Secondary metabolisms terpenes

Conventionally, central and special metabolic pathways are distinguished. Central pathways are common to the decomposition and synthesis of major macromolecules. Actually, they are much alike in all representatives of the living world. Special cycles are characteristic of the synthesis and decomposition of individual monomers, macromolecules, cofactors, etc. Special cycles are extremely diversified, especially in the plant kingdom. For this reason, the plant metabolism is conventionally classified into primary and secondary metabolisms. The primary metabolism includes the classical processes of synthesis and deeradation of major macromolecules (proteins, carbohydrates, lipids, nucleic acids, etc.), while the secondary metabolism ensuing from the primary one includes the conversions of special biomolecules (for example, alkaloids, terpenes, etc.) that perform regulatory or other functions, or simply are metabolic end byproducts. 

Secoiridoids are complex phenols produced from the secondary metabolism of terpenes as precursors of several indole alkaloids (Soler-Rivas and others 2000). They are characterized by the presence of elenolic acid, in its glucosidic or aglyconic form, in their molecular structure. Oleuropein, the best-known secoiridoid, is a heterosidic ester of elenolic acid and 3,4- dihydroxyphenylethanol containing a molecule of glucose, the hydrolysis of which yields elenolic acid and hydroxytyrosol (Soler-Rivas and others 2000). 

Mihaliak, C.A., Karp, F. and Croteau, R. (1993) Cytochrome P450 terpene hydroxylases, in Enzymes of Secondary Metabolism (ed. P.J. Lea). Academic Press, London, pp. 261-79. 

Despite the thousands of secondary metabolites made by microorganisms, they are synthesized from only a few key precursors in pathways that comprise a relatively small number of reactions and which branch off from primary metabolism at a limited number of points. Acetyl-CoA and propionyl-CoA are the most important precursors in secondary metabolism, leading to polyketides, terpenes, steroids, and metabolites derived from fatty acids. Other secondary metabolites are derived from intermediates of the shikimic acid pathway, the tricarboxylic acid cycle, and from amino acids. The regulation of the biosynthesis of secondary metabolites is similar to that of the primary processes, involving induction, feedback regulation, and catabolite repression [6]. 

The production of higher levels of terpenes can be ascribed to the onset of secondary metabolism triggered by a lack of nutrients. 

These same features appear to be responsible for creating diversity in other secondary metabolic pathways. For example, in both polyketide and teipene formation, repetitive addition of either C2 or C5 carbon subunits leads to the formation of a variety of carbon skeletons. In addition, in nearly all groups of secondary metabolites, including alkaloids, phenylpropanoids, and terpenes, the initially-formed products are subjected to a wide variety of oxidative modifications. Thus, despite the seemingly large and chaotic assemblage of secondary metabolites found in plants, their formation may he governed by a few common principles. 

Secondary metabolism is, by contrast, chemistry less fundamental to the workings of life and restricted to smaller groups of organisms. Later in this chapter you will meet alkaloids produced by some plants and terpenes produced by others. Humans produce neither of these, but we do make steroids, as do other animals (and a few plants). All of these molecules are the products of secondary metabolism. 

Natural products often seem to have little value to the organism itself, and are made by the processes of secondary metabolism. They are classified by the way they are made into terpenes and steroids, alkaloids, and polyketides. 

This has led to an acceleration in the last decade in secondary metabolism research and generated a wealth of new knowledge related with terpene metabolism and other plant secondary pathways [5]. 

Plant metabolism can be separated into primary pathways that are found in all cells and deal with manipulating a uniform group of basic compounds, and secondary pathways that occur in specialized cells and produce a wide variety of unique compounds. The primary pathways deal with the metabolism of carbohydrates, lipids, proteins, and nucleic acids and act through the many-step reactions of glycolysis, the tricarboxylic acid cycle, the pentose phosphate shunt, and lipid, protein, and nucleic acid biosynthesis. In contrast, the secondary metabolites (e.g., terpenes, alkaloids, phenylpropanoids, lignin, flavonoids, coumarins, and related compounds) are produced by the shikimic, malonic, and mevalonic acid pathways, and the methylerythritol phosphate pathway (Fig. 3.1). This chapter concentrates on the synthesis and metabolism of phenolic compounds and on how the activities of these pathways and the compounds produced affect product quality. 

In order for allelochemicals to enter the body of a herbivore, absorption must occur across the gut lining. Curtailing the initial absorption of dietary allelochemicals may be a herbivore s first line of defense against plant toxins. Studies have citied the lack of absorption or metabolism of lipophilic plant secondary metabolites (i.e., terpenes), conducive to phase I or II detoxification, in the gut of terrestrial herbivores rather these compounds are excreted unchanged in the feces (Marsh et al. 2006b). While physical barriers or surfactants have been used to explain this limited adsorption in both marine and terrestrial herbivores (Lehane 1997 Barbehenn and Martin 1998 Barbehenn 2001 for review of marine herbivores, see Targett and Arnold 2001), active efflux of plant allelochemicals out of enterocytes into the gut lumen has received limited attention until now. 

Mycotoxins are, in general, low molecular weight, non-antigenic fungal secondary metabolites formed by way of several metabolic pathways, e.g. the polyketide route (aflatoxins), the terpene route (trichothecenes), the amino acid... 

There is also the possibility that pollutants alter susceptibility of the plant to pathogens (36) or insect attack. Of the latter there is the decreased resistance of ponderosa pine to bark beetle attack caused by ambient oxidant exposure (37). The investigations of others with respect to the effects of fluoride on ponderosa pine indicated that although foliar injury was associated with increased resin exudation pressure, which could be interpreted as an increased capacity of the tree to overcome bark beetle attack, degree of insect infestation was not associated with amount of foliar injury (38). As more is known about pheromones, the botanical investigation of the secondary products of metabolism, such as terpenes and phenolics, may become more important in investigating the mode of action of pollutants in the entire plant. The switch to alternate pathways, while resulting in the same products, may reduce the intermediates needed in biosynthesis and thereby affect the plants resistance to disease or attractiveness to insects. 

In 1982, female dry weight production was inversely related to bornyl acetate amd an unidentified sesquiterpene (Table III). Neither of these were associated with reduced biomass production in the 1981 model. Greater internode growth, evenness in the quantitative distribution of terpenes, myrcene, camphene, and larger crown diameters were positively associated with female biomass production. Five of the seven variables (71%) which entered into the model were secondary metabolites, and none of the variables estimating nitrogen metabolism entered the model as Important determinants of female biomass production. 

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