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Read this article to learn about Triterpenoid Saponins. After reading this article you will learn about 1. Biosynthesis of Triterpenoid Saponins 2. Chemistry of Triterpenoid Saponins.
Biosynthesis of Triterpenoid Saponins:
The biosynthesis of triterpenoid saponins are initiated from mevalonic acid and further proceed via isoprenoid pathway. In Isoprenoid pathway, at first 3 isoprene units of 5 carbon atoms are producing 5 carbon atom molecule farnesyl pyrophosphate through geranyl pyrophosphate. Two molecules of this farnesyl pyro-phosphate linked in a tail-to-tail manner to form squalene of 30 carbon atoms.
This squalene oxidized to 2, 3 oxidosqualene and followed by enzymatic cyclisation to produce various skeletons of triterpenoid and sterols (Fig. 1) as aglycone parts of saponins.
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The enzymatic cyclization process is regulated by diverse type of 2, 3 oxidosqualene cyclases (OSCs) like lanosterol synthase, cycloartenol synthase, dammarenediol synthase, β-amyrin synthase.
The biosynthesis of triterpenoid or sterol follows Ruzicka deduced “biogenetic rule of isoprene”, which is governed by cation activated oxidosqualene cyclization of “chair-chair-chair” or “chair-boat-chair” conformation, rearrangement and de-protonation reaction.
The tetracyclic dammarenyl cation may undergo to further rearrangements to form tetracyclic C17 baccharenyl carbocation and can be followed by reaction to produce pent acyclic C25 lupenyl carbocation. The lupeol synthase (LuS) catalyses the conversion of pent acyclic C25 lupenyl carbocation to Iupane triterpenoid.
Further, the rearrangement of lupenyl carbocation lead to produce oleanane type or β-amyrin triterpenoids and ursane type or α- amyrin triterpenoids, which is mediated by β- amyrin synthase (PAS) and α-amyrin synthase(aAS) respectively.
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On the other hand, enzymatic cyclisation of “chair-boat-chair “conformation of 2,3-oxidosqualene produces tetracyclic protosteryl C-20 carbocation, which is then converted to sterols like cycloartenol (in plant) and lanosterol (in animals and fungi) by cycloartenol synthase (CS) and lanosterol synthase (LS), respectively.
In the final step of saponin biosynthesis, all sapogenins (triterpenoid and sterols) go through the various chemical decorations (oxidation, substitution, acylation, glycosylation, benzoylation, and so on) mediated by cytochrome P450-dependent monooxygenases, glycosyltransferases, acyltransferases, benzoyltransferases, and other enzymes. But the detail mechanisms of these reactions are still unknown.
Various sugar moieties like glucose, galactose, rhamnose, glucouronic acid will be attached to carbon skeletons by glycosylation reaction. In case of triterpenoid saponins, sapogenins attach to chain of sugar and/or uronic acid units at C3 position, unlike steroidal saponins.
Whereas oxidation reaction causes the attachment of functional groups (hydroxyl groups, carbonyl groups and carboxylic acids) at various positions of carbon skeletons.
Chemistry of Triterpenoid Saponins:
Saponins are surface active glycosides that are widely available in nature. Chemically saponins are composed of aglycone and sugar moieties which are produced by hydrolysis of saponins.
The structure of saponins are varied depend on variety of aglycone or sapogenins are attached. Commonly triterpenoid and steroids are served as aglycones which lead to name saponins as triterpenoid saponins and steroidal saponins, respectively.
Triterpenoid saponins are generally occur in higher plant kingdom whereas steroidal saponins are common in wide range of plant species and in some bacteria. The diverse chemical class of saponins also include steroidal glycoalkaloids, which consists of glycosidically linked steroidal alkaloid with monosaccharide moieties.
The non-polar aglycone part of triterpenoid saponins include diverse skeletons of triterpenoids and that is attached to one or more polar sugar moieties.
In last the few decades, several works have been carried out on biosynthesis, isolation and characterization of naturally occurring triterpenoid saponins, gave an extensive knowledge about the various classes of triterpenoids which boost up the further classification process for this class of saponins.
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Therefore, based on triterpenoid carbon skeletons which produced in biosynthetic pathway (see biosynthesis of triterpenoid saponins); triterpenoid saponins are classified in various classes like dammarene, lupanes, oleananes and ursanes. These classes of saponins are distributed in various medicinal plants.
The sugar moieties which are attached to these carbon skeletons, are found to be mono-saccharides (e.g. glucose, xylose, arabinose, pyranose, rhamnose, etc.) only, but differ in number. Based on the number of sugar chains attached, triterpenoid saponins are also classified, like, monodesmoside (containing single sugar chain), bidesmoside (containing two sugar chains), tridesmoside (containing three sugar chains).
It has seen that 18 monosaccharide residues can be attached through sugar chains to oleanane skeleton. During biosynthesis several substituents such as -OH, -COOH, -CHO, -CH2OH and -CH3 are also introduced to different position of aglycone skeleton.