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