Isoprenoids insects

Abstract Pheromones are utilized by many insects in a complex chemical communication system. This review will look at the biosynthesis of sex and aggregation pheromones in the model insects, moths, flies, cockroaches, and beetles. The biosynthetic pathways involve altered pathways of normal metabolism of fatty acids and isoprenoids. Endocrine regulation of the biosynthetic pathways will also be reviewed for the model insects. A neuropeptide named pheromone biosynthesis activating neuropeptide regulates sex pheromone biosynthesis in moths. Juvenile hormone regulates pheromone production in the beetles and cockroaches, while 20-hydroxyecdysone regulates pheromone production in the flies. 

Coleoptera comprise the largest order of insects and accordingly pheromone structures and biochemical pathways are diverse [98, 99]. Beetle pheromone biosynthesis involves fatty acid, amino acid, or isoprenoid types of pathways. In some cases dietary host compounds can be converted to pheromones, but it is becoming apparent that most beetle pheromones are synthesized de novo. 

Bark beetles primarily utilize isoprenoid derived pheromones [100,101] and have been the most studied regarding their biosynthesis [8,98]. Earlier work indicated that the isoprenoid pheromones could be produced by the beetle altering host derived isoprenoids however more recent work indicates that for the most part bark beetles are producing pheromones de novo. The production of isoprenoids follows a pathway outlined in Fig. 4 which is similar to the isoprenoid pathway as it occurs in cholesterol synthesis in mammals. Insects cannot synthesize cholesterol but can synthesize farnesyl pyrophosphate. Insects apparently do not have the ability to cyclize the longer chain isoprenoid compounds into steroids. The key enzymes in the early steps of the isoprenoid... 

In this discussion, we have restricted ourselves to the consideration of only a few examples of arthropod chemistry. From these alone, it is evident that insects synthesize defensive compounds by using all of the major biosynthetic pathways, producing acetogenins, simple aromatics and quinones, isoprenoids, and alkaloids. In addition, some of the millipedes, coccinellid beetles, and spiders we have studied utilize biosynthetic pathways that have yet to be characterized. 

Hick, A. J., Luszniak, M. C. and Pickett, J. A. (1999). Volatile isoprenoids that control insect behaviour and development. Natural Product Reports 16 39-54. 

In addition to its role as an intermediate in cholesterol biosynthesis, isopentenyl pyrophosphate is the activated precursor of a huge array of biomolecules with diverse biological roles (Fig. 21-48). They include vitamins A, E, and K plant pigments such as carotene and the phytol chain of chlorophyll natural rubber many essential oils (such as the fragrant principles of lemon oil, eucalyptus, and musk) insect juvenile hormone, which controls metamorphosis dolichols, which serve as lipid-soluble carriers in complex polysaccharide synthesis and ubiquinone and plastoquinone, electron carriers in mitochondria and chloroplasts. Collectively, these molecules are called isoprenoids. More than... 

Insects communicate through the use of a great variety of volatile pheromones. As mentioned in Chapter 8, Section A,l, some moths utilize acetate esters of various isomers of A7 and A11 unsaturated C14 fatty acids as sex pheromones. Some other moths convert the trans-l 1 -tetradecenyl acetate into the corresponding C14 aldehyde or alcohol, while others use similar compounds of shorter (Cn - C12) chain length.143 Some ants use ketones, such as 4-methyl-3-heptanone, as well as various isoprenoid compounds and pyrazines as volatile signaling compounds.144 Other insects also utilize isoprenoids,145 alkaloids,146 and aromatic substances as pheromones. 

Terpenoids are structurally based on the isoprenoid (C5) unit and include monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, steroids and carotenoids. These compounds can be further modified to generate greater structural complexity. Thus the saponins are surface active amphiphiles deriving from the glycosylation of steroid (C27) or triterpenoid (C30) entities. Plant triterpenoids with very specific biochemical effects include those that mimic the effects of mammalian steroid hormones or of insect developmental hormones. 

A number of compounds important to animal physiology have been identified as isoprenoid compounds. Notable examples are vitamin A, retinal (Section 28-7), and squalene (Table 30-1). Also, terpene hydrocarbons and oxygenated terpenes have been isolated from insects and, like famesol, show hormonal and pheromonal activity. As one example, the juvenile hormone isolated from Cecropia silk moths has the structure shown in 3 ... 

Figure 6.10 De novo biosynthesis of isoprenoid pheromone components by bark and ambrosia beetles through the mevalonate biosynthetic pathway. The end products are hemiterpenoid and monoterpenoid pheromone products common throughout the Scolytidae and Platypodidae (Figure 6.9A). The biosynthesis is regulated by juvenile hormone III (JH III), which is a sesquiterpenoid product of the same pathway. The stereochemistry of JH III is indicated as described in Schooley and Baker (1985). Although insects do not biosynthesize sterols de novo, they do produce a variety of derivatives of isopentenyl diphosphate, geranyl diphosphate, and farnesyl diphosphate. Figure adapted from Seybold and Tittiger (2003).

Wiygul et al. (1982) reported the site of pheromone biosynthesis in male boll weevils as the fat body. To date this is the only insect where the fat body has been proposed as a site for pheromone biosynthesis, and it is an obvious contradiction that in most species of the closely related Scolytidae, isoprenoid pheromone biosynthesis has been reported to occur in the alimentary canal (see section 6.6.2). Wiygul et al. (1982) observed that the level of pheromone production... 

Volatile isoprenoids that control insect behavior and development have been reviewed.476 Information on the biosynthesis of terpenoids with special emphasis on beetle pheromones has been compiled by Seybold and... 

Several hundred volatile isoprenoid substances that interfere with insect larval moulting have so far been resolved from plants and such phytoecdysones will be dealt with in Chapter 11, which is concerned with development-perturbing compounds binding to cytosolic hormone receptors. It must also be noted that plant cells within a plant (and indeed whole plants themselves) can communicate with each other via volatile signalling bioactives... 

A unique role is played by chemical communication in the interactions between plants and insects. About half a million insect species feed on plants. The process of reproduction in many plant species is critically dependent upon pollination by insects. It is not surprising, then, to find among the numerous natural products of plants both attractants for useful insects and repellents or even insecticides for plant-eating insects. The remarkable diversity of the these compounds (the list includes acyclic and polycyclic compounds, isoprenoids, aromatic derivatives, heterocyclic compounds, etc.) illustrates the non-selectivity in the structure of the chemical mediators for biological applications. The intimate mechanism of their action is, unfortunately, still insufficiently understood. 

T. Hartmann D. Ober, Biosynthesis and Metaboiism of Pyrroiizidine Aikaioids in Piants and Speciaiized insect Herbivores, in Biosynthesis Aromatic Polyketides, Isoprenoids, Alkaloids F. J. Leeper, J. C. Vederas, Eds. Topics in Current Chemistry Springer-Veriag Beriin, Heideiberg, New York, 2000 Voi. 209, pp 207-243. 

HMG-CoA reductase activity has been detected in mammals, birds, insects, reptiles, fish, higher plants, moulds, yeast and bacteria [112]. HMG-CoA reductase probably is present in any life form capable of synthesizing isoprenoids. In mammals, HMG-CoA reductase activity has been detected in many tissues (Table 3). The highest quantities are present in liver and intestine, which together provide 2/3-3/4... 

Cholesterol serves as the biosynthetic precursor for several vital compounds, including a variety of steroid hormones and bile acids. Many of these compounds, and many other polyisoprenoid compounds biosynthetically related to cholesterol, act biologically as important regulatory compounds [105]. In mammals such regulatory compounds include steroid hormones and vitamins A and D. Steroids and other isoprene derivatives also play important regulatory roles in other phyla. Several insect hormones, for example, are isoprenoid derivatives [106] (cf. Chapter 8). Memy of the floral scents of plants are isoprene derivatives. 

Schmialek (1963) obtained radioactive farnesol and famesal from silkworm moth treated with [2- ]-mevalonic acid. The experiments of Sridhara and Bhat (1965), Happ and Mainwald (1966), Karlson (1970), Meyer et al. (1970) also prove that isoprenoid biosynthesis in insects proceeds via mevalonic acid as intermediate product, i.e. it is analogous to the early stages of mammalian steroid biosynthesis prior to the formation of the sterane skeleton. Early intermediates include acetate (Schooley et al.. 1973), acetyl-CoA (Baker and Schooley, 1978), (3S)-HMG-CoA (Bergot et al., 1979) and (3/ )-mevalonate (Schooley et al., 1973 Lee et al., 1978 Feyereisen et al., 1981). 

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