Terpenoids (Isoprenoids)

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Biogenetic Outline. The basal, central building block for any biosynthesis of terpenoids (isoprenoids), isopentenyl diphosphate, may be generated in plants by two different pathways, the cytosolic mevalonate and the plastidic methylerythritol (deoxyxylulose) pathways. The cytosolic pathway was assumed to provide the precursors for sesquiterpenoids and triterpenoids/steroids, the plastidic one for mono-, di-, and tetraterpenoids. However, it has been proven recently that this strict separation of both pathways does not exist. Cross-talk between both path-ways has been discovered (De-Eknamkul and Potduang 2003; Bartram et al. 2006 and references therein). The transformation of isopentenyl diphosphate to dimethylallyl diphosphate is catalyzed by a corresponding isomerase. A condensation of both isomers catalyzed by a plastidic prenyltransferase leads to the common precursor of all monoterpenoids (C10), geranyl diphosphate (“head-to-tail” condensation). Condensation of geranyl diphosphate and another isopentenyl diphosphate generates farnesyl diphosphate (C15), the common precursor of all sesquiterpenoids. Farnesyl diphosphate in turn acts as a prenyl donor for isopentenyl diphosphate to form geranylgeranyl diphosphate (C20), the common precursor of all diterpenoids. In contrast to all these “head-to-tail” condensations, “head-to-head” condensation of two molecules of farnesyl diphosphate is responsible for the formation of squalene (C30), the common precursor of all triterpenoids. The analogous reaction of two molecules of geranylgeranyl diphosphate yields phytoene (C40), the common precursor of all tetraterpenoids. Almost all of these precursors are generated as all-trans isoprenoids (exception: phytoene is yielded predominantly in the 15-cis configurated form). Nevertheless, they may change their configuration in the course of specific pathways leading to certain secondary metabolites.