no aromaticity
- Complete methylation of all hydroxyls
Correct Answer: Presence of phenolic hydroxyl groups and conjugated aromatic systems
Q25. Which synthetic modification of podophyllotoxin led to clinically used anticancer agents like etoposide?
- Introduction of a glycosidic‑like side chain and conversion to a semisynthetic derivative
- Simple methylation of all phenols
- Hydrogenation of aromatic rings to alkanes
- Cleavage of the lactone ring only
Correct Answer: Introduction of a glycosidic‑like side chain and conversion to a semisynthetic derivative
Q26. Which biosynthetic precursor supplies the aromatic ring and propanoid side chain for lignan assembly?
- Phenylalanine via the shikimate pathway
- Acetyl‑CoA via fatty acid synthesis
- Mevalonate via isoprenoid pathway
- Glucose via glycolysis
Correct Answer: Phenylalanine via the shikimate pathway
Q27. Which laboratory observation would suggest glycosylation of a lignan during LC analysis?
- Increase in molecular mass by 162 Da (hexose) or multiples and more polar retention time
- Decrease in UV absorbance only
- Complete disappearance in MS due to ion suppression only
- Formation of insoluble precipitate
Correct Answer: Increase in molecular mass by 162 Da (hexose) or multiples and more polar retention time
Q28. Which lignan subclass is characterized by a straight‑chain dibenzylbutane skeleton?
- Dibenzylbutane lignans (e.g., secoisolariciresinol)
- Arylnaphthalene lignans
- Dibenzylbutyrolactone lignans
- Neolignans
Correct Answer: Dibenzylbutane lignans (e.g., secoisolariciresinol)
Q29. Which safety/pharmacological consideration is important when working with podophyllotoxin and its derivatives in drug development?
- They are cytotoxic and require careful handling and dosing due to narrow therapeutic windows
- They are inert and safe for ingestion without limits
- They universally act as broad‑spectrum antibiotics
- They are benign topical nutrients
Correct Answer: They are cytotoxic and require careful handling and dosing due to narrow therapeutic windows
Q30. In plant metabolic engineering to increase lignan content, which strategy would most directly increase flux toward lignan biosynthesis?
- Overexpression of PAL, C4H, 4CL and dirigent proteins or PLR enzymes
- Knockout of photosynthetic genes
- Overexpression of proteases
- Suppression of sugar transporters only
Correct Answer: Overexpression of PAL, C4H, 4CL and dirigent proteins or PLR enzymes
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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