Triterpenes and Steroids


Triterpenes and steroids


There are at least 4000 known triterpenes. They are derived from mevalonic acid, not from the DOXP pathway.

Triterpenes are precursors to steroids in both plants and animals. Steroids are hormonal substances in animals, but they are components of membranes in most organisms.

Many triterpenes occur free, but others occur as glycosides (saponins) or in special combined forms.

Biosynthesis


Triterpenes are derived from a triterpene squalene that is formed by condensation of two units of farnesyl pyrophosphate. Squalene is then converted to 2,3-squalene epoxide and cyclized by two different pathways to make most triterpenes.

squalene ----> 2,3-squalene epoxide ----> cyclization


However, some triterpenes, like tetrahymanol, are condensed directly from squalene.

In contrast to animals, cycloartenol is the key intermediate in the formation of some triterpenes and all steroids in plants.


Squalene synthetase


A single enzyme, squalene synthetase, carries out all the steps from FPP to squalene, but two catalytic sites may be involved.

MVA and related precursors are incorporated at least 100 times more efficiently than in monoterpenes and sesquiterpenes. This is not as surprising as it once was, not that it has been established that monoterpenes and some sesquiterpenes are derived by another pathway.


Squalene epoxidase


The two most important types of triterpenes are derived by orientation of the precursor squalene epoxide in two distinct arrangements followed by condensation. One type leads to triterpenes and to steroids. The other main type leads to pentacyclic triterpenes (see below).

chair-boat-chair-boat ----> cycloartenol and triterpenoids ----> steroids


Phytosterols


Steroids are derivatives of triterpenes with 26 or more carbon atoms that have undergone a characteristic type of rearrangement.


Cholesterol is an important sterol in animals. This compound is an intermediate in the synthesis of other steroids in plants, but is rarely, if ever, accumulated in plants.

β-Sitosterol, stigmasterol, and campesterol are the most important plant sterols. They are found in a large number of plant species.


Prostagens


Progestagens are derived from cholesterol in plants and animals. Most reports of progestagens from plants are probably not valid however.

Progestagens are mostly C21- compounds.


Ecdysteroids


Ecdysteroids are hormonal substances in insects, molluscs, and crustaceans. They are involved in moulting. Various developmental processes require change in the ratios of ecdysones and juvenile hormones.

Many plants synthesize ecdysteroids. Although these are called phytoecdysteroids, some are identical in structure to the compounds from animals.

There is a requirement for phytosterols (or other sources of sterols) in the diet for these organisms because they lack the ability to synthesize sterols de novo.

Chemotaxis in fungi


A series of sterols including antheridiol and oogoniols, derived from fucosterol, are involved in attraction of gametes and concomitant developmental changes in members of the genus Achlya.

Cucurbitacins


Cucurbitacins are triterpenes. They are extremely bitter compounds and also are highly toxic. Although they are common in the family Cucurbitaceae, they also are the bitter principles in at least 20 other families. They serve as powerful kairomones for certain chrysomelic beetles.

Triterpenes by chair-chair-chair-boat intermediate


Pentacyclic triterpenes are produced by arrangement of squalene epoxide in a chair-chair-chair-boat arrangement followed by condensation. These compounds are also extremely common and are found in most plants.

Common representatives of this group are α-amyrin, β-amyrin, friedelin, oleanolic acid and taraxerane.


Related Images

Assorted triterpenes

Presqualene pyrophosphate biosynthesis

Compounds similar to triterpenes in synthesis

Cycloartenol formation

Cholesterol biosynthesis

Phytosterol biosynthesis

Bioactive triterpenes

Progesterone, pregnenolone and related compounds

Ecdysones

Ecdysone biosynthesis

Assorted pentacyclic triterpenes

Pentacyclic triterpene biosynthesis

More pentacyclic triterpene biosynthesis

Triterpenoid chemotactic compounds

More bioactive triterpenes


Lecture Slides

Plants with Triterpenes and Steroids



© David S. Seigler, Integrative Biology 425, Plant Secondary Metabolism, Department of Plant Biology, 265 Morrill Hall, 505 S. Goodwin Ave., University of Illinois, Urbana, Illinois 61801, USA. 217-333-7577. seigler@life.uiuc.edu.