Saponins pathway

Saponins maybe classified into two groups, the triterpenoid saponins, which will not be considered here (for reviews, see Mahato et al, 1992 Conolly et al, 2008 Abe, 2007 Vincken et al, 2007 Liang and Zhao, 2008), and the steroid saponins. Steroid alkaloids behave like saponins but are sometimes treated as alkaloids, although these compounds are formed from intermediates of the steroid saponin pathway. Steroid saponins constitute a vast group of plant-borne glycosides present almost exclusively in the monocotyledonous angiosperms and occurring in only a few dicotyledonous families, such as the... 

SCHEME 19. Retrosynthetic pathway for the glycon of the anti-Leishmania triterpenoid saponin 81. 

SUZUKI, S., ACHNINE, L., HUHMAN, D., SUMNER, L.W., DIXON, R.A., A functional genomics approach to the triterpene saponin biosynthetic pathway in Medicago truncatula, Phytochemical Society of North America, 2001, Oklahoma City, OK... 

Saponins are glycosylated secondary metabolites that are widely distributed in the Plant Kingdom.3,4 They are a diverse and chemically complex family of compounds that can be divided into three major groups depending on the structure of the aglycone, which may be a steroid, a steroidal alkaloid, or a triterpenoid. These molecules have been proposed to contribute to plant defense.3 6 Saponins are also exploited as drugs and medicines and for a variety of other purposes.4 Despite the considerable commercial interest in this important group of natural products, little is known about their biosynthesis. This is due in part to the complexity of the molecules, and also to the lack of pathway intermediates for biochemical studies. 

A more detailed understanding of the biochemical pathways and enzymes involved in saponin biosynthesis will facilitate the development of plants with altered saponin content. In some cases, enhanced levels of saponins or the synthesis of novel saponins may be desirable (for example, for drug production 4 or improved disease resistance3,5,6), while for other plants, reduction in the content of undesirable saponins would be beneficial (for example, for legume saponins that are associated with antifeedant properties in animal feed7). This chapter is concerned with recent progress that has been made in the characterization of the enzymes and genes involved in the synthesis of these complex molecules and focuses on triterpenoid saponins. 

Triterpenoid saponins are synthesized via the isoprenoid pathway.4 The first committed step in triterpenoid saponin biosynthesis involves the cyclization of 2,3-oxidosqualene to one of a number of different potential products (Fig. 5.1).4,8 Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons although lupanes are also common 4 This cyclization event forms a branchpoint with the sterol biosynthetic pathway in which 2,3-oxidosqualene is cyclized to cycloartenol in plants, or to lanosterol in animals and fungi. 

FIGURE 1.4 Proposed biosynthetic route for the biosynthesis of (A) squalene oxide (squalene-2,3-oxide) via the isoprenoid pathway and (B) triterpene saponins of the dammarane-type and oleanane-type from squalene oxide. PP, diphosphate group GPS, geranyl phosphate synthase FPS, farnesyl phosphate synthase NADPH, nicotinamide adenine dinucleotide phosphate. 

Saponins have been variously attributed with a diverse range of properties, some of which include both beneficial and detrimental effects on human health, piscidical, insecticidal and molluscicidal activity, allelopathic action, antinutri-tional effects, sweetness and bitterness, and as phytoprotectants that defend plants against attack by microbes and herbivores [2-11]. A more detailed understanding of the biochemical pathways and enzymes involved in saponin... 

Cell suspension cultures of Gypsophila paniculata and Saponaria officinalis produce very closely related triterpenoid saponins. Pretreatment of cell suspension cultures of G. paniculata with gypsogenin 3,0-glucuxonide (a triterpenoid saponin precursor in G. paniculata) followed by administration of [ C] acetate resulted in a marked reduction in incorporation of radioactivity into saponins and their precursors, but not into sterols and steryl glycosides [26]. Measurements of OSC activities revealed that there was no effect of elicitor treatment on CS levels in either species, but in G. paniculata AS levels went down while in S. officinalis they increased. This suggests that in these two species OSCs are regulating steps in the isoprenoid pathway and control the flux to sterols and triter-penes. 

Protection of rats from devl. of Pseudomonas aeruginosa infections Activation of multiple effector pathwaye (Tcells, Bcells, macrophages) Protection of mice from devl. of C. albicans infection (macrophages) Polysaccharides as antineoplastic immunostimulators (NK cells) Proliferative responses of blood lymphocytes Protection of ginseng saponins of immunosuppression in mice Inhibition on bacterial endotoxin-induced embryolethality in rats Induction of neutrophil accumulation in mice... 

Penetration enhancers act by increasing the permeability of the corneal cell membrane and/or loosening the tight junctions between the epithelial cells, which primarily restrict the entry of molecules via the paracellular pathway. Classes of penetration enhancers include surfactants, bile salts, calcium chelators, preservatives, fatty acids, and some glycosides such a saponin. 

Keywords biosynthesis cardiac glycosides ecdysteroids metabolic pathways phylogeny pregnanes saponins secondary metabolites withanolides... 

Plant sterols are products of primary metabolism, but they may also be regarded as direct precursors of many secondary plant metabolites, such as the cardiac glycosides, saponins and steroid alkaloids. All of the compounds mentioned share the same basic skeleton therefore, the accumulation of a particular compound can only be achieved if (1) enzymes with a high degree of substrate specificity are involved in their biosynthesis, (2) metabolites can be channelled efficiently to the respective pathways and (3) products can be transported, sequestered and/or stored in specific compartments. 

These are C27 sterols in which the side chain has been metabolized to a spiroketal. They are formed from cholesterol by a pathway illustrated for diosgenin in Fig. 18. Sapogenins are the aglycones of the naturally occurring saponins which are 3-<7-gly-cosides. 

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