Synthesis of terpenes from isopentenyl diphosphate. Isopentenyl
diphosphate (IPP) is isomerized to form dimethylallyl diphosphate.A
prenyltransferase creates the 10-carbon geranyl diphosphate, from which
monoterpenes are derived.Addition of further IPP units to geranyl
diphosphate via prenyltransferases produces 15- and 20-carbon compounds,
which give rise to the sesquiterpenes and diterpenes, respectively.Terpenes
of 30, 40, or more carbons are produced from addition of 15-carbon and
20-carbon units. Examples of the multitude of terpenoid compounds are
listed (green).Some, such as gibberellins, cholesterol, and carotenoids, are
found in essentially all plants.Others, such as the essential oil limonene
(found in lemon fruits), occur in only a small number of species.The roles
of terpenoids as defense compounds and phytoalexins are discussed
in Chapter 8.