Table of Contents
Overview
Terpenes (or isoprenoids) are a very large and diverse group of natural substances built from simple 5‑carbon units called isoprene units. They occur in all domains of life and are especially abundant in plants, where they function as pigments, hormones, defense compounds, and signaling molecules.
In this chapter, the focus is on what makes terpenes special as a class of biological molecules: their basic building principle, how they are classified, and key biological functions and examples.
The Isoprene Unit and the “Isoprene Rule”
The structural basis of terpenes is the isoprene unit, a 5‑carbon building block. As a simplified formula, isoprene is:
$$$$
\text{Isoprene: } \mathrm{C_5H_8}
$$$$
Many terpenes have structures that can be mentally “cut” into isoprene‑like units. This observation is summarized in the isoprene rule:
- Terpenes are constructed from multiple isoprene units, usually linked “head‑to‑tail”.
“Head‑to‑tail” means that the end of one isoprene unit (the “head”) is joined to the beginning of the next (the “tail”), which gives rise to long chains and ring structures. Because they derive from isoprene units, terpenes are also called isoprenoids.
Classification by Number of Isoprene Units
Terpenes are categorized by how many isoprene units they contain. This is a useful way to relate structure to size and often to function.
- Hemiterpenes
- 1 isoprene unit
- Molecular formula typically: $$\mathrm{C_5H_8}$$
- Smallest terpenes, relatively simple structures.
- Monoterpenes
- 2 isoprene units
- Typically: $$\mathrm{C_{10}H_{16}}$$
- Often volatile, major components of many essential oils (for example, fragrances in flowers and herbs).
- Sesquiterpenes
- 3 isoprene units
- Typically: $$\mathrm{C_{15}H_{24}}$$
- Less volatile than monoterpenes, often bitter or pungent, frequently involved in plant defense.
- Diterpenes
- 4 isoprene units
- Typically: $$\mathrm{C_{20}H_{32}}$$
- Include many important biological molecules such as plant growth regulators and photosynthetic pigments (when further modified).
- Sesterterpenes
- 5 isoprene units
- Typically: $$\mathrm{C_{25}H_{40}}$$
- Relatively rare, often found in marine organisms and some fungi.
- Triterpenes
- 6 isoprene units
- Typically: $$\mathrm{C_{30}H_{48}}$$
- Precursors for many steroids; can form complex cyclic structures and resins.
- Tetraterpenes
- 8 isoprene units
- Typically: $$\mathrm{C_{40}H_{64}}$$
- Include carotenoids, important pigments in plants and many microorganisms.
- Polyterpenes
- Many isoprene units (polymeric)
- General formula: $$(\mathrm{C_5H_8})_n$$ with large $$n$$
- Natural rubber is the most famous example.
When oxygen-containing groups (like hydroxyl, carbonyl, or carboxyl groups) are added, one often speaks of terpenoids. In practice, “terpenes” and “terpenoids” are frequently used together or interchangeably.
Biosynthetic Origin (Outline Only)
Terpenes are not taken up as such from the environment in most organisms; they are synthesized from universal metabolic intermediates. Two central 5‑carbon activated building blocks are:
- Isopentenyl pyrophosphate (IPP)
- Dimethylallyl pyrophosphate (DMAPP)
These are combined enzymatically to build larger isoprenoid chains by adding 5‑carbon units stepwise. Different pathways (for example, the mevalonate pathway, or the MEP/DOXP pathway) supply IPP/DMAPP in different organisms and compartments, but the isoprene‑based construction principle is universal for terpenes.
Details of these pathways are covered in metabolism chapters; here it is important only that all terpenes share this common origin from IPP and DMAPP.
Structural Diversity
Although all terpenes are based on the same 5‑carbon units, they show enormous structural diversity:
- Linear (acyclic) terpenes
- Straight or branched carbon chains without rings.
- Example: some monoterpenes and polyterpenes.
- Monocyclic terpenes
- Contain one ring.
- Example: limonene (a monoterpene with a single ring).
- Polycyclic terpenes
- Contain two or more rings.
- Example: many sesquiterpenes and triterpenes.
This diversity is created by enzymes that:
- Join the isoprene units in different sequences and orientations.
- Fold and cyclize the chains into rings.
- Introduce additional functional groups (oxygen, sugars, etc.).
Because of this, terpenes can act as volatile scents, rigid membrane components, strongly colored pigments, flexible polymers, or complex signaling molecules.
Selected Biological Roles and Examples
Terpenes participate in numerous biological processes. Below are important roles with characteristic examples.
1. Plant Volatiles: Fragrances and Essential Oils
Many of the smells we associate with plants come from terpene mixtures:
- Monoterpenes and sesquiterpenes are major components of essential oils (volatile, aromatic mixtures).
- Examples:
- Limonene (monoterpene) gives citrus fruits their characteristic smell.
- Menthol (a monoterpenoid) contributes to the scent and cooling sensation of mint.
- Pinene (monoterpene) smells of pine and resin.
Functions for the plant include:
- Attracting pollinators and seed dispersers (pleasant scents, floral fragrances).
- Repelling herbivores or inhibiting microbial growth (toxic or deterrent compounds).
- Communicating with neighboring plants (volatile signals inducing defense responses).
2. Pigments: Carotenoids (Tetraterpenes)
Carotenoids are tetraterpenes that serve as important pigments:
- Provide yellow, orange, and red colors in many flowers, fruits, and leaves (for example, in carrots, tomatoes, autumn leaves).
- In photosynthetic organisms:
- Assist in light harvesting by absorbing light wavelengths not absorbed efficiently by chlorophyll.
- Protect against photooxidative damage by quenching reactive oxygen species and excess energy.
Typical carotenoids:
- β‑Carotene: a precursor (provitamin) of vitamin A in animals.
- Lutein and zeaxanthin: important in plant photosystems and in animal eyes.
3. Hormones and Growth Regulators
Certain terpenes function as hormones or hormone precursors:
- Gibberellins (gibberellic acids)
- Diterpenoids that act as plant growth regulators.
- Influence stem elongation, seed germination, flowering, and fruit development.
- Abscisic acid (ABA)
- Often classified as a terpenoid in terms of biosynthetic origin.
- Involved in stress responses and stomatal closure in plants.
- Brassinosteroids
- Steroid-like plant hormones derived from triterpenes.
- Regulate cell expansion and division.
In animals, some terpenoid derivatives are also involved in hormone systems; those are covered in more detail in hormone or steroid-specific chapters.
4. Sterols and Steroids (Triterpene Derivatives)
Many biologically important sterols and steroids are derived from triterpenes:
- Squalene is a linear triterpene that serves as a key precursor of sterols.
- From squalene, cyclic sterols are formed, such as:
- Cholesterol in animals.
- Phytosterols (for example, sitosterol) in plants.
- Ergosterol in fungi.
Key roles:
- Components of biological membranes (affecting fluidity, stability, and permeability).
- Precursors for:
- Steroid hormones (in animals: for example, sex hormones, corticosteroids).
- Bile acids (important in fat digestion in vertebrates).
- Certain vitamins.
While the detailed function of steroids is treated elsewhere, it is important here that they belong to the isoprenoid/terpenoid family via their triterpene precursors.
5. Polyterpenes: Natural Rubber and Related Materials
Polyterpenes are long-chain polymers made from many isoprene units:
- Natural rubber (latex) from the rubber tree (Hevea brasiliensis) is primarily cis‑1,4‑polyisoprene.
- The repeating unit of rubber can be represented as:
$$$$
\left[\mathrm{–CH_2–C(CH_3)=CH–CH_2–}\right]_n
$$$$
Biological and practical roles:
- In plants:
- Rubber can seal wounds and act as a physical and chemical defense.
- For humans:
- Basis for many industrial products (tires, elastic bands, gloves, etc.).
Other polyisoprenoids include dolichols and polyprenols, which are involved in membrane processes and protein modification in cells.
6. Defense Compounds and Toxins
Many terpenes are defensive substances produced by plants, fungi, and some animals:
- Bitter, pungent, or toxic terpene compounds deter herbivores and pathogens.
- Examples:
- Some sesquiterpene lactones in the Asteraceae (daisy family) are strongly bitter and can be toxic.
- Certain diterpenes in conifer resins protect against insects and microbes.
- Terpenoid toxins in some insects and amphibians originate from their diet or symbiotic microorganisms and help deter predators.
These substances can also have medicinal or pharmacological effects in humans, making terpenes an important source of drugs and natural remedies.
7. Isoprenoids in Cellular Processes
Beyond pigments, hormones, and volatiles, some isoprenoids have more subtle but essential roles:
- Ubiquinone (coenzyme Q)
- An isoprenoid with a long polyisoprenoid tail.
- Functions in the electron transport chain of cellular respiration by shuttling electrons in membranes.
- Plastoquinone
- Similar to ubiquinone but active in photosynthesis in chloroplasts.
- Prenylated proteins
- Some proteins are modified by covalent attachment of short isoprenoid chains (for example, farnesyl or geranylgeranyl groups).
- This “prenylation” helps anchor proteins to cell membranes and can affect their function and localization.
These examples show that isoprenoids are not only structural and defensive but also involved in fundamental energy and signaling processes.
Ecological and Practical Significance
Because of their volatility, color, and biological activity, terpenes have wide-ranging ecological and human relevance:
- Ecological roles:
- Mediate plant–pollinator and plant–herbivore interactions.
- Contribute to communication between organisms (for example, alarm signals, attraction of predators of herbivores).
- Influence atmospheric chemistry (volatile terpenes can affect formation of aerosols and cloud condensation nuclei).
- Human uses:
- Fragrances and flavors (perfumes, food flavoring).
- Medicinal substances (for example, anti-inflammatory, antimicrobial, or anticancer agents derived from terpenoids).
- Industrial materials (rubber, resins, turpentine, solvents).
Understanding terpenes as isoprenoids built from a simple 5‑carbon unit helps to connect their chemical structure, biosynthetic origin, and wide array of functions in living organisms and ecosystems.