If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Secondary Metabolites Biosynthesis → Phenylpropanoid Derivatives Biosynthesis → Flavonoids Biosynthesis → Isoflavonoids Biosynthesis|
Some taxa known to possess this pathway include : Cicer arietinum , Glycine max , Glycyrrhiza echinata , Glycyrrhiza glabra , Lotus japonicus , Medicago sativa , Medicago truncatula , Phaseolus vulgaris , Pisum sativum , Pueraria montana lobata
Expected Taxonomic Range: Fabaceae
Pterocarpans constitute a group of natural isoflavonoids, which are typically involved as phytoalexins in the defense against pathogens of leguminous plants. In contrast to many other isoflavonoids, pterocarpans are mostly not constitutively expressed but induced by biotic and abiotic stress [Dewick94].
Formononetin is a crucial intermediate in the biosynthesis of phytoalexins, and as such the starting point for the formation of various pterocarpans. The recent discovery of a new 4'-O-methyltransferase [Akashi03], which facilitates an additional biosynthetic route to synthesize formononetin, sheds new light on the intermediary steps of phytoalexin production in legumes.
The biosynthetic route of pterocarpans leads to various compounds distinctive for certain leguminous species. The simple pterocarpans medicarpin (alfalfa), maackiain (chickpea) and pisatin (pea) are characterized by methoxy, methylendioxy and 6a-hydroxyl ring substitutents. On the other side there are the complex type pterocarpans, such as phaseollins (bean) and gluceollins (soybean) bearing prenyl substituents on their furan ring structure [Dixon99] [Tahara].
About This Pathway
The biosynthesis of pterocarpans is salient by the stereo-specificity of many of the enzymes involved. Pterocarpan synthase, the enzyme converting 2'-hydroxyisoflavanones [e.g. (-)-vestitone] to the corresponding pterocarpans [e.g. (6aR, 11aR)-medicarpin] has been demonstrated in alfalfa to consist of in fact two enzymes, a reductase and a dehydratase [Guo94a] [Guo94]. The precedent enzyme, isoflavone reductase, catalyzes the stereo-specific formation of the 2'-hydroxyisoflavanones (6aS or 6aR configuration), which determines the stereochemistry of the subsequent pterocarpans, e.g. (+)-maackiain in pea and (-)-maackiain in chickpea.
Pterocarpan phytoalexins are antimicrobial compounds in leguminous plants formed upon pathogenic attacks and similar biotic stress-related events. Maackiain is together with medicarpin the main pterocarpan phytoalexin in chickpea and occurs there exclusively in a (-)-(6aR,11aR)-configuration [Guo94a]. Maackiain was allocated within the two subtribes Diocleinae and Erythrininae to 12 species representing 4 genera [Ingham90].
The biosynthetic route of maackiain starts with formononetin, a 4'-methoxylated isoflavone, which is also the origin for the pterocarpans medicarpin in chickpea and pisatin in pea. The biosynthetic route differs in essentially two additional steps, one introducing a hydroxyl group to the 3'-position of the isoflavone B-ring [Dewick78] followed by another one forming the methylendioxy bridge of maackiain [Clemens96].
The question of how the final stereochemistry is applied to the various pterocarpans is still under discussion. In leguminous plants pterocarpans have been found with different stereochemical orientation. Chick pea and alfalfa accumulate only (-)- or (6aR,11aR)-pterocarpans, whereas peanuts accumulate pterocarpans of the opposite stereochemistry (+)- (6aS, 11aS). The stereochemistry of this phytoalexins can be crucial since it has been found that pathogens cannot break down the opposite enantiomer of the pterocarpans usually found in their target plants [VanEtten83].
There are two possibilities to introduce stereo-specificity to pterocarpans, one is that the reduction of the first chiral center (6a), catalyzed by the isoflavanone reductase, determines the final stereochemistry of the pterocarpans. It has been argued that once this center is fixed the second chiral center (11a) cannot invert the stereochemistry which would cause undue ring strain [Guo94]. This would require isoflavone reductase(s) catalyzing two different stereospecific reactions.
The other possibility is that the stereochemistry alters after the first reduction. The isoflavones isolated to date possess only one configuration, which is the (-)- or (6aR,11aR). It is not clear yet how this configuration is converted into the opposite form such as in plants like pea and peanut, where the (+)-pterocarpans accumulate. It has been suggested that a special epimerase exists in this plants, which carries out the epimerization on the stage of either the isoflavanones or pterocarpanes [Dixon95]. However the existence of such an enzyme remains to be shown [Guo94a] [Guo94] [Guo95].
Akashi03: Akashi T, Sawada Y, Shimada N, Sakurai N, Aoki T, Ayabe S (2003). "cDNA cloning and biochemical characterization of S-adenosyl-L-methionine: 2,7,4'-trihydroxyisoflavanone 4'-O-methyltransferase, a critical enzyme of the legume isoflavonoid phytoalexin pathway." Plant Cell Physiol 44(2);103-12. PMID: 12610212
Dixon99: Dixon RA, 1999 "Isoflavonoids: biochemistry, molecular biology, and biological functions." In: Comprehensive natural products chemistry Vol. 1: Sankawa, U. (editor), Polyketides and other secondary metabolites including fatty acids and their derivatives. Amsterdam, New York: Elsevier 1999, 773-82323.
Guo94: Guo L, Dixon RA, Paiva NL (1994). "The 'pterocarpan synthase' of alfalfa: association and co-induction of vestitone reductase and 7,2'-dihydroxy-4'-methoxy-isoflavanol (DMI) dehydratase, the two final enzymes in medicarpin biosynthesis." FEBS Lett 356(2-3);221-5. PMID: 7805842
Guo94a: Guo L, Dixon RA, Paiva NL (1994). "Conversion of vestitone to medicarpin in alfalfa (Medicago sativa L.) is catalyzed by two independent enzymes. Identification, purification, and characterization of vestitone reductase and 7,2'-dihydroxy-4'-methoxyisoflavanol dehydratase." J Biol Chem 269(35);22372-8. PMID: 8071365
Guo95: Guo L, Paiva NL (1995). "Molecular cloning and expression of alfalfa (Medicago sativa L.) vestitone reductase, the penultimate enzyme in medicarpin biosynthesis." Arch Biochem Biophys 320(2);353-60. PMID: 7625843
Ingham90: Ingham JL, (1990) "Systematic aspects of phytoalexin in formation within tribe Phaseoleae of the Leguminosae (subfamily Papilionoideae)." Biochemical Systematics and Ecology (1990), 18(5), 329-343.
VanEtten83: VanEtten HD, Matthews PS, Mercer EH, (1983) "(+)-maackiain and (+)-medicarpin as phytoalexins in Sophora japonica and identification of the (-) isomers by biotransformation." Phytochemistry (1983), 22(10), 2291-2295.
Akashi00: Akashi T, Sawada Y, Aoki T, Ayabe S (2000). "New scheme of the biosynthesis of formononetin involving 2,7,4'-trihydroxyisoflavanone but not daidzein as the methyl acceptor." Biosci Biotechnol Biochem 64(10);2276-9. PMID: 11129614
Akashi05: Akashi T, Aoki T, Ayabe S (2005). "Molecular and biochemical characterization of 2-hydroxyisoflavanone dehydratase. Involvement of carboxylesterase-like proteins in leguminous isoflavone biosynthesis." Plant Physiol 137(3);882-91. PMID: 15734910
Akashi06: Akashi T, VanEtten HD, Sawada Y, Wasmann CC, Uchiyama H, Ayabe S (2006). "Catalytic specificity of pea O-methyltransferases suggests gene duplication for (+)-pisatin biosynthesis." Phytochemistry 67(23);2525-30. PMID: 17067644
Akashi98: Akashi T, Aoki T, Ayabe S (1998). "CYP81E1, a cytochrome P450 cDNA of licorice (Glycyrrhiza echinata L.), encodes isoflavone 2'-hydroxylase." Biochem Biophys Res Commun 251(1);67-70. PMID: 9790908
Barz: Barz W, Welle R, (1992) "Biosynthesis and metabolism of isoflavones and pterocarpan phytoalexins in chickpea, soybean and phytopathogenic fungi." In: Stafford, H.A., Ibrahim, R.K. (editors) Phenolic metabolism in plants, recent advances in phytochemistry, Vol 26, Plenum Press New York and London, 139-164.
Barz89: Barz W, Bless W, Daniel S, Gunia W, Hinderer W, Jaques U, Kessmann H, Meier D, Tiemann K, Wittkampf U, (1989) "Elicitation and supression of isoflavones and pterocarpan phytoalexins in chickpea (Cicer arietinum L.) cell cultures." In: Kurz WGW (editor) Primary and secondary metabolism of plant cell cultures II, Springer Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong, (1989), 208-218.
Bhandari92b: Bhandari, P., Crombie, L., Daniels, P., Holden, I., Van Bruggen, N., Whiting, D. (1992). "Biosynthesis of the A/B/C/D-Ring System of the Rotenoid Amorphigenin by Amorpha fruticosa SeedIings." Journal of the Chemical Society, Perkin Transactions 1: 839 - 849.
Bless88: Bless W, Barz W, (1988) "Isolation of pterocarpan synthase, the terminal enzyme of pterocarpan phytoalexin biosynthesis in cell suspension cultures of Cicer arietinum." FEBS Letters (1988), 235(1), 47-50.
Crombie98: Crombie L, Whiting DA (1998). "Review article number 135 biosynthesis in the rotenoid group of natural products: applications of isotope methodology." Phytochemistry 49(6);1479-1507. PMID: 11711058
Daniel90: Daniel S, Tiemann K, Wittkampf U, Bless W, Hinderer W, Barz W, (1990) "Elicitor-induced metabolic changes in cell cultures of chickpea (Cicer arietinum L.) cultivar resistant and susceptible to Ascochyta rabiei." Planta (1990), 182(2), 270-278.
Gunia: Gunia W, Hinderer W, Wittkampf U, Barz W, (1991) "Elicitor induction of cytochrome P-450 monooxygenases in cell suspension cultures of chickpea (Cicer arietinum L.) and their involvement in pterocarpan phytoalexin biosynthesis." Z. Naturforsch. 46c, 58-66.
Hagmann84: Hagmann ML, Heller W, Grisebach H (1984). "Induction of phytoalexin synthesis in soybean. Stereospecific 3,9-dihydroxypterocarpan 6a-hydroxylase from elicitor-induced soybean cell cultures." Eur J Biochem 142(1);127-31. PMID: 6540173
Hakamatsuka98: Hakamatsuka T, Mori K, Ishida S, Ebizuka Y, Sankawa U, (1998) "Purification of 2-hydroxyisoflavanone dehydratase from the cell cultures of Pueraria lobata." Phytochemistry (1998), 49(2), 497-505.
Heller94: Heller W, Forkmann G (1994). "Biosynthesis of flavonoids." In: Harborne JB (editor) The flavonoids. Advances in research since 1986. Chapman & Hall, London Glasgow New York Tokyo Melbourne Madras, 499-537.
Hinderer: Hinderer W, Flentje U, Barz W, (1987) "Microsomal isoflavone 2'- and 3'-hydroxylases from chickpea (Cicer arietinum L.) cell suspensions induced for pterocarpan phytoalexin formation." FEBS Letters, 214(1), 101-106.
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