Sesquiterpenes are terpenes with a basic 15-carbon framework, corresponding to three five-carbon units. Plants make them mainly from farnesyl diphosphate. Their structures range from open chains to complex rings, and their functions include chemical defence and communication.
The name becomes easier to remember when compared with a monoterpene: a monoterpene has ten carbons, while a sesquiterpene has fifteen—one and a half times that number. Carbon count identifies the class; it does not tell you the number of rings or the biological activity.
Sesquiterpenes and sesquiterpenoids: the difference
In a strict chemical usage, terpenes are hydrocarbons, whereas terpenoids include oxygenated or otherwise modified derivatives. Biological writing sometimes uses these terms broadly, so always consider the actual structure.
Many hydrocarbon sesquiterpenes have the molecular formula C15H24. It is not a universal formula for every sesquiterpenoid. Farnesol, for example, contains oxygen; artemisinin is a much more highly modified compound. The activated building blocks are isopentenyl diphosphate and dimethylallyl diphosphate, rather than free isoprene molecules joining inside a plant. [1,2]
Classification by carbon skeleton
| Structural group | Example | What to notice |
|---|---|---|
| Acyclic | Farnesene | An open-chain hydrocarbon framework |
| Monocyclic | α-Humulene | One large carbon ring |
| Bicyclic | β-Caryophyllene | Two rings, including a distinctive four-membered ring |
| Tricyclic | α-Cedrene | A compact three-ring framework |
| Oxygenated derivatives | Farnesol; artemisinin | A chemical-modification category that can overlap with ring-based classes |
Ring count and oxygenation are different classification axes. A compound can be both bicyclic and oxygenated; these are not mutually exclusive labels. [1,3]
Where are sesquiterpenes synthesized?
The usual precursor supply is the mevalonate pathway, associated primarily with the cytosolic metabolic system. The plastidial 2-C-methyl-D-erythritol 4-phosphate pathway generally supplies monoterpenes and diterpenes.
This division is a useful first model, not an absolute rule. Precursors can be exchanged between compartments, and enzyme location and substrate availability can change the products a plant makes. Some terpene synthases also accept more than one substrate. [2,4]
Sesquiterpene biosynthesis: step by step
1. Produce the activated five-carbon building blocks
Three molecules of acetyl–coenzyme A contribute to formation of 3-hydroxy-3-methylglutaryl–coenzyme A through acetoacetyl–coenzyme A. Reduction produces mevalonate. Phosphorylation and decarboxylation then produce isopentenyl diphosphate, which can be isomerized to dimethylallyl diphosphate. Energy and reducing power are required. [2]
2. Assemble the fifteen-carbon precursor
Farnesyl diphosphate synthase adds two isopentenyl diphosphate units to one dimethylallyl diphosphate starter. A ten-carbon intermediate forms after the first addition; the second addition yields the fifteen-carbon precursor.
Carbon bookkeeping: 5 + 5 + 5 = 15. The diphosphate group helps activate the molecule for subsequent reactions; it is not part of the fifteen-carbon skeleton. [1,5]
3. Create different skeletons
Sesquiterpene synthases act on farnesyl diphosphate to generate diverse products. Depending on the enzyme, the reaction may produce an open chain or involve cyclization and rearrangements. Metal ions commonly support the catalytic process. Enzyme structure controls how the reactive intermediate is positioned and transformed. [3]
4. Modify the initial product
Oxidation, reduction and other enzymatic reactions can add functional groups or alter the skeleton. Thus, making a hydrocarbon precursor is not the same as completing the biosynthesis of a complex sesquiterpenoid. Artemisinin biosynthesis, for example, passes through amorpha-4,11-diene and additional intermediates before the final compound appears. [5]
Functions in plants
Some sesquiterpenes are components of volatile blends released by leaves or flowers. These blends may influence herbivores, natural enemies of herbivores or other organisms. A compound's ecological effect depends on concentration, the surrounding blend and the responding species; not every sesquiterpene repels every insect. [2,6]
Sesquiterpenoids also contribute to specialized chemical defence. Their roles differ between tissues and species, and production can respond to developmental signals or stress. In metabolic engineering, increasing one product requires attention to precursor competition, enzyme activity and cellular compartmentation. More precursor alone does not guarantee a proportional rise in the target compound. [2,7]
Monoterpenes versus sesquiterpenes
| Feature | Monoterpenes | Sesquiterpenes |
|---|---|---|
| Basic carbon number | 10 | 15 |
| Five-carbon units | 2 | 3 |
| Typical immediate precursor | Geranyl diphosphate | Farnesyl diphosphate |
| Usual precursor pathway | Plastidial methylerythritol phosphate pathway | Mevalonate pathway |
| Hydrocarbon example | Limonene | β-Caryophyllene |
Five revision questions with explanations
1. How many five-carbon units form a sesquiterpene?
Three. Together they provide the basic fifteen-carbon framework.
2. What is the usual immediate precursor?
Farnesyl diphosphate, an activated fifteen-carbon molecule.
3. Is C₁₅H₂₄ the formula of every sesquiterpenoid?
No. Oxygenation and other modifications change the molecular formula.
4. Why can one precursor produce many compounds?
Different synthases direct different cyclizations, rearrangements and termination reactions.
5. Are all plant sesquiterpenes produced exclusively in the cytosol?
No. The usual compartmental model has exceptions and precursor exchange.
References and further reading
- Bohlmann and colleagues: Plant terpenoid synthases.
- Mani and colleagues: Metabolic perturbation and synthetic biology strategies for plant terpenoid production.
- Sesquiterpene synthases: passive catalysts or active players?
- Multi-substrate terpene synthases: occurrence and physiological significance.
- The biosynthesis of artemisinin and the phytochemistry of Artemisia annua.
- Diversity, regulation and genetic manipulation of plant mono- and sesquiterpenoid biosynthesis.
- Experimental study of precursor competition in Artemisia annua.
20 multiple-choice questions with answers
Use these questions for revision. Each answer includes a short explanation.
1. What is the basic carbon number of a sesquiterpene?
- 15
- 5
- 10
- 20
Show answer and explanation
Answer: A. 15 Sesquiterpenes have a basic fifteen-carbon framework.
2. How many five-carbon units build this framework?
- Two
- Three
- Four
- Six
Show answer and explanation
Answer: B. Three Three five-carbon units provide fifteen carbons.
3. What is the usual immediate precursor of plant sesquiterpenes?
- Geranyl diphosphate
- Glucose
- Farnesyl diphosphate
- Oxaloacetate
Show answer and explanation
Answer: C. Farnesyl diphosphate Farnesyl diphosphate is the usual fifteen-carbon precursor.
4. Which pathway usually supplies precursors for cytosolic sesquiterpene synthesis?
- Glycolysis alone
- Calvin cycle alone
- Citric acid cycle alone
- Mevalonate pathway
Show answer and explanation
Answer: D. Mevalonate pathway The mevalonate pathway is the usual source, although compartment exchange creates exceptions.
5. Which distinction between a terpene and a terpenoid is useful?
- Terpenes are hydrocarbons; terpenoids include modified derivatives
- Terpenoids always have fewer than five carbons
- Terpenes always contain nitrogen
- The terms identify different kingdoms
Show answer and explanation
Answer: A. Terpenes are hydrocarbons; terpenoids include modified derivatives Oxygenation and other modifications produce terpenoid derivatives.
6. Which is a sesquiterpene hydrocarbon?
- Limonene
- Beta-caryophyllene
- Glucose
- Cholesterol
Show answer and explanation
Answer: B. Beta-caryophyllene Beta-caryophyllene is a fifteen-carbon sesquiterpene hydrocarbon.
7. Which compound is a sesquiterpene alcohol?
- Sucrose
- Citrate
- Farnesol
- Glyoxylate
Show answer and explanation
Answer: C. Farnesol Farnesol is an oxygenated sesquiterpenoid with an alcohol group.
8. Which is an oxygenated sesquiterpenoid rather than a hydrocarbon terpene?
- Isoprene
- Limonene
- Ethylene
- Artemisinin
Show answer and explanation
Answer: D. Artemisinin Artemisinin is a highly modified oxygenated sesquiterpenoid.
9. What does acyclic mean in structural classification?
- An open-chain skeleton without rings
- Exactly one ring
- Exactly two rings
- A skeleton made only of oxygen
Show answer and explanation
Answer: A. An open-chain skeleton without rings Acyclic structures contain no carbon rings.
10. What does monocyclic mean?
- No rings
- One ring
- Two rings
- Three rings
Show answer and explanation
Answer: B. One ring Monocyclic structures contain one ring.
11. Which feature is used in classification as bicyclic or tricyclic?
- Leaf shape
- Flower colour
- Number of rings
- Number of seed leaves
Show answer and explanation
Answer: C. Number of rings These categories describe two-ring and three-ring skeletons.
12. Which activated units combine to form farnesyl diphosphate?
- Three glucose molecules
- Two succinate molecules
- One citrate and one malate
- One dimethylallyl diphosphate and two isopentenyl diphosphates
Show answer and explanation
Answer: D. One dimethylallyl diphosphate and two isopentenyl diphosphates A five-carbon starter plus two five-carbon extenders gives fifteen carbons.
13. Which enzyme joins the five-carbon units to form farnesyl diphosphate?
- Farnesyl diphosphate synthase
- Isocitrate lyase
- Malate synthase
- Rubisco
Show answer and explanation
Answer: A. Farnesyl diphosphate synthase Farnesyl diphosphate synthase catalyses the sequential additions.
14. What happens to the diphosphate group during carbon bookkeeping?
- It adds five carbon atoms
- It is not part of the fifteen-carbon skeleton
- It becomes a fourth isoprene unit
- It doubles the carbon number
Show answer and explanation
Answer: B. It is not part of the fifteen-carbon skeleton The diphosphate group activates the molecule but contributes no carbon to the skeleton.
15. What is a major role of sesquiterpene synthases?
- Producing glucose directly
- Hydrolysing starch
- Generating diverse carbon skeletons from the precursor
- Fixing nitrogen
Show answer and explanation
Answer: C. Generating diverse carbon skeletons from the precursor Cyclization, rearrangements and termination reactions generate different products.
16. Why can different synthases produce different products from the same precursor?
- The precursor has a different carbon number each time
- Only temperature matters
- No enzymes are involved
- Their active sites guide different reaction paths
Show answer and explanation
Answer: D. Their active sites guide different reaction paths Enzyme structure controls how reactive intermediates are positioned and transformed.
17. Which statement about plant functions is accurate?
- Some sesquiterpenes contribute to defence and communication
- Every sesquiterpene repels every insect
- All sesquiterpenes are photosynthetic pigments
- None can be volatile
Show answer and explanation
Answer: A. Some sesquiterpenes contribute to defence and communication Effects depend on the compound, concentration and interacting organism.
18. How many carbons does a monoterpene usually contain?
- Five
- Ten
- Fifteen
- Thirty
Show answer and explanation
Answer: B. Ten Monoterpenes have a basic ten-carbon framework.
19. Is the formula C15H24 valid for every sesquiterpenoid?
- Yes, without exceptions
- Only for sugars
- No; modifications can change the molecular formula
- Yes, because oxygen has no effect on formula
Show answer and explanation
Answer: C. No; modifications can change the molecular formula Oxygenation and other modifications alter molecular formulas.
20. Does increasing precursor supply always proportionally increase the final product?
- Yes, in every plant
- Yes, without enzymes
- Only if light is absent
- No; enzyme activity, competing pathways and compartmentation also matter
Show answer and explanation
Answer: D. No; enzyme activity, competing pathways and compartmentation also matter Product formation depends on more than precursor availability.
Interactive quiz: test your understanding
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