Biosynthetic pathways of major phytoconstituent classes MCQs With Answer

Biosynthetic pathways of major phytoconstituent classes MCQs With Answer

This quiz collection is designed for M.Pharm students to deepen understanding of the biosynthetic logic behind major plant secondary metabolite classes. It emphasizes enzymology, subcellular localization, precursor–product relationships and branching points in pathways such as shikimate, phenylpropanoid, mevalonate (MVA), MEP (non-mevalonate), polyketide and terpenoid biosynthesis. The MCQs test application-level knowledge: enzyme names and functions, molecular donors (e.g., IPP/DMAPP), cyclization and tailoring reactions (oxidation, glycosylation, prenylation), and biosynthetic origins of alkaloids, flavonoids, lignins, saponins and carotenoids. Answers are provided for rapid self-assessment and revision.

Q1. Which amino acid is the primary precursor that enters the phenylpropanoid pathway via the action of phenylalanine ammonia‑lyase (PAL)?

• Tyrosine
• Phenylalanine
• Tryptophan
• Histidine

Correct Answer: Phenylalanine

Q2. In plants the two distinct pathways for isoprenoid (terpenoid) precursor biosynthesis are localized to different compartments. Which enzyme catalyzes the committed, often rate‑limiting step of the plastidic MEP pathway?

• HMG‑CoA reductase (HMGR)
• 1‑deoxy‑D‑xylulose‑5‑phosphate synthase (DXS)
• DXR (1‑deoxy‑D‑xylulose‑5‑phosphate reductoisomerase)
• Mevalonate kinase

Correct Answer: 1‑deoxy‑D‑xylulose‑5‑phosphate synthase (DXS)

Q3. Formation of farnesyl diphosphate (FPP; C15) in isoprenoid biosynthesis requires condensation of which combination of 5‑carbon units?

• Three DMAPP units
• Two DMAPP + one IPP
• One DMAPP + two IPP
• Three IPP units

Correct Answer: One DMAPP + two IPP

Q4. Carotenoids are synthesized from which C20 precursor by the action of phytoene synthase?

• Farnesyl diphosphate (FPP)
• Geranyl diphosphate (GPP)
• Geranylgeranyl diphosphate (GGPP)
• Isopentenyl diphosphate (IPP)

Correct Answer: Geranylgeranyl diphosphate (GGPP)

Q5. Chalcone synthase (CHS), a type III polyketide synthase, condenses p‑coumaroyl‑CoA with how many malonyl‑CoA extender units to form a chalcone scaffold?

• One malonyl‑CoA
• Two malonyl‑CoA
• Three malonyl‑CoA
• Four malonyl‑CoA

Correct Answer: Three malonyl‑CoA

Q6. Which enzyme catalyzes the first committed step of the general phenylpropanoid pathway by deaminating phenylalanine to cinnamic acid?

• Cinnamate 4‑hydroxylase (C4H)
• Phenylalanine ammonia‑lyase (PAL)
• Tyrosine aminotransferase
• Cinnamyl alcohol dehydrogenase (CAD)

Correct Answer: Phenylalanine ammonia‑lyase (PAL)

Q7. The biosynthesis of benzylisoquinoline alkaloids begins with enzymatic condensation of dopamine and 4‑hydroxyphenylacetaldehyde to form (S)‑norcoclaurine. Which enzyme catalyzes this stereoselective Pictet‑Spengler‑like reaction?

• Norcoclaurine synthase
• Tryptophan decarboxylase
• Berberine bridge enzyme
• Monoamine oxidase

Correct Answer: Norcoclaurine synthase

Q8. Triterpenoid saponins are derived from cyclization of 2,3‑oxidosqualene. Which enzyme class performs that cyclization to produce β‑amyrin or other triterpene skeletons?

• Terpene synthase (monoterpene synthase)
• Oxidosqualene cyclase (OSC), e.g., β‑amyrin synthase
• Prenyltransferase
• Polyketide synthase

Correct Answer: Oxidosqualene cyclase (OSC), e.g., β‑amyrin synthase

Q9. In the flavonoid pathway, which enzyme reduces dihydroflavonols to leucoanthocyanidins, a key step en route to anthocyanins and condensed tannins?

• Flavanone 3‑hydroxylase (F3H)
• Chalcone isomerase (CHI)
• Dihydroflavonol 4‑reductase (DFR)
• Anthocyanidin synthase (ANS)

Correct Answer: Dihydroflavonol 4‑reductase (DFR)

Q10. The shikimate pathway generates chorismate as a central branch point. Chorismate is the common precursor for which set of aromatic amino acids?

• Alanine, valine, leucine
• Phenylalanine, tyrosine, tryptophan
• Histidine, lysine, arginine
• Glycine, serine, cysteine

Correct Answer: Phenylalanine, tyrosine, tryptophan

Q11. In the cytosolic mevalonate (MVA) route, which enzyme catalyzes the NADPH‑dependent reduction of HMG‑CoA to mevalonate and is commonly considered a major regulatory point?

• Mevalonate kinase
• HMG‑CoA reductase (HMGR)
• Mevalonate 5‑pyrophosphate decarboxylase
• Isopentenyl diphosphate isomerase (IDI)

Correct Answer: HMG‑CoA reductase (HMGR)

Q12. Which characteristic distinguishes plant type III polyketide synthases (PKSs), such as chalcone synthase, from type I/II PKSs?

• They are large multi‑domain megasynthases similar to fatty acid synthases
• They use acyl carrier protein (ACP)‑bound substrates exclusively
• They are homodimeric enzymes that use CoA‑linked substrates without ACP
• They perform oxidative decarboxylation reactions to generate alkaloids

Correct Answer: They are homodimeric enzymes that use CoA‑linked substrates without ACP

Q13. Condensed tannins (proanthocyanidins) are oligomers or polymers of which flavonoid subunits?

• Anthocyanidins (e.g., cyanidin)
• Flavanones (e.g., naringenin)
• Flavan‑3‑ols (e.g., catechin, epicatechin)
• Isoflavones (e.g., genistein)

Correct Answer: Flavan‑3‑ols (e.g., catechin, epicatechin)

Q14. Prenylated flavonoids arise by transfer of a prenyl group to the flavonoid core. Which prenyl donor is most commonly used by plant aromatic prenyltransferases?

• UDP‑glucose
• Geranyl diphosphate (GPP)
• Dimethylallyl diphosphate (DMAPP)
• Farnesyl diphosphate (FPP)

Correct Answer: Dimethylallyl diphosphate (DMAPP)

Q15. Which enzyme type catalyzes the stereospecific cyclization of 2,3‑oxidosqualene to diverse triterpene or sterol skeletons in plant secondary metabolism?

• Terpene cyclase (monoterpene cyclase)
• Oxidosqualene cyclase (OSC)
• Polyprenyl synthase
• Polyketide cyclase (PKC)

Correct Answer: Oxidosqualene cyclase (OSC)

Q16. Conversion of tryptophan to tryptamine, a precursor for many indole alkaloids, is catalyzed by which enzyme?

• Tryptophan synthase
• Tryptophan decarboxylase (TDC)
• Tryptophan hydroxylase
• Indoleamine 2,3‑dioxygenase

Correct Answer: Tryptophan decarboxylase (TDC)

Q17. The MEP (non‑mevalonate) pathway that supplies plastidic isoprenoid precursors is localized predominantly in which cellular compartment?

• Cytosol/Endoplasmic reticulum
• Plastids (chloroplasts)
• Mitochondria
• Vacuole

Correct Answer: Plastids (chloroplasts)

Q18. Biosynthesis of the plant hormone abscisic acid (ABA) from carotenoids involves cleavage of 9‑cis‑epoxycarotenoids by which enzyme family member commonly denoted NCED?

• 9‑cis‑epoxycarotenoid dioxygenase (NCED)
• Carotenoid isomerase
• Phytoene desaturase
• β‑Carotene hydroxylase

Correct Answer: 9‑cis‑epoxycarotenoid dioxygenase (NCED)

Q19. Conjugation (glycosylation) of many plant secondary metabolites (e.g., flavonoids, saponins) is performed by which enzyme class using UDP‑sugars as donors?

• Glycosyl hydrolases
• UDP‑glycosyltransferases (UGTs)
• Glycosyl phosphotransferases
• Phosphoglycosyltransferases

Correct Answer: UDP‑glycosyltransferases (UGTs)

Q20. Iterative Claisen condensations of malonyl‑CoA to assemble plant polyketide backbones are catalyzed primarily by which enzyme family in plants?

• Type I multifunctional PKSs (mammalian type)
• Type II modular PKSs (bacterial type)
• Type III polyketide synthases (CHS‑like enzymes)
• Fatty acid synthases (FAS)

Correct Answer: Type III polyketide synthases (CHS‑like enzymes)

Download