If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Terpenoids Biosynthesis → Triterpenoids Biosynthesis|
Expected Taxonomic Range: Magnoliophyta
Saponins are glycosides of triterpenoid (this pathway) or steroidal aglycones (see plant sterol biosynthesis) with a varying number of sugar side chains. Saponins are especially enriched in plant epidermal cells forming a protective surfactant that forms a soapy froth when mixed with water [Wegner02]. Found among a wide variety of plants legumes constitute a major source of saponins [Waller96] [Dixon03]. The saponins isolated from soybean and other legumes, also referred to as soyasaponins, are derived from triterpenoid aglycones such as soyasapogenol A, B, E, medicagenic acid, hederagenin and bayogenin which have been identified through metabolic profiling [Huhman02].
Saponins possess a number of biological activities that have been intensively investigated over the last few decades. Extracts of plants rich on saponins have been demonstrated to have allelopathic, antiflammatory, hemolytic [Waller96], cholesterol lowering [Sauvaire96] [Sowmya99] and anticancer properties [Kerwin04] but are also known to reduce the digestibility in ruminants [Klita96, Cheeke96, Oleszek99] and to be generally toxic to cold blooded animals and insects [Tava96].
The diversity in conjugation of saponins increases the biological activity of those compounds compared to their aglycone (sapogenin) complements [Haridas01]. In addition, saponin has been successfully tested as a cleaning agent for remediation of heavy metal contaminated soils [Hong02]. However, the physiological significance of saponins in plants remains poorly understood and requires further research.
About This Pathway
The first committed step in the biosynthesis of triterpenoid saponins is the cyclization of 2,3-oxidosqualene, a common intermediate in the biosynthesis of terpenoids that marks both a branching point for phytosterols and triterpenoid biosyntheses [Hayashi01] and primary (sterols) and secondary (triterpenoids) metabolism [Kushiro98]. The reaction is carried out by a specific oxidosqualene cyclase (OSC), i.e. β-amyrin synthase (bAS) that catalyzes the formation of β-amyrin and is inducible upon treatment with methyljasmonate [Hayashi03]. β-amyrin is the central compound that represents another branch point in the saponin biosynthesis and can be converted into at least five different triterpenoid aglycones in legumes [Suzuki02a].The cyclization of 2,3-oxidosqualene can generate more than 80 different skeleton types leading to tetra- and pentacyclic compounds via multiple (8-10) asymmetric centres in one single enzymatic step [Xu04a]. That clearly indicates the high stereospecificity of the enzyme involved in a series of intermediate reactions which mechanism has been thoroughly investigated [Kushiro98].
The further decoration of β-amyrin towards the synthesis of soyasaponins involves hydroxylation and glycosylation reactions. In analogy to a series of similar oxidative reactions involved in the biosynthesis of brassinosteroids (brassinosteroid biosynthesis I, brassinosteroid biosynthesis II) the involvement of cytochrome P450 dependent monooxygenases have been expected for the formation of soysaponin [Dixon03] [Xu04a]. Just recently the characterization of a 24-hydroxylase being able to hydroxylate both β-amyrin to form olean-12-ene-3beta,24-diol and sophoradiol to generate soyasapogenol B has been reported [Shibuya06]. The other steps of the pathway (C-22-hydroxylation) leading to soyasapogenol and/or relevant intermediates are also anticipated to be carried out by cytochrome P450's.
The remaining reaction of the pathway concerns the attachment of glycosidic residues to the sapogenin, leading to the formation of a saponin. The first step is the attachemnt of β-glucuronate, resulting in soyasapogenol B-3-O-β-glucuronide, which is catalyzed by soyasapogenol glucuronosyltransferase (UGASGT). This is followed by attachment of β-galactose and L-rhamnose, forming soyasaponin I [Shibuya10].
Unification Links: PlantCyc:PWY-5203
Cheeke96: Cheeke PR (1996). "Biological effects of feed and forage saponins and their impacts on animal production." In: Saponins used in Food and Agriculture. Waller GR, Yamasaki K (eds.), Advances in experimental medicine and biology, Vol. 405, 377-385.
Haridas01: Haridas V, Higuchi M, Jayatilake GS, Bailey D, Mujoo K, Blake ME, Arntzen CJ, Gutterman JU (2001). "Avicins: triterpenoid saponins from Acacia victoriae (Bentham) induce apoptosis by mitochondrial perturbation." Proc Natl Acad Sci U S A 98(10);5821-6. PMID: 11344312
Hayashi01: Hayashi H, Huang P, Kirakosyan A, Inoue K, Hiraoka N, Ikeshiro Y, Kushiro T, Shibuya M, Ebizuka Y (2001). "Cloning and characterization of a cDNA encoding β-amyrin synthase involved in glycyrrhizin and soyasaponin biosyntheses in licorice." Biol Pharm Bull 24(8);912-6. PMID: 11510484
Hong02: Hong KJ, Tokunaga S, Kajiuchi T (2002). "Evaluation of remediation process with plant-derived biosurfactant for recovery of heavy metals from contaminated soils." Chemosphere 49(4);379-87. PMID: 12365835
Huhman02: Huhman DV, Sumner LW (2002). "Metabolic profiling of saponins in Medicago sativa and Medicago truncatula using HPLC coupled to an electrospray ion-trap mass spectrometer." Phytochemistry 59(3);347-60. PMID: 11830145
Kurosawa02: Kurosawa Y, Takahara H, Shiraiwa M (2002). "UDP-glucuronic acid:soyasapogenol glucuronosyltransferase involved in saponin biosynthesis in germinating soybean seeds." Planta 215(4);620-9. PMID: 12172845
Kushiro98: Kushiro T, Shibuya M, Ebizuka Y (1998). "β-amyrin synthase--cloning of oxidosqualene cyclase that catalyzes the formation of the most popular triterpene among higher plants." Eur J Biochem 256(1);238-44. PMID: 9746369
Sauvaire96: Sauvaire Y, Baissac Y, Leconte O, Petit P, Ribes G (1996). "Steroid saponins from fenugreek and some of their biological properties." In: Saponins used in Food and Agriculture. Waller GR, Yamasaki K (eds.), Advances in experimental medicine and biology, Vol. 405, 37-46.
Shibuya06: Shibuya M, Hoshino M, Katsube Y, Hayashi H, Kushiro T, Ebizuka Y (2006). "Identification of β-amyrin and sophoradiol 24-hydroxylase by expressed sequence tag mining and functional expression assay." FEBS J 273(5);948-59. PMID: 16478469
Shibuya10: Shibuya M, Nishimura K, Yasuyama N, Ebizuka Y (2010). "Identification and characterization of glycosyltransferases involved in the biosynthesis of soyasaponin I in Glycine max." FEBS Lett 584(11);2258-64. PMID: 20350545
Suzuki02a: Suzuki H, Achnine L, Xu R, Matsuda SP, Dixon RA (2002). "A genomics approach to the early stages of triterpene saponin biosynthesis in Medicago truncatula." Plant J 32(6);1033-48. PMID: 12492844
Tava96: Tava A, Odoardi M (1996). "Saponins from Medicago ssp.: chemical characterization and biological activity against insects." In: Saponins used in Food and Agriculture. Waller GR, Yamasaki K (eds.), Advances in experimental medicine and biology, Vol. 405, 97-109.
Basyuni06: Basyuni M, Oku H, Inafuku M, Baba S, Iwasaki H, Oshiro K, Okabe T, Shibuya M, Ebizuka Y (2006). "Molecular cloning and functional expression of a multifunctional triterpene synthase cDNA from a mangrove species Kandelia candel (L.) Druce." Phytochemistry 67(23);2517-24. PMID: 17078982
Haralampidis01: Haralampidis K, Bryan G, Qi X, Papadopoulou K, Bakht S, Melton R, Osbourn A (2001). "A new class of oxidosqualene cyclases directs synthesis of antimicrobial phytoprotectants in monocots." Proc Natl Acad Sci U S A 98(23);13431-6. PMID: 11606766
HusselsteinMull01: Husselstein-Muller T, Schaller H, Benveniste P (2001). "Molecular cloning and expression in yeast of 2,3-oxidosqualene-triterpenoid cyclases from Arabidopsis thaliana." Plant Mol Biol 45(1);75-92. PMID: 11247608
Liu09: Liu Y, Cai Y, Zhao Z, Wang J, Li J, Xin W, Xia G, Xiang F (2009). "Cloning and Functional Analysis of a beta-amyrin synthase gene associated with oleanolic acid biosynthesis in Gentiana straminea MAXIM." Biol Pharm Bull 32(5);818-24. PMID: 19420748
Wang10: Wang Z, Yeats T, Han H, Jetter R (2010). "Cloning and characterization of oxidosqualene cyclases from Kalanchoe daigremontiana: enzymes catalyzing up to 10 rearrangement steps yielding friedelin and other triterpenoids." J Biol Chem 285(39);29703-12. PMID: 20610397
Wang11a: Wang Z, Guhling O, Yao R, Li F, Yeats TH, Rose JK, Jetter R (2011). "Two oxidosqualene cyclases responsible for biosynthesis of tomato fruit cuticular triterpenoids." Plant Physiol 155(1);540-52. PMID: 21059824
Yu13: Yu F, Thamm AM, Reed D, Villa-Ruano N, Quesada AL, Gloria EL, Covello P, De Luca V (2013). "Functional characterization of amyrin synthase involved in ursolic acid biosynthesis in Catharanthus roseus leaf epidermis." Phytochemistry 91;122-7. PMID: 22652241
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