Terpenoids: Classification, isoprene rule; mono-, di-, tri-terpenoids MCQs With Answer

Introduction: Terpenoids: Classification, isoprene rule; mono-, di-, tri-terpenoids MCQs With Answer is a focused quiz resource designed for M.Pharm students studying MPC 104T Chemistry of Natural Products. This collection reviews fundamental and advanced concepts of terpenoid chemistry—covering the isoprene rule, biosynthetic precursors (IPP/DMAPP), chain-building (GPP, FPP, GGPP), cyclization mechanisms, common enzyme types, and representative mono-, di- and triterpenoids. Questions emphasize structural logic, biosynthetic pathways, stereochemistry, and pharmaceutical relevance to reinforce classroom learning and exam preparation with practice items that probe both conceptual understanding and applied knowledge.

Q1. Which statement best describes the isoprene rule in terpene biosynthesis?

• Terpenes are assembled from isoprene units in a head-to-tail fashion such that C1 of one isoprene bonds to C4 of the next
• Terpenes are randomly linked carbon units without any specific pattern
• Isoprene rule states all terpenes are linear chains only
• Terpenes are formed exclusively by tail-to-tail linkages of isoprene units

Correct Answer: Terpenes are assembled from isoprene units in a head-to-tail fashion such that C1 of one isoprene bonds to C4 of the next

Q2. What is the primary chemical distinction between a terpene and a terpenoid?

• Terpenoids contain oxygen-containing functional groups while terpenes are hydrocarbons
• Terpenes contain oxygen while terpenoids are pure hydrocarbons
• Terpenoids are always larger molecules than terpenes
• There is no chemical difference; names are interchangeable

Correct Answer: Terpenoids contain oxygen-containing functional groups while terpenes are hydrocarbons

Q3. What is the immediate C10 biosynthetic precursor for monoterpene formation?

• Geranyl pyrophosphate (GPP)
• Farnesyl pyrophosphate (FPP)
• Geranylgeranyl pyrophosphate (GGPP)
• Isopentenyl phosphate (IP)

Correct Answer: Geranyl pyrophosphate (GPP)

Q4. Which prenyl diphosphate serves as the direct precursor for diterpene biosynthesis?

• Geranylgeranyl pyrophosphate (GGPP)
• Geranyl pyrophosphate (GPP)
• Farnesyl pyrophosphate (FPP)
• Squalene

Correct Answer: Geranylgeranyl pyrophosphate (GGPP)

Q5. Which C30 compound is the principal linear precursor of triterpenes and sterols?

• Squalene
• Lanosterol
• Geranyl pyrophosphate
• Mevalonate

Correct Answer: Squalene

Q6. Which two metabolic pathways supply the five-carbon units IPP and DMAPP for terpenoid biosynthesis?

• Mevalonate (MVA) pathway and MEP/DOXP (non-mevalonate) pathway
• Glycolysis and Krebs cycle
• Shikimate pathway and polyketide pathway
• Beta-oxidation and fatty acid synthesis

Correct Answer: Mevalonate (MVA) pathway and MEP/DOXP (non-mevalonate) pathway

Q7. Formation of squalene from two FPP molecules is an example of which type of coupling that is an exception to the simple head-to-tail isoprene rule?

• Head-to-head coupling
• Strict head-to-tail coupling
• Sequential methylation coupling
• Radical polymerization coupling

Correct Answer: Head-to-head coupling

Q8. Which of the following is a commonly cited monoterpenoid with cooling and analgesic properties?

• Menthol
• Cholesterol
• Squalene
• Lanosterol

Correct Answer: Menthol

Q9. Which enzyme class catalyzes the conversion of geranyl pyrophosphate into cyclic monoterpenes like limonene?

• Monoterpene synthase (terpene cyclase)
• Oxidosqualene cyclase
• Polyketide synthase
• Cytochrome P450 reductase

Correct Answer: Monoterpene synthase (terpene cyclase)

Q10. Paclitaxel (Taxol) is classified as which type of terpenoid?

• Diterpenoid
• Monoterpenoid
• Triterpenoid
• Sesquiterpenoid

Correct Answer: Diterpenoid

Q11. During terpenoid biosynthesis, what is the mechanistic role of the pyrophosphate (PPi) leaving group from prenyl diphosphates?

• It departs to generate an allylic carbocation that initiates cyclization or rearrangement
• It becomes permanently bound to the final terpene scaffold
• It acts as a strong nucleophile to attack the substrate
• It oxidizes the substrate to form an alcohol

Correct Answer: It departs to generate an allylic carbocation that initiates cyclization or rearrangement

Q12. Match the terpene class to its carbon count: monoterpenes, diterpenes, triterpenes.

• Monoterpenes C10, Diterpenes C20, Triterpenes C30
• Monoterpenes C5, Diterpenes C10, Triterpenes C15
• Monoterpenes C15, Diterpenes C30, Triterpenes C45
• Monoterpenes C20, Diterpenes C30, Triterpenes C40

Correct Answer: Monoterpenes C10, Diterpenes C20, Triterpenes C30

Q13. How many carbon atoms are present in farnesyl pyrophosphate (FPP), the precursor of sesquiterpenes?

• 15 carbons
• 10 carbons
• 20 carbons
• 5 carbons

Correct Answer: 15 carbons

Q14. Which enzyme catalyzes the cyclization of 2,3-oxidosqualene to produce lanosterol in sterol biosynthesis?

• Oxidosqualene cyclase (lanosterol synthase)
• Squalene synthase
• Geranyltransferase
• Monoterpene synthase

Correct Answer: Oxidosqualene cyclase (lanosterol synthase)

Q15. The two enantiomers of limonene show different odors. Which description is correct for (+)-limonene?

• (+)-Limonene has a citrus (orange) aroma
• (+)-Limonene has an earthy, mushroom-like scent
• (+)-Limonene is odorless
• (+)-Limonene smells like mint

Correct Answer: (+)-Limonene has a citrus (orange) aroma

Q16. Conversion of a terpene hydrocarbon to a terpenoid typically involves which type of chemical transformation?

• Oxidation to introduce oxygenated functional groups (alcohols, ketones, acids)
• Hydrogenation to remove double bonds exclusively
• Polymerization to make larger hydrocarbon chains
• Halogenation as the primary step

Correct Answer: Oxidation to introduce oxygenated functional groups (alcohols, ketones, acids)

Q17. Which of the following is a diterpenoid widely used as an anticancer drug?

• Paclitaxel (Taxol)
• Menthol
• Squalene
• Cholesterol

Correct Answer: Paclitaxel (Taxol)

Q18. How many isoprene units are incorporated into a typical triterpene molecule?

• Six isoprene units
• Two isoprene units
• Four isoprene units
• Ten isoprene units

Correct Answer: Six isoprene units

Q19. Which oxygenated monoterpenoid is classically used as a topical rubefacient and has a bicyclic ketone structure?

• Camphor
• Squalane
• Geraniol
• Phytol

Correct Answer: Camphor

Q20. In the isoprene unit nomenclature used for terpene assembly, which carbon is designated the ‘head’ and which the ‘tail’ in the common head-to-tail linkage?

• Head = C1, Tail = C4
• Head = C5, Tail = C2
• Head = C3, Tail = C1
• Head = C4, Tail = C5

Correct Answer: Head = C1, Tail = C4

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