Lipids Derived from Isoprene Terpenes

The name terpene was applied originally to the steam-distillable oils obtained from turpentine (an extract of pine). It was recognized that  

Most of the compounds present in the oil have the formula C10H15. 

Terpenes with more than 10 carbons exist, the number of carbons being usually a multiple of five. The structures are extraordinarily diverse. 

Many similar water-insoluble compounds are distributed very widely particularly large quantities are found in many plants, but they exist also in most other living organisms. 

Terpenes with IOC atoms are known as monoterpenes. Two examples are  

---

Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenicaHy derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the perhydro-l,2-cyclopentenophenanthrene ring system (1) and are found in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primarily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring junctions. 

The ultimate precursor of all the carbon atoms in cholesterol and in the other steroids that are derived from cholesterol is the acetyl group of acetyl-GoA. There are many steps in the biosynthesis of steroids. The condensation of three acetyl groups produces mevalonate, which contains six carbons. Decarboxylation of mevalonate produces the five-carbon isoprene unit frequently encountered in the structure of lipids. The involvement of isoprene imits is a key point in the biosynthesis of steroids and of many other compounds that have the generic name terpenes. Vitamins A, E, and K come from reactions involving terpenes that humans cannot carry out. That is why we must consume these vitamins in our diets vitamin D, the remaining lipid-soluble vitamin, is derived from cholesterol (Section 8.8). Isoprene units are involved in the biosynthesis of ubiquinone (coenzyme Q) and of derivatives of proteins and tRNA with specific five-carbon units attached. Isoprene units are often added to proteins to act as anchors when the protein is attached to a membrane. 

---