Plant terpenes

Terpenes are composed of isoprenyl (C-,) units and are conveniently grouped as monoter-penes (skeletal basis CI0 = 2X C-,), sesquiterpenes (G13 = 3X C3), diterpenes (C20 = 4X C-,), triterpenes (C3o = 6X C-,) and tetraterpenes (C40 = 8X G-j. The structures of some representative terpenes are shown in the Appendix (Section 3). Terpenes ultimately derive biosynthetically from acetate (C2) via the activated acetyl thioester (CH3—CO—S—X) acetyl-coenzyme A (acetylCoA CH3-CO-S-C0A) as outlined below (enzymes catalysing key steps being indicated in parentheses). 

Excess acetate (C2) can be converted to the mobile ketone body energy source aceto-acetate (C4) and thence its reduced form hydroxybutyrate (C,) for transport throughout the body. Excess acetate can be carboxylated (via acetylCoA carboxylase) to form malonylCoA (C3), the donor for further C2 additions (with C02 elimination) in the anabolic synthesis of long chain fatty acids. Fatty acids are components of the phospholipids of cellular membranes and are also stored as triacylglycerols (triglycerides) for subsequent hydrolysis and catabolic fatty acid oxidation to yield reduced coenzymes and thence ATP (see Chapter 2). 

AcetylCoA (C2) can also react with acetoacetylGoA (Cj) to generate hydroxy-methylglutarylCoA (HMGCoA) (C6) and thence the isoprenoid precursor mevalonate (C6). Mevalonate (C(i) ultimately yields the key C-, isoprenoids isopentenylpyrophosphate (CH3G(=GH2)—CH2—CH2—O—P03—P03) (IP-PP) and dimethylallylpyrophosphate (CH3— C(GH3)=GH—CH2—O—P03—P03) (DMA-PP), the immediate precursors of cholesterol and 

IP-PP and DMA-PP can yield volatile C3 hemiterpenes. At the other extreme, extensive polymerization of the C3-pyrophosphates (with release of pyrophosphate, PP ) yields the formation of the plant latex polymers such as eis-polyisoprenes (rubber) and trans-polyisoprenes (gutta-percha). In between these extremes, a variety of monoterpenes, sesquiterpenes, triterpenes and C3() carotenes derive from these C3-pyrophosphate precursors. 

Head to tail condensation of IP-PP (C3) and DMA-PP (C3) with release of PP forms ger-anylpyrophosphate CH3-C(CH3)=CH-CH2-CH2-C(CH3)=CH—CH2—O—P03—P03, that is, H(GH2-C(CH3)=CH-GH2)2 0-P03-P03 (Cig-PP), the starting point for plant monoterpenes. Further, head-to-tail reaction of geranylpyrophosphate (C q—PP) with isopentenylpyrophosphate (C3—PP) yields farnesylpyrophosphate H(CH2—C(CH3)=CH— CH2)3-0-P03-P03 (Ci3-PP), the parent of plant sesquiterpenes. Head to tail condensation of farnesylpyrophosphate (CI 3-PP) with IP-PP (C3 PP) yields geranylgeranylpyrophosphate H(CH2-C(CH3)=CH-CH2) -0-P03-P03 (C2o PP), the parent of plant diterpenes. 

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In plants, terpenes (see Chapter 6 Focus On) are biosynthesized by a pathway that involves loss of CO2 from 3-phosphomevalonate 5-diphosphate to yield isopentenyl diphosphate. Use curved arrows to show the mechanism of this reaction. 

Until rather recently, our choice among chemicals repellant to insects was very limited (9), and some of the available remedies seem to have been almost equally repellent to their human users. The most familiar of them undoubtedly is the classical oil of citronella, a mixture of plant terpenes which consists principally of geraniol, citronellol, and citronellal. It is a remarkable coincidence that at least one insect species, an ant discussed by Dr. Happ, also makes use of some of these same terpenes as repellents against other insects. It biosynthesizes them de novo rather than simply taking them from plant sources. Many other examples of insect repellency have been observed (9), and Roth and Eisner (28) list over 30 compounds which have been identified as defense substances of anthropods. 

Alicyclic hydrocarbons are saturated carbon chains that form ring structures. Naturally occurring alicyclic hydrocarbons are common (Chap. 1). For example, alicyclic hydrocarbons are a major component of crude oil, comprising 20-67 vol.%. Other examples of complex, naturally occurring alicyclic hydrocarbons include camphor (a plant terpene) and cyclohexyl fatty acids (components of microbial lipids). Anthropogenic sources of alicyclic hydrocarbons to the environment include fossil-fuel processing and oil spills, as well as the use of such agrochemicals as the pyrethrin insecticides (Chap. 1, and references therein). 

Elucidating the Metabolism of Plant Terpene Volatiles Alternative Tools for Engineering Plant Defenses ... 

Cytochrome P-450 Involvement in the Interactions Between Plant Terpenes and Insect Herbivores... 

Figure I. Outline of the biosynthetic relationships of plant terpenes.

Mammalian PLANT RESISTANCE TO INSECTS Table I acute toxicities of plant terpenes ... 

Rabdosia lasiocarpus (Flayata) Mao Guo Yan Ming Cao (whole plant) Terpenes, oridonin, rubescensins, 5-fluoro uracil.50 This herb is toxic. For carcinomas of esophagus and stomach, antiarthritic, antidotal, febrifuge. 

FIGURE 12-13 Interaction of Gs with adenylyl cyclase. (PDB ID 1AZS) The soluble catalytic core of the adenylyl cyclase (AC, blue), severed from its membrane anchor, was cocrystallized with G,, (green) to give this crystal structure. The plant terpene forskolin (yellow) is a drug that strongly stimulates the enzyme, and GTP (red) bound to Gsa triggers interaction of Gsa with adenylyl cyclase. 

Plant terpenes may deter herbivores and attract pollinators. They may participate in competition among plants and may act as antibiotics, called phytoalexins, to protect plants from bacteria and fungi.84 In invertebrate animals terpenes serve as hormones, pheromones, and defensive repellants (Figs. 22-3,22-4). The terpene squalene is the precursor to sterols. Some terpenes are toxic. For example, thujone (Fig. 22-3), which is present in the liqueur absinthe, causes serious chronic poison-... 

As is indicated in Fig. 22-3, the same intermediate cation can yield a variety of end products. For example, pure geranyl diphosphate pinene cyclase catalyzes formation of several other terpenes in addition to a-pinene.89 Another aspect of terpene synthesis is that insects may convert a plant terpene into new compounds for their own use. For example, myrcene, which is present in pine trees, is converted by bark beetles to ipsenol (Fig. 22-3), a compound that acts as an aggregation pheromone.90... 

If as above we simply represent alicyclic rings sharing two Gs by a vertical line, then we can represent the basic tetracyclic structure of lanosterol as G61G61 G6 C5 (noting that there are two double bonds and various alkyl substituents and also a 3-hydroxyl on the first of the alicyclic rings). Many subsequent reactions yield cholesterol, a major triterpene membrane component that modifies the fluidity of animal cell membranes and is a precursor for synthesis of animal bile acids (fat solubilizing amphipathic detergents) plant triterpenes and steroid hormones such as the corticosteroids cortisol and cortisone, the mineralocorticoid aldosterone and the sex hormones testosterone and 17-(3-oestradiol. The structure and bioactivity of the plant terpenes is sketched below. 

The recently published crystal structure of tobacco epi-aristolochene synthase (a sesquiterpene synthase) has provided the first look at the three-dimensional configuration of a plant terpene s)mthase (Starks et al, 1997). 

Moreover, bark beetles convert host plant terpenes into oxygenated products that can serve as aggregation pheromones [227]. When host kairomones (chemicals released from one organism that induce an adaptively favourable response by an individual of another species) are released in combination with these pheromones, there ean be a synergistic or additive effeet. One of the first reports of such an effect is the... 

Biological sources Humans, moulds, bacteria, plants (terpenes, glycoesters alcohols esters aldehydes). 

In plants, terpenes are typically found as a component of the essential oils. The name of this fluid suggests an important characteristic of this class of molecules. If humans have found these oils important enough to refer to them as essential, there is a reasonable chance that they have useful properties. Many terpenes such as menthol (shown in Figure 1) and camphor have medicinal values. You may have used a throat lozenge with menthol to help clear your sinuses the last time you had a cold. 

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