Secondary metabolisms alkaloids

Keywords Amaryllidaceae alkaloid plant secondary metabolism alkaloid biosynthesis natural products systems biology galanthamine... 

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). 

Perhaps the most interesting arthropodan defensive compounds from the point of view of structural diversity are the alkaloids. While alkaloids had long been believed to arise only as a consequence of plant secondary metabolism, it has become apparent over the last few decades that arthropods are both prolific and innovative alkaloid chemists. The millipede Polyzonium rosalbum, once thought to secrete camphor (20), in fact gives off a camphoraceous/earthy aroma produced by the spirocyclic isoprenoid imine polyzonimine (21). 

Alkaloid biosynthesis needs the substrate. Substrates are derivatives of the secondary metabolism building blocks the acetyl coenzyme A (acetyl-CoA), shikimic acid, mevalonic acid and 1-deoxyxylulose 5-phosphate (Figure 21). The synthesis of alkaloids starts from the acetate, shikimate, mevalonate and deoxyxylulose pathways. The acetyl coenzyme A pathway (acetate pathway) is the source of some alkaloids and their precursors (e.g., piperidine alkaloids or anthraniUc acid as aromatized CoA ester (antraniloyl-CoA)). Shikimic acid is a product of the glycolytic and pentose phosphate pathways, a construction facilitated by parts of phosphoenolpyruvate and erythrose 4-phosphate (Figure 21). The shikimic acid pathway is the source of such alkaloids as quinazoline, quinoline and acridine. 

Alkaloids as non-final products of the secondary metabolism are very different in their structure and life functions in organisms. Many different groups of alkaloids are known. As mentioned, they have different precursors and rings. They also have different sub-pathways and intermedia. 

Hartmann, T. 1988. Secondary metabolism of lupins Biosynthesis, translocation and accumulation of the quinolizidine alkaloids. In Proceedings of the 5th International Lupin Conference. (Twardowski, T., ed.) pp. 64-78. Poznari, Polish Academy of Sciences, Institute of Bioorganic Chemistry. 

Overall, however, the immensity of temperate land corresponds to a most various secondary metabolic production, different from that of tropical land. The most renowned alkaloids belong to the morphine class (Chart 6.2.A1), and, in combination with isoprenoids, to the ergot and triterpene classes (Chart 6.2. A2). Prominent in the peptides are the cyclosporins (the first of which was isolated from a fiingus collected in Norway), streptogramins, and P-lactams (Chart 6.2.P). The isoprenoids are represented by pyrethrin monoterpenes, cedrane sesquiterpenes, ginkgolide and taxane diterpenes, ophiobolane sesterterpenes, and arborane and amyrin-like triterpenes (Chart 6.2.1). In the polyketides, epothilones, recently discovered from Myxobacteria, and the long known rapamycin, are two prominent classes of macrolides (Chart 6.2.FA/PO/C). 

Although the participation of amino acids in the biosynthesis of some shikimate metabolites and particularly in the pathways leading to alkaloids has already been explored in Chapters 4 and 6, amino acids are also the building blocks for other important classes of natural products. The elaboration of shikimate metabolites and alkaloids utilized only a limited range of amino acid precursors. Peptides, proteins, and the other compounds considered in this chapter are synthesized from a very much wider range of amino acids. Peptides and proteins represent another grey area between primary metabolism and secondary metabolism, in that some materials are widely distributed in nature and found, with subtle variations, in many different organisms, whilst others are of very restricted occurrence. 

Legumes are able to fix atmospheric nitrogen via symbiotic Rhizobia in root nodules. Thus, nitrogen is easily available for secondary metabolism, and it is probably not surprising that nitrogen-containing SM (alkaloids, nonprotein amino acids [NPAAs], cyanogens, protease inhibitors, lectins) are a common theme in... 

Methylation is one of the most common enzymatic modifications in plant specialized (secondary) metabolism. Almost all classes of plant specialized metabolites are known to be methylated, including amino acids, alkaloids, phenylpropanoids, sugars, purines, sterols, thiols, and flavonoids. The methyl transfer most commonly occurs on C, N, S, or O atoms. 

Synthesis of senecionine, a pyrrolizidine alkaloid (Secondary metabolism)... 

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