Terpenoid Metabolism

Abstract

INTRODUCTIONThe terpenoids constitute the largest family of natural products; over 22,000 individual compounds of this class have been described (Connolly and Hill, 1991), and the number of defined structures has doubled every decade since the 1970s (Devon and Scott, 1972;Glasby, 1982). The terpenoids play diverse functional roles in plants as hormones (gibberellins, abscisic acid), photosynthetic pigments (phytol, carotenoids), electron carriers (u biquinone, plastoquinone), mediators of polysaccharide assembly (polyprenyl phosphates), and structural components of membranes (phytosterols). In addition to these universal physiological, metabolic, and structural functions, many specific terpenoid compounds (commonly in the CIO, C15, and C20 families) serve in communication and defense, for example, as attractants for pollinators and seed dispersers, competitive phytoxins, antibiotics, and herbivore repellents and toxins (Harborne, 1991). Terpenoids available in relatively large amounts as essential oils, resins, and waxes are important renewable resources and provide a range of commercially useful products, including solvents, flavorings and fragrances, adhesives, coatings, and synthetic intermediates (Zinkel and Russell, 1989; Dawson, 1994). Members of the terpenoid group also include industrially useful polymers (rubber, chicle) and a number of pharmaceuticals (artemisinin, taxol) and agrochemicals (pyrethrins, azadirachtin).In spite of the economic significance of the terpenoids and their many essential functions, relatively little is known about terpenoid metabolism and its regulation in plants. There are severa1 reasons for this. An overwhelming problem with the terpenoids is their sheer number. A given plant may synthesize (and catabolize) many different terpenoid types (from C5 to C40 and higher) at different times and locations for many different purposes throughout the course of development. Because all terpenoids are produced by a common biosynthetic pathway, sophisticated control mechanisms must exist to ensure the production of appropriate levels of these often structurally complex compounds in the proper metabolic, developmental, and environmental context. Pathway elucidation for highly functionalized terpenoid metabolites is not trivial, and determining the enzymology of terpenoid metabolism has proven very challenging, both because relatively little enzymatic machinery is dedicated to terpenoid metabolism and because many of the reaction types involved (carbonium ion To whom correspondence should be addressed. mechanisms) are unlike those of primary metabolism of carbohydrates, proteins, and lipids. Moreover, the genetics (let alone the molecular genetics) of terpenoid metabolism are insufficiently developed to provide generally useful tools for examining control of metabolism at the cell, enzyme, transcript, or DNA levels. Of comparable significance is the paucity of suitable model systems for examining either the induced regulation of multiple terpenoid pathways or the developmental regulation of ...

Medicinal chemistry and pharmacology of genus Tripterygium (Celastraceae)

“…FPP can then give rise to a range of linear and cyclic sesquiterpenes (Beale, 1990). Two molecules of FPP are joined tail-to-tail to yield the precursor of triterpenes, squalene (C 30 ), from which other triterpenes arise (McGarvey and Croteau, 1995).…”

“…GPP can produce the fundamental diterpene precursor (C 20 ), geranylgeranyl diphosphate (GGPP), with the successive additions of a further two IPPs (Lichtenthaler, 1999). Two molecules of GGPP are joined tail-to-tail to form a tetraterpene compound phytoene (C 40 ), a precursor for other tetraterpenes (McGarvey and Croteau, 1995). The two biosynthetic pathways of terpenoids are summarized in Figs.…”

The contribution of reactive carbon emissions from vegetation to the carbon balance of terrestrial ecosystems

“…These compounds act as a solvent for creating a physical barrier and/or as feeding deterrents to pests and are dominated by monoterpenes (C 10 ) with a large number, but small quantity, of sesquiterpenes (C 15 ) found within leaves. The biochemistry of these compounds and the genetic and ecological controls of these plant defense mechanisms have been studied for many economically significant plants (McGarvey and Croteau, 1995). Diurnal variations in emissions of these stored compounds typically range from about a factor of two to four and are primarily controlled by temperature.…”

Biosynthesis and Therapeutic Properties ofLavandulaEssential Oil Constituents

“…Like other terpenoids, these low molecular weight biochemicals are derived from condensation of the universal terpene precursors isopentenyl diphosphate (IPP, C 5 ) and dimethylallyl diphosphate (DMAPP, C 5 ). The condensation reactions initially form geranyl diphosphate (GPP) and farnesyl diphosphate (FPP), which can be modified by terpene synthases to produce over 1000 monoterpenes and approximately 5000 sesquiterpenes, respectively [55][56][57][58][59][60][61]. Monoterpenes are derived from GPP by the action of monoterpene synthases, and sesquiterpene synthases transform FPP to various sesquiterpenes [62].…”