Terpenoid pathway

The late cannabinoid pathway starts with the alkylation of ohvetolic acid (3.2 in Fig. 4) as polyketide by geranyl diphosphate (3.1) as the terpenoid unit. Terpenoids can be found in all organisms, and in plants two terpenoid pathways are known, the so called mevalonate (MEV) and non-mevalonate (DXP) pathway as described by Eisenrich, lichtenthaler and Rohdich [23,24,29,30]. The mevalonate pathway is located in the cytoplasm of the plant cells [30], whereas the DXP pathway as major pathway is located in the plastids of the plant cells [29] and delivers geranyl diphosphate as one important precursor in the biosynthesis. 

The repertoire of chemicals that can be used for communication is limited by the biosynthetic ability of the insect. Compared to other insect orders, pheromone biosynthesis in Hymenoptera has received little study [191]. However, the biosynthetic origins of chemically diverse hymenopteran semiochemicals likely include aromatic, fatty acid, and terpenoid pathways as well as simple modifications of host-derived precursors. Notable recent studies include the biosynthesis of the fatty acid components (2 )-9-oxodec-2-enoic acid 52 and (2 )-9-hydroxydec-2-enoic acid of the honeybee queen mandibular pheromone from octadecanoic acid [192,193], and the aliphatic alcohol and ester... 

Figure 3.3. The isoprenoid/terpenoid pathways start hy joining together units in multiple ways to generate carhon skeletons made up of multiple units (e.g., Cj , 0,5, C o, and C ). Each of these carhon skeletons can then he tailored to generate further chemical diversity.

Having produced many different carbon skeletons via the first phase of the isoprenoid/terpenoid pathway, the second phase of diversity generation occurs in all organisms when a number of versatile enzymes carry out a series of chemical additions, rearrangements and deletions by acting on different carbons in the basic skeleton. The... 

FIGURE 63.1 Starting with mevalonate, carotenoids are biosynthesized by a special branch of the terpenoid pathway. The first C-40 hydrocarbon unit formed is phytoene, a carotenoid with three conjugated double bonds, which then is enzymatically desaturated to successively yield (3-carotene, neurosporene, and lycopene. Other carotenoids such as (3-carotene and oxocarotenoids are produced from lycopene following cyclization and hydroxylation reactions. Thus, lycopene is a central molecule in the biosynthesis pathway of carotenoids. 

The prenyl transferase from avian liver has been crystallized,40 and was found to be a dimer of molecular weight 86 000 dalton the subunits could not be resolved by SDS electrophoresis. The enzyme catalysed the formation of FPP from IPP and either DMAPP or GPP, and this was accompanied by the synthesis of small amounts of geranylgeranyl pyrophosphate (GGPP). This is the first stable crystalline enzyme of the steroid and terpenoid pathways to be prepared. 

Alkaloids thus represent one of the largest groups of natural products, with over 10,000 known compounds at present, and they display an enormous variety of structures, which is due to the fact that several different precursors find their way into alkaloid skeletons, such as ornithine, lysine, phenylalanine, tyrosine, and tryptophan (38-40). In addition, part of the alkaloid molecule can be derived from other pathways, such as the terpenoid pathway, or from carbohydrates (38-40). Whereas the structure elucidation of alkaloids and the exploration of alkaloid biosynthetic pathways have always commanded much attention, there are relatively few experimental data on the ecological function of alkaloids. This is the more surprising since alkaloids are known for their toxic and pharmacological properties and many are potent pharmaceuticals. 

Many of the enzymes involved in the mevalonic acid pathway have been described. Fig. (6) however, studies on the genetic modifications of these enzymes are scarce in the literature. Here an attempt has been made to present the most relevant data on the terpenoid pathway, the enzymes involved and the genetic engineering of them. 

Unlike primary metabolites, the genes that regulate the formation of the enzymes of seeondary metaboUte biosynthesis are often clustered. In several eases the loei of these genes have been determined. This has considerable sig-nifieanee in the control of secondary metabolite biosynthesis. The genes that eode for several important polyketide pathways such as those leading to the aflatoxins and the statins have been identified. Similar work has also been reported for penicillin biosynthesis and some non-ribosomal peptides as well as terpenoid pathways such as that leading to the gibberellins. 

Biosynthetic studies showed that the aromatic ring was formed from a poly-ketide whilst the side chain was derived from the mevalonate terpenoid pathway. The C-5 O-methyl and C-4 methyl groups were derived from methionine. The high incorporation of 4,6-dihydroxy-2,3-dimethyl-[l- C]benzoic acid (4.37) suggested that the methylation at C-4 occurred at the polyketide stage prior to the formation of the phthalide. 5,7-Dihydroxy-4-methyl-[7- C]phthalide (4.38) and its 6-farnesyl analogue were also incorporated eiSciently into mycophenolic acid. The farnesyl precursor was detected as a metabolite of P. brevi-compactum and shown to be formed from 5,7-dihydroxy-4-methyl-[7- C]phthalide by a trapping... 

Information on derivatives of the alkaloidal types considered so far is as follows (o) there is good evidence that the biosynthesis of strychnine (51) in Strychnos nux vomica plants follows the terpenoid pathway and it is proved that carbons x and y of (51) are derived from acetate the Wieland-Gumlich aldehyde (52) is not incorporated into strychnine by these plants b) apparicine... 

Radioactive tracer work has shown that nomilin (3) is biosynthesized by the terpenoid pathway from acetate in the phloem region of stems and translocated to other parts of plants such as leaves, fruit tissues and seeds (13,14). At those locations, nomilin is further biosynthesized to other limonoids. Limonoid biosynthesis occurs at each location independently, thus the composition of limonoids in fruit tissues, seeds and leaves are different from each of the others. Limonin is biosynthesized from nomilin via obacunone (4), obacunoate (5) and ichangin (6) (15-17) (Fig. 2)... 

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

All carotenoids are derived from the isoprenoid or terpenoid pathway. From prenyl diphosphates of different chain lengths, specific routes branch off into various terpenoid end products. The prenyl diphosphates are formed by different prenyl transferases after isomerization of IPP to DMAPP by successive T-4 condensations with IPP molecules. Condensation of one molecule of dimethylallyl diphosphate (DMADP) and three molecules of isopentyl diphosphate (IDP) produces the diter-pene geranylgeranyl diphosphate (GGDP) that forms one-half of all C40 carotenoids. The head-to-head condensation of two GGDP molecules results in the first colorless carotenoid, phytoene. Phytoene synthesis is the first committed step in C40 carotenoid biosynthesis (Britton et al. 1998, Sandmann 2001). 

Interest in enzyme stereospecificity and the stereochemistry of prochiral centres, such as the methylene groups of mevalonic acid, has necessitated more precise definitions of the stereochemistry of the various molecules involved and of the enzymological consequences. The use of multiply labelled mevalonic acid in terpenoid and steroid biosynthesis has been reviewed by Hanson. The Proceedings of the 1970 Phytochemical Society symposium have been published. They include a general discussion of terpenoid pathways of biosynthesis by Clayton and specific chapters on monoterpenoids, diterpenoids, eedysones, carotenoids, isoprenoid quinones, and chromanols. Other reviews concerning biosynthesis have appeared on furanocoumarins, indole alkaloids, monoterpenoids, and diterpenoids. ... 

Tissue and Cell-Specific Localization of Terpenoid Pathways in Defense... 

Our recent research suggests organ-, tissue-, and cell-specific localization of constitutive and induced terpenoid defense pathways in conifers. For example, linalool synthase (PaTPS-Lin) seems to be preferentially expressed in needles of Norway spruce and Sitka spruce with little or no expression in sterns. ft is also likely that expression of PaTPS-Lin in spruce needles is not associated with resin ducts but could reside in other cells involved with induced terpenoid emission. In contrast, we can speculate that most other mono-TPS and di-TPS are associated with epithelial cells of constitutive and induced resin ducts. The possible localization of conifer sesqui-TPS is difficult to predict. Furthermore, the exact spatial and temporal patterns of terpenoid pathway gene expression associated with traumatic resin duct development in the cambium zone and outer xylem remain to be studied at the tissue and cell level. In situ hybridization and immuno-localization of TPS will address these open questions. These methods have worked well in identifying cell type specific gene and protein expression of alkaloid formation in opium poppy Papaver somniferum) As the biochemistry of induced terpene defenses and the development of traumatic resin ducts have been well described in spruce, this system is ideal for future studies of tissue- and cell-specific localization of transcripts and proteins associated with oleoresin defense and induced volatile emissions in conifers. In addition, the advent of laser dissection microscopy techniques presents a fascinating means by which to further address RNA and protein analysis in a tissue-and cell-specific manner. These techniques, when applied to the cambium zone, xylem mother cells, and the epithelial cells that surround traumatic resin ducts, and will allow a temporal and spatial analysis of cellular functions occurring in the traumatic resin response. 

While the understanding of induced conifer terpenoid defenses has increased dramatically with much information now available from terpenoid metabolite profiling and the study of associated terpenoid pathways, there is still much to be... 

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