Figure 6.

The major subclasses of terpenoids are biosynthesized from the basic five-carbon unit, IPP, and from the initial prenyl (allylic) diphosphate, dimethylallyl diphosphate, which is formed by isomerization of IPP. In reactions catalyzed by prenyltransferases, monoterpenes (C10), sesquiterpenes (C15), and diterpenes (C20) are derived from the corresponding intermediates by sequential head-to-tail addition of C5 units. Triterpenes (C30) are formed from two C15 (farnesyl) units joined head-to-head, and tetraterpenes (C40) are formed from two C20 (geranylgeranyl) units joined head-to-head