The reaction of
geranyl pyrophosphate with
isopentenyl pyrophosphate results in the 15-carbon
farnesyl pyrophosphate (FPP), which is an intermediate in the biosynthesis of sesquiterpenes such as
farnesene. Cyclic sesquiterpenes are more common than cyclic monoterpenes because of the increased chain length and additional double bond in the sesquiterpene precursors. In addition to common six-membered ring systems such as the ones found in
zingiberene and
bisacurone, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as
humulene. File:Longifolene plus acsv.svg|
Longifolene, found in terpentine File:Zingiberene.png|
Zingiberene, abundant in
ginger File:Cadinene.svg|δ-
Cadinene, found in
cade juniper File:Humulene.png|
Humulene, found in
hops. File:Caryophyllene.svg|
Caryophyllene, which is pervasive but especially in cloves. The
cadinenes contain two fused six-membered rings.
Caryophyllene, a component of many
essential oils such as clove oil, contains a nine-membered ring fused to a
cyclobutane ring. Rishitin is another example of a cadinene, which is found in potatoes and tomatoes.
Vetivazulene and
guaiazulene are aromatic bicyclic sesquiterpenoids. With the addition of a third ring, the possible structures become increasingly varied. Examples include
longifolene,
copaene and the alcohol
patchoulol.
Sesquiterpenoids The FPP backbone can be rearranged in several different ways and further decorated with different functional groups, hence the large variety of sesquiterpenoids.
Geosmin, the volatile compound that gives an earthy taste and musty odor in drinking water and the characteristic odor on a rainy day, is a sesquiterpenoid, produced by bacteria, especially
cyanobacteria, that are present in the soils and water supplies. Oxidation of farnesene then provides the sesquiterpenoid
farnesol.
Sesquiterpene lactones are a common class of sesquiterpenoids that contain a
lactone ring, hence the name. They are found in many plants and can cause allergic reactions and toxicity if consumed excessively, particularly in grazing livestock. The term merosesquiterpenoids was coined in 1968 to describe molecules of this class that have a mixed biosynthetic origin, meaning isoprenoid precursors like isopentenyl pyrophosphate are derived from both the
mevalonate and
non-mevalonate pathways. ==References==