Mevalonate is the key precursor for synthesis of cholesterol and related isoprenoid compounds. Isoprenoid s are compounds which appear to be formed by polymerization of multiple units of isoprene, C 5 H 8 . These compounds are relatively volatile plant products which are important flavour and aroma components of food. Triterpenoids are the basis for compounds such as cholesterol or diosgenin although the loss of three methyl groups makes the isoprene origin less obvious. While higher plants have the greatest variety of isoprenoids, cholesterol and related sterols are important components of biological membranes.
Cholesterol is also the precursor of the steroid hormones in animals.
Although isoprenoids have been known chemically for over a century, the biosynthetic origin was a mystery until the discovery of mevalonate as an essential growth factor for certain strains of lactobacilli. This yields isopentenyl pyrophosphate which is clearly recognisable as an isoprene unit. The first condensation generates the C 10 product geranyl pyrophosphate, which is the precursor of monoterpenoids in plants. Additional isopentenyl pyrophosphate units can join by this head to tail condensation, elongating the chain by one C 5 prenyl unit each time. Sterols such as cholesterol are derived from the C 15 product farnesyl pyrophosphate.