Terpenes
Terpenes are a huge family of natural products built from repeating 5-carbon units called isoprene (2-methyl-1,3-butadiene). The number of isoprene units classifies the terpene.
The Isoprene Rule
Every terpene is assembled from isoprene units, head-to-tail. The number of units gives the class name:
| Class | Isoprene units | Carbons | Example |
|---|---|---|---|
| Monoterpene | 2 | 10 | Menthol, limonene (citrus) |
| Sesquiterpene | 3 | 15 | Farnesol |
| Diterpene | 4 | 20 | Retinol (vitamin A), phytol |
| Triterpene | 6 | 30 | Squalene (cholesterol precursor) |
| Tetraterpene | 8 | 40 | Beta-carotene |
| Polyterpene | Many | Large | Rubber |
Terpene vs. Terpenoid
- Terpene: pure hydrocarbon made of isoprene units.
- Terpenoid: a terpene with added oxygens or rearrangements (which is most of them in real biology).
These terms are often used interchangeably, but technically “terpenoid” is broader.
MCAT-Relevant Examples
- Squalene (C30) is the direct precursor of cholesterol. Cyclization of squalene builds the four-ring steroid skeleton.
- Beta-carotene (C40) is a tetraterpene found in carrots, spinach, and other orange/green vegetables. It is cleaved into two molecules of vitamin A (retinol, C20) in your intestines.
- Vitamin A (retinol) (C20) is a diterpene alcohol. Its aldehyde form (retinal) is the visual pigment in rhodopsin; light-induced cis-to-trans isomerization of retinal is the first step of vision.
- Coenzyme Q (ubiquinone) has a long polyprenyl side chain (isoprenoid), giving it lipid solubility to shuttle electrons through the inner mitochondrial membrane.
What is the structural building block of a terpene, and how many carbons does it have?
Click to reveal answer
Isoprene (2-methyl-1,3-butadiene), a 5-carbon unit. Terpenes are built by joining isoprene units head-to-tail. The number of isoprenes determines the class: monoterpene (2 isoprenes = 10 carbons), sesquiterpene (3 = 15), diterpene (4 = 20), triterpene (6 = 30, e.g., squalene), and so on.
What is the difference between a terpene and a terpenoid?
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Terpenes are pure hydrocarbons built from isoprene units. Terpenoids (sometimes called isoprenoids) are terpene derivatives with added oxygens, functional groups, or rearranged skeletons. Most biologically important "terpenes" you encounter are technically terpenoids. The terms are often used interchangeably.
Which terpene is the direct precursor to cholesterol?
Click to reveal answer
Squalene - a 30-carbon triterpene. Cyclization of squalene builds the four-ring steroid nucleus, from which cholesterol and all other steroids are derived. Mevalonate, upstream of squalene, is the product of HMG-CoA reductase - the enzyme blocked by statins.