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8. Shikimates and Phenylpropanoids RA Macahig FM Dayrit O H CO 2 H O HO HO OH OH O H shikimic acid O luteolin O H Introduction Shikimic acid is the key intermediate of a large group aromatic natural products. The isolation of shikimic acid was first reported from aniseed (Illicium anisatum) and the fruit of I. religiosum, whose Japanese name was “shikimi-no-ki” (shi four; kimi seasons; no of; ki tree, literally “tree of four seasons”). Shikimic acid has since been found in many plants, bacteria, yeasts and moulds. It is estimated that this group accounts for 35% CO 2 H of plant dry mass, and that one-fifth of the carbon fixed by plants is channeled to shikimic acid metabolites, such as the lignins. HO Shikimic acid is the precursor of three important aromatic amino acids: phenylalanine, tyrosine, and tryptophan. 8.0 Shikimates & phenylpropanoids (Dayrit) OH OH shikimic acid 2 Overview OP H+ _ O2C phosphoenol pyruvate glycolysis O _ O2C D-glucose H pentose phosphate cycle OH H H-O PO O H HO OH OH heptulose D-erythrose-4-phosphate Shikimic acid is produced directly from two D-glucose metabolites: phosphoenol pyruvate (C3) and Derythrose-4-phosphate (C4) condense to form the seven carbon heptulose. Heptulose cyclizes to form 5dehydroquinate which loses water to form shikimic acid. H H _ CO2 O HO _ CO2 O OH OH OH OH 5-dehydroquinate 3-dehydroshikimate _ CO2 HO OH OH shikimate 8.0 Shikimates & phenylpropanoids (Dayrit) 3 HOCH2 O H H HO Shikimic acid is converted to 5-enolpyruvylshikimate-3-phosphate, which loses phosphate to yield chorismic acid and prephenic acid. Chorismic acid is the branch point to a group of benzoic acid metabolites and tryptophan. Prephenic acid leads to the phenyl propanoids (C6-C3) and the flavonoids (C6-C3-C6), and the amino acids phenylalanine and tyrosine. OH OH H H H OH D-glucose CH3 CO2H O CO2H 1 CO2H 3 EPSPS HO OH OH shikimic acid CO2H benzoic acids, tryptophan 5 PO O CO2H OH 5-enolpyruvylshikimate -3-phosphate O CO2H OH chorismic acid EPSPS: 5-enolpyruvyl shikimate-3-phosphate synthase phenylalanine, tyrosine O HO2C lignans, lignins CO2H + (3 x Ac-CoA) flavonoid phenyl propanoids OH phenols prephenic acid 8.0 Shikimates & phenylpropanoids (Dayrit) 4 styrenesbenzoic acids In microorganisms: [O] polyketide aromatic compound quinone In plants: [O] Overview of biosynthesis of quinones. Depending on the organism, quinones can arise via the polyketide or shikimate pathways. polyketide aromatic compound quinone quinone shikimate aromatic compound + terpene [O] quinone (mixed metabolite) quinone from shikimate: OH quinones from shikimate + terpene: OH O OH O H OH CO2H OH homogentisic acid alkarinin O R H 5. Polyketides (Dayrit) O n ubiquinones: R = H, CH3 ; n = 4-13 5 Shikimates comprise a large group of aromatic natural products. CO2H CO2H R NH2 O phenylalanine, R=H tyrosine, R=OH cinnamic acid O coumarin CO2H O CO2H phenylacetic acid benzoic acid O flavonoid O O lignan 8.0 Shikimates & phenylpropanoids (Dayrit) 6 Shikimic acid picks up three carbons from of phosphoenol pyruvate to form chorismic acid and prephenic acid. Chorismic acid is converted to prephenic acid via a concerted [3,3]-sigmatropic shift. _ CO2 _ CO2 _ CO2 H H HO 1. ATP OH PO OH 2. PEP shikimate _ CO 2 _ CO2 O OH 3 2 _ 1 HO 2 1 OH CO2 O2C O 3 [3,3]sigmatropic shift _ CO2 _ _ CO2 2 _ CO2 3 O 3 3 2 3 1 _ CO2 OH chorismate 5-EPSP 1 _ O2C PO O H 1 O 2 1 OH 2 _ CO2 OH p-hydroxybenzoic acid prephenate o-aminobenzoic acid, p-aminobenzoic acid phenylpropanoids 8.0 Shikimates & phenylpropanoids (Dayrit) 7 a _ O _ _ CO2 O Prephenate is the precursor of phenylalanine and tyrosine. NH2 O CO2 CO2H O a -CO2, -H2O Pyridoxamine transaminase OH phenylalanine prephenate b [O] _ _ O2C CO2 NH2 O _ CO2 CO2H O -CO2 Pyridoxamine transaminase O OH OH tyrosine 8.0 Shikimates & phenylpropanoids (Dayrit) 8 C6, C6-C1 and C6-C2 The shikimate metabolites can be grouped according to the number of carbons atoms in the side chain. • C6: phenols and quinones • C6-C1: benzoic acid derivatives, including tannins • C6-C2: phenyl ethyl compounds 8.0 Shikimates & phenylpropanoids (Dayrit) 9 Routes to benzoquinones: A. Polyketides Hydroxybenzoic acids B. Shikimatic acid Benzoquinones Homogentisic acid Benzoquinones Chroismic acid p-Hydroxybenzoic acid Benzoquinones Phenylalanine From p-Hydroxybenzoic acid: CO 2H CO 2H R= 1. -CO 2 2. [O] 3. O-methylation n from terpenes R MeO OH OH O O Me Me MeO MeO O Ubiquinone n R O 5-Demethoxy ubiquinone [O] C6 Metabolites Quinones are formed mainly from the polyketide and shikimate pathways, although other groups may produce quinones from extensive modification. The biosynthesis of R OH ubiquinone, the 1. [O] compound which 2. C-methylation assists in biological electron transfer, is OH Me shown. MeO R OH 10 C6-C1. Tannins are a group of benzoic acid plant metabolites which precipitate proteins. The gallotanins, which are hydrolyzable tannins, contain glucose and gallic acid joined by ester linkages. HO O HO OH O HO HO O O OH OH corilagin OO HO O O OH OH O HO HO O OH OH OH O O OH OH O HO O OH HO O O OH O HO O HO O OH OH O O HO HO O OH Turkish tannin HO OH 8.0 Shikimates & phenylpropanoids (Dayrit) 11 Two benzoic acid derivatives of some interest are gallic acid and ellagic acid. Gallic acid and ellagic acid are constituents of hydrolyzable tannins but these are found in many other natural products. These compounds are natural antioxidant in aqueous and micellar environments. Ellagic acid is also a naturally-occuring phytochemical pesticide and antimicrobial. O H O H H O O O H H O H O H O O H O H H O H O O Gallic acid O O O O O H O H O O H H O O H Ellagic acid 8.0 Shikimates & phenylpropanoids (Dayrit) 12 Ellagitannins are hydrolyzable tannins which are formed from the condensation of a sugar core and ellagic acid units. Ellagitannis from banaba, Lagerstroemia speciosa, were shown to increase uptake of adipocytes in rats, and could be responsible for lowering the blood glucose level. (Hayashi, et al., “Ellagitannins from Lagerstroemia speciosa as Activators of Glucose Transport in Fat Cells,” Planta Med., 2002, 68, 173-5.) OH HO OH OH OH R1 HO O R2 H O O H H O H O O O HO HO HO O OH OH OH OH CH 2 O O HO O HO HO O O O O Lagerstroemin : R 1 =OH; R 2 =H Flosin B : R 1 =H; R 2 =OH 8.0 Shikimates & phenylpropanoids (Dayrit) OH OH 13 -o xidation PAL The benzoic and cinnamic acids are biosynthesized in a metabolic grid. These compounds occur widely in plants. CO 2H CO 2H CO 2H NH2 cinnamic acid phenylalanine [O] [O] CO 2H NH2 HO be nzoic acid [O] CO 2H TAL HO p-hydroxybenzoic acid [O] [O] HO CO 2H HO [O] CO 2H -o xidation NH2 HO HO HO CO 2H de carbo xylase, -CO2 CO 2H pro tocatechuic acid OH gallic acid [CH3] CH3O HO HO HO caffe ic acid 3,4-dihydroxyphenylalanine (DOPA) HO chorismic acid HO 4-hydroxycinnamic acid (p-coumaric acid) tyrosine CO 2H -o xidation [CH3] CO 2H CH3O CO 2H -o xidation NH2 HO HO HO ferulic acid do pamine vanillic acid 1. [O] 2. [Me] 1. [O] 2. [Me] 1. [O] 2. [Me] OH CH3O HO CH3O CO 2H CO 2H -o xidation NH(CH3) HO adrenaline HO HO OCH3 sinapic acid 14 OCH3 syring ic ac id Cyanogenic glycosides: C6-C2 • A cyanogenic glycoside has an aglycone with a cyanide group and an attached sugar. Cyanogenic glycosides release the poisonous hydrogen cyanide enzymatically. • Cyanogenic glycosides are found in cassava, and the fruits and wilting leaves of the rose family (including cherries, apples, plums, almonds, peaches, apricots, and raspberries). Sorghum (Sorghum bicolor) expresses cyanogenic glycosides in its roots and thus is resistant to pests such as rootworms. • The dotted arrow is a metabolic link to aromatic glucosinolates. (Jorgensen et al., Curr Opinion in Plant Biol 2005, 8:280–291) 8.0 Shikimates & phenylpropanoids (Dayrit) 15 Glucosinolates OH O HO HO HO HO • The glucosinolates are found in Brassicales and contain sulfur, nitrogen and a group derived from OSO3 glucose. S N • About 120 glucosinolates are known in plants, where the R group is alkyl, derived from methionine, alanine, (R group variable) leucine, or valin; aromatic, derived from phenylalanine and tyrosine; or indolic derived from tryptophan. • Glucosinolates act as natural pesticides and defense against herbivores. These substances are also responsible for the bitter or sharp taste of many common foods such as mustard, radish, horseradish, cress, cabbage, Brussels sprouts, cauliflower, broccoli, and turnip. 8.0 Shikimates & phenylpropanoids (Dayrit) 16 Phenyl propanoids: C6-C3 • The phenylpropanoid metabolism is unique to plants. • Many intermediates and end products of the phenylpropanoid pathway play important roles in plants as phytoalexins, antioxidants, antiherbivory compounds, UV protectants, pigments, and aroma compounds. Phenylpropanoids polymerize to form lignins, which are essential components of the cell wall stability. • Phenylpropanoid biosynthesis is one of the best-studied pathways in plants. The enzymes of the phenylpropanoid pathway are organized in multi-enzyme complexes and there is evidence for the coordinated expression of genes and enzymes. Genes encoding enzymes of this pathway are developmentally and tissue-specifically regulated and may be induced by environmental stresses such as nutrient deficiency, exposure to cold, UV light, and pathogen attack. 8.0 Shikimates & phenylpropanoids (Dayrit) 17 Phenylalanine and tyrosine are deaminated by phenylalanine ammonia lyase (PAL) or tyrosine ammonia lyase (TAL) to cinnamic acids. The cinnamic acids (C6-C3) are the precursors to the phenyl propanoids, coumarins, styrenes, benzoic acids, phenols, and flavonoids. A multi-enzyme complex enables coordinated action of PAL and cinnamate-4-hydroxylase (C4H) which control the flux of intermediates in phenylpropanoid biosynthesis. NH2 CO2H CO2H CO2H Ar - C3: cinnamaldehydes cinnamyl alcohols aryl propanes PAL or TAL Ar - C3: coumarins C4H R R1 R3 R phenylalanine, R=H tyrosine, R=OH R2 + 3 x Ac cinnamic acids Ar - C2: styrenes acetophenones flavonoids Ar: phenols quinones Ar - C1: benzoic acids benzyl aldehydes benzyl alcohols 8.0 Shikimates & phenylpropanoids (Dayrit) 18 HO CO2H Examples of cinnamic acids and the C6-C3 derivatives. Cinnamic acids themselves do not usually occur in the free form but are isolated as glycosides. Cinnamic acids are precursors of lignans and lignins. O OH HO O 3-caffeoylquinic acid (chlorogenic acid). First isolated in 1846 from coffee, it has since been found to be a common plant compound. It functions as an allelopathic substance in sunflower. OH OH OH HO O O OH HO HO OC CH3O OH RO OH coniferyl alcohol, R=H coniferin, R=glucosyl O 2-caffeoylarbutin CH3O eugenol Eugenol and saffrole are well-known constituents of flavor and spice plants. They are precursors to lignins and lignans. HO O saffrole O 8.0 Shikimates & phenylpropanoids (Dayrit) 19 Examples of cinnamic acids and the C6-C3 derivatives. Rosmarinic acid is a dimer of two different C6-C3 units. C O H 2 H O C O H 2 O N H 2 H O O H H O O tyrosine caffeic acid H O C H C H C O H 2 2 O H H O H O C O H 2 C O H 2 O H N H 2 H O phenylalanine rosmarini Rosmarinic ac anti-histamine tsaang Ehretia gubat ). m Coumarins are aromatic lactone C6-C3 derivatives. They are widely distributed in plants, particularly the Umbelliferae and Rutacea families. C O H 2 [O] R O C O H 2 C O H 2 R O glucose O H O g lu R O E R1R2 umbelliferone R H H 2 4 herniarin H CH 5 3 6 3 skimmin glu H 7 2 1 aesculetin8.0 Shikimates & phenylpropanoids H OH 8 (Dayrit) R 1 O CH O O scopoletin OCH R O 3 3 Z C O H 20 2 O g lu Examples of coumarin compounds. O O- H O C O H 2 O H O O O C H O H O H O O- H O O O O Some microorganisms Aspergillus (such)fumigatus as C H 2 transform coumarin from plants (such as grass) to produce dicoumarol. Dicoumarol is a powerful blood anticoagulant and can cause fatal hemmorages in cattle that eat the hay. O O O O dicoumarol O H O H H O H Furobinorden O O O HO OH 8.0 Shikimates & phenylpropanoids (Dayrit) 21 Lignans are phenylpropanoid dimers. They can be rationalized by formation by coupling of resonance-stabilized by radicals with the assistance of “dirigent” proteins. OH oxidase with guiding (dirigent) protein oxidase only OCH3 OH H OH . O CH3O HO CH3O OH H O O OH . OCH3 OCH3 OH H Proposed coupling intermediate (+/-)-Dehydrodiconiferyl alcohols + OCH3 OCH3 OH OH O O H H O O HO HO OCH3 (+/-)-Pinoresinols OCH3 (+)-Pinoresinol + Other dimers 8.0 Shikimates & phenylpropanoids (Dayrit) (N.G. Lewis, et al. in Science, 275, 362 (1997); Plant Physiol. , 123, 453 (2000);Chem. Biol. , 6, 143 (1999)) 22 The diversity of lignan structures can be rationalized by bond formation via different radical sites. CH2OH CH2OH CH2OH . [O] CH3O CH3O CH2OH O CH2OH CH3O O. OH CH3O O OCH3 O H+ O [H] HO OCH3 OCH3 HO O CH3O CH2OH HO CH2OH OH O OCH3 O H+ OCH3 OCH3 HO (+)pinoresinol O H+ CH3O CH2OH HO CH2OH 8.0 Shikimates & phenylpropanoids (Dayrit) OCH3 OH (+)isolariciresinol 23 The diversity of lignan structures can be rationalized by bond formation via different radical sites. H . C H O 3 . C H O 3 O O O O C H 3 (from C H O 3 O O C H 3 + H H O O O C H 3 licarin eugenol) 8.0 Shikimates & phenylpropanoids (Dayrit) 24 A Lignins The majority of carbon in the phenylpropanoid group is channeled toward the synthesis of lignin, a complex three-dimensional polymer that is a principal structural component of plant cell walls. (/www.scielo.cl) Lignin has far-reaching impacts on agriculture, industry and the environment, making phenylpropanoid metabolism a globally important part of plant biochemistry. Lignin is the second most abundant polymer on earth, next to cellulose. Lignin is a major carbon sink in the biosphere, accounting for about 30% of the more than 1.4 × 1012 kg of carbon sequestered into terrestrial plant material each year. 8.0 Shikimates & phenylpropanoids (Dayrit) 25 O OH CH3O O HO OCH3 O Ph. + Ph. O OCH3 alkyl. + alkyl. O. + alkyl. HO O O OCH3 OH Ph. + alkyl. HO OCH3 OH HO Lignins are extensively crosslinked phenylpropanoid polymers. Similar to the lignans, they are formed by coupling between different radicals arising from phenylpropanoids. Lignins contribute to the strength and robustness of plants and as a protective barrier against biochemical degradation by microorganisms. CH3O O 8.0 Shikimates & phenylpropanoids (Dayrit) 26 Intermediates and enzymes of the lignin pathway. 4CL, 4-(hydroxy)cinnamoyl CoA ligase C3H, p-coumarate 3-hydroxylase C4H, cinnamate 4-hydroxylase CAD, cinnamyl alcohol dehydrogenase CCoAOMT, caffeoyl CoA O-methyltransferase CCR, cinnamoyl CoA reductase COMT, caffeic acid/5-hydroxyferulic acid O-methyltransferase CQT, hydroxycinnamoyl CoA:quinate hydroxycinnamoyltransferase CST, hydroxycinnamoyl CoA:shikimate hydroxycinnamoyltransferase F5H, ferulate 5-hydroxylase PAL, phenylalanine ammonia-lyase pCCoA3H, p-coumaryl CoA 3-hydroxylase SAD, sinapyl alcohol dehydrogenase. 8.0 Shikimates & phenylpropanoids Humphreys and Chapelle, (Dayrit) Current Opinion in Plant Biology 2002,27 5:224–229. Flavonoids The flavonoids are a very widely occurring group of natural products. There are over 5,000 flavonoids identified in plants. Most flavonoids, except for the catechins, are naturally found in their glycosylated form. Flavonoids are universally distributed among vascular plants and are found throughout the plant- roots, bark, fruit, seeds, leaves and flowers—where they play important functions. Flavonoids are physiologically important because of their many antioxidant properties. O H 4’ Flavonoids have a characteristic “C6-C3C6” structure, with three rings: an aromatic A and B ring, and a -pyrone C ring. 1 O HO 7 8.0 Shikimates & phenylpropanoids (Dayrit) B 1’ H 3’ O A C 5 4 O H O luteolin 28 O H O HO Flavonoids O H O H O luteolin Flavonoids have attracted wide interest due to the following: a. Distribution of flavonoids. The flavonoids are one of the most numerous and widespread groups of secondary metabolites. They are present in most parts of the plant, often in various methylated and glycosylated forms. b. Functions of flavonoids in plants. The colored pigments in flowers of angiosperms (blues, purples, reds, and yellows) are produced by the flavonoid group known as the anthocyanidins. In leaves, the chlorophylls mask these colors. The highly oxygenated flavonoids are UV-active and are thought to protect the plant against UV radiation. Various flavonoids are believed to play specific roles in the physiology of plants. A number of flavonoids, for example, are known to be antifungal, antibacterial, and allelopathic. 8.0 Shikimates & phenylpropanoids (Dayrit) 29 O H O HO Flavonoids O H O H O luteolin c. Biological activity of flavonoids. A number of flavonoids have been shown to protect cells against oxidative attack. This is thought to explain its anti-inflammatory and cancerprotective properties. In addition, flavonoids have been shown to strengthen blood capillaries, and have found use in protection against weak arteries, strokes and hemorrhoids. • Flavonoids are formed from the condensation of a phenylpropanoid + triketide. The flavonoids can be subdivided according into oxidation level and position at the rings.biosynthetic studies have been carried out using • three Extensive tissue culture of parsley. Chalcone is the entry point to the flavonoids. Further oxidation and methylation occur after flavonoid ring formation. 8.0 Shikimates & phenylpropanoids (Dayrit) 30 R R 1. PAL / TAL 2. CoA ligase CoA -S -O C 2 NH 2 O CO 2 - phenylalanine, R=H tyrosine, R=OH - 1. R O * CoA -S 2. -CO 2, -CoASH R 1. NADPH 2. -CO 2, -CoASH benzal acetone Overview of the condensation of phenylpropanoid + (3 x acetyl CoA). O CoA -S * R O O CO 2 - -CoASH O O - * 1. O * CoA -S 2. -CO 2, -CoASH OH dihydropyrone R R CoA -S O O * O O -CoASH * * O * styryl pyrone CO 2 - 1. * O CoA -S 2. -CO 2, -CoASH R R OH HO to the flavonoids * * -CoASH O * O * O chalcone S -CoA * O * OH chalcone synthase O 31 R R 1. PAL / TAL 2. CoA ligase CoA -S -O C 2 NH 2 O CO 2 - phenylalanine, R=H tyrosine, R=OH - 1. R O * CoA -S 2. -CO 2, -CoASH R 1. NADPH 2. -CO 2, -CoASH benzal acetone O CoA -S R * O O CO 2 - -CoASH O O - * 1. * O CoA -S 2. -CO 2, -CoASH OH dihydropyrone R R CoA -S O O Flavonoids arise from condensation of a phenylpropanoid, (C6C3), with three acetyl CoA units (3 x C2). The addition of acetyl CoA probably occurs stepwise yielding intermediates with the following structures: (C6-C3) + C2; (C6-C3) + C4; (C6-C3) + C6. The first member of the flavonoid group is chalcone. * O O -CoASH * * O * 32 styryl pyrone CO 2 - 2. -CO 2, -CoASH benzal acetone O CoA -S * R O O CO 2 - -CoASH O O - * 1. O * CoA -S 2. -CO 2, -CoASH OH dihydropyrone R R CoA -S O O Flavonoids arise from condensation of phenylpropanoid + triketide. The first member formed is chalcone. * O O -CoASH * * O * styryl pyrone CO 2 - 1. * O CoA -S 2. -CO 2, -CoASH R R OH HO to the flavonoids * * -CoASH O * O * O chalcone S -CoA * O * OH chalcone synthase (polyketide cyclization) O 33 5' Oxidation Level 1 8 9 7 1 4' 6' O 2 3' 1' 2' 6 3 10 5 4 flavone O 2 OH O dihydrochalcone flavan-3-ol (catechin) O O 3 The principal structural feature of the flavonoids is C6-C3-C6. The flavonoids can be subdivided by oxidation level and oxidation position. O O chalcone O flavanone isoflavanone + O O OH OH flavan-3,4-diol (leucoanthocyanidinin) flavylium 8.0 Shikimates & phenylpropanoids (Dayrit) 34 Oxidation level O O 4 O O isoflavone O flavone aurone + O O OH O OH anthocyanidin flavanonol (dihydroflavonol) The principal structural feature of the flavonoids is C6-C3-C6. Flavonoids can be subdivided by oxidation level at the C ring. O 5 OH O flavonol 8.0 Shikimates & phenylpropanoids (Dayrit) 35 Six flavonoid subgroups 1. Flavones: Apigenin Luteolin Kaempferol Quercetin 2. Flavonols: 3. Flavanones: Naringenin 8.0 Shikimates & phenylpropanoids (Dayrit) 36 Six flavonoid subgroups 4. Isoflavones: 5. Flavan-3-ols: Epicatechin Epigallocatechin 6. Anthocyanidins: Cyanidin 8.0 Shikimates & phenylpropanoids (Dayrit) 37 OH OH B O HO OH HO 7 OH O 4' A C 5 4 O naringenin (flavanone) OH chalcone flavones flavonols 4' 4' O HO 5 4 O apigenin (flavone) 5 4 OH OH O kaempferol (flavonol) anthocyanin OH OH 4' 3' O OH 2 7 5 OH OH luteolin O HO OCH3 2 7 5 OH O OH O quercetinin 2 7 OH 4' 3' O HO 5 4 4 [Me] OH 4' 3' OH 2 7 5 [Me] 4' 3' O HO 4 O OH OH [O] [O] HO O 7 7 OH OCH3 4 OH 8.0 Shikimates & phenylpropanoids (Dayrit) chrysoeriol 4' + HO O HO 7 5 OH OH OH OH Biosynthetic studies in tissue culture of parsley: Chalcone is the entry to the flavonoids. Flavonoids can be further grouped according to the specific position of oxidation at the three rings. Note that further oxidation and methylation occur after flavonoid ring formation. O isohamnetin 38 Occurrence of flavonoids in various plants (Wink, Biochemistry of Secondary Metabolites 2010). There are some taxonomic patterns but many questions remain. OH HO OH Glc-O O O OH OH OH UDP-glucose O UDP OH O UDP-glucose UDP OH OH Glc-O HO O OH OH O-Glc O O 2x CoASH O O 2x malonyl CoA OH O Numerous flavonoids are formed by further modifications of the basic flavonoid rings. For example, glycosylation and acetylation are commonly observed modifications. O O CO2 OH - O - CO2 O O O HO OH HO bismalonyl isorhamnetin-3,7-bisglucoside 8.0 Shikimates & phenylpropanoids (Dayrit) 40 Isoflavonoids Isoflavonoids are formed from flavonoids by 1,2-migration of the B-ring from the 2- to the 3-position. Studies using labeled phenylalanine are consistent with the proposed origin of the isoflavonoids. A. Labeling studies on isoflavonoid O H H O * ^ # H O C N H 2 2 H O O * ^ # O * ^ # O O 8.0 Shikimates & phenylpropanoids (Dayrit) O C H 3 form onon 41 H O Isoflavonoids O O O C H 3 • The isoflavonoids, which are produced in legumes, are essential for plant-microbe interactions, such as chemoattractants and signal molecules for symbiotic Rhizobium bacteria. The nodD gene-encoded proteins of Rhizobia have been shown to physically bind to flavonoids and isoflavonoids, and this ligand association initiates transcription of the nod operon leading to root nodule formation. • Different isoflavonoids are also either precursors to, or are themselves the major phytoalexins in legumes, which play key roles in non-specific plant defense against bacterial and fungal pathogens. The activation of isoflavonoid synthesis during the disease resistance response is important for providing these many defense compounds. 8.0 Shikimates & phenylpropanoids (Dayrit) 42 H O Isoflavonoids O O O C H 3 • Isoflavonoid synthesis is a branch of the general phenylpropanoid pathway that exists in all higher plants. An enzyme found almost exclusively in the legumes, isoflavone synthase (IFS), converts the phenylpropanoid pathway intermediates tetrahydroxy-chalcone (naringenin) and trihydroxy-chalcone (isoliquiritigenin) into the isoflavones genistein and daidzein, respectively. • Isoflavones are produced at significant levels only in tissues where the phenylpropanoid pathway activity is elevated, such as in floral tissues, in UV-treated tissues, and in tissues where expression of a heterologous transcription factor was used to activate genes of the phenylpropanoid pathway. 8.0 Shikimates & phenylpropanoids (Dayrit) 43 O O Flavonoids are converted pterocarpan enzymatically to the isomeric isoflavonoids. dehydropterocarpan The isoflavonoids are found mostly in the coumestan Leguminosae family. isoflavanone O O O O isoflavone O O O O O O 3-arylcoumarin O O O O O rotenoid dehydrorotenoid O O 8.0 Shikimates & phenylpropanoids (Dayrit) 44 B. Biosynthesis of rotenone OPP [O] HO 1. [O] 2. ATP H3C O O isoflavone HO H2C O O O O OCH3 OCH3 [H-] OCH3 OCH3 NADPH H HO H O HO O O O OPP H H O O OCH3 OCH3 OCH3 OCH3 Rotenone is one of the best known of the isoflavonoids. Rotenone is found in mangrove plants and is a fish poison, but is considered safe for mammals. another closely related compound: milletone H O O s s H O O O O H O OCH3 rotenone OCH3 H O 8.0 Shikimates & phenylpropanoids (Dayrit)O O 45 Anthocyanins are the sugar- derivatives of the anthocyanidins (the aglycone). Anthocyanins are water-soluble pigments in flowers, leaves and fruits. They also impart many of the colors of fruit juices and wines. A. List of the best-known anthocyanins and anthocyanidins; A. Substituents and Color R: 3 5 3' 5' Name Color R 5 ' H H O H H H + H O O H H R 3 ' Glc Glc - O R 3 O R 5 8.0 Shikimates & phenylpropanoids (Dayrit) 46 B. Anthocyanin color is produced in compounds which can achieve a planar conformation which gives maximum delocalization. B. Structure and Color R 5' H O + O R 5 ' + O H O H R 3 ' H O O H H O R 3 - O R 3 O R 5 R 3 ' O R 5 extended reso non-planar conformation is colorle this conformat this form predominates if R3 is la 8.0 Shikimates & phenylpropanoids (Dayrit) 47 Anthocyanins and flower color (“The Big Bloom: Birth of Flowering Plants, National Geographic, July 2002) • The yellow day lily has a near uniform hue in the visible spectral range. However, in the UV range which is invisible to most animals but visible to bees, the lily has a two-toned pattern. • The UV pattern of many flowers is due to anthocyanins. 8.0 Shikimates & phenylpropanoids (Dayrit) 48 Leaf color is due to several Anthocyanins and autumn leaves compounds: Why do leaves turn yellow, orange, • Chlorophyll, which is green, gives leaves its predominant color. It is red, purple and magenta during responsible for photosynthesis. As autumn? the temperature drops and the days shorten, it is destroyed enzymatically, revealing the presence of the other leaf pigments. • Carotenoids are responsible for the yellow, orange and gold hues. Different species of trees contain various types of carotenoids. • Anthocyanins give rise to the reds, purple and magenta hues. In contrast to the carotenoids, which are always present, anthocyanins are actively formed in the leaves during autumn. 8.0 Shikimates & phenylpropanoids (Dayrit) 49 Anthocyanins and autumn leaves Why do leaves turn yellow, orange, What is the special role of anthocyanins? red, purple and magenta during autumn? It is hypothesized that anthocyanins protect the leaf from free radicals which are produced by the destruction of the photosynthetic system. They provide some environmental protection from UV radiation and nutrient deficiency. The net effect is that anthocyanins slow down leaf death and abscission. 8.0 Shikimates & phenylpropanoids (Dayrit) 50 A. OH + O HO Catechins are OH HO O OH OH OH catechin anthocyanidin OH B. HO O OH HO O OH OH OH OH OH OH (-)-epicatechin (-)-catechin OH HO O OH HO O OH OH OH OH OH OH (+)-epicatechin (+)-catechin 8.0 Shikimates & phenylpropanoids (Dayrit) flavan-3-ols which are believed to be biosynthetically related to the anthocyanidins and may be produced during the extraction of anthocyanidins. The best known catechins are ()catechin and ()epicatechin which are major constituents of tea. 51 Condensed tannins are polymeric compounds made up of catechin monomers. OH R HO O OH OH OH OH OH HO O OH n x HO O OH OH OH OH OH OH (-)-epicatechin HO O OH OH OH 8.0 Shikimates & phenylpropanoids (Dayrit) R 52 8.0 Shikimates & phenylpropanoids (Dayrit) 53 MK biosynthetic pathways. (A) Classical pathway. Chorismate is converted into isochorismate by MenF (isochorismate synthase) and then into 2-succinyl-6-hydroxy-2,4cyclohexadiene-1-carboxylate by MenD. This compound is dehydrated by MenC to osuccinylbenzoate, followed by the attachment of coenzyme A to yield o-succinylbenzoylCoA by MenE. o-Succinylbenzoyl-CoA is then converted into 1,4-dihydroxy-2-naphthoate by MenB. In the last two steps of the pathway, MK is converted by MenA and MenG, which catalyze prenylation and methylation, respectively. Red and black bold lines show carbons originated from erythrose-4-phosphate and phosphoenolpyruvate, respectively. MK biosynthetic pathways. (B) Alternative pathway. Green and blue bold lines indicate two carbon units derived from C-5 and C-6 of glucose via different metabolic pathways. Based on the annotation of the open reading frames of S. coelicolor A3(2), we presumed that SCO4491 (prenylation), SCO4556 (methylation), SCO4490 (decarboxylation), and SCO4492 (decarboxylation) would be involved in the late step of the MK biosynthetic pathway. Metabolons of Phenylpropanoid Genes • A metabolon is a temporary functional complex of several sequential enzymes held together by noncovalent interactions in a given metabolic pathway. • Organization of the branched pathways of phenylpropanoid metabolism within separate individual metabolons. • Enzymes that participate in multiple branches are shown in red and blue, whereas enzymes that are thought to be unique to specific pathways are shown in other colors. • Enzymes that are known to be present as multiple isoforms are marked with an asterix. Jorgensen et al., Curr Opinion in Plant Biol 2005, 8:280–291 4CL, 4-(hydroxy)cinnamoyl CoA ligase C3H, p-coumarate 3-hydroxylase C4H, cinnamate 4-hydroxylase CAD, cinnamyl alcohol dehydrogenase CCoAOMT, caffeoyl CoA O-methyltransferase 8.0 CCR, cinnamoyl CoA reductase COMT, caffeic acid/5-hydroxyferulic acid O-methyltransferase CQT, hydroxycinnamoyl CoA:quinate hydroxycinnamoyltransferase CST, hydroxycinnamoyl CoA:shikimate hydroxycinnamoyltransferase F5H, ferulate 5-hydroxylase PAL, phenylalanine ammonia-lyase Shikimates phenylpropanoids (Dayrit) pCCoA3H& , p-coumaryl CoA 3-hydroxylase SAD, sinapyl alcohol dehydrogenase. 56 Metabolons and the Biosynthesis of Natural Products • Metabolon formation and metabolic channeling in plant secondary metabolism enable plants to effectively synthesize specific natural products and to avoid metabolic interference. • Channeling can involve different cell types, take advantage of compartmentalization within the same cell or proceed directly within a metabolon. New experimental approaches document the importance of channeling in the synthesis of isoprenoids, alkaloids, phenylpropanoids, flavonoids and cyanogenic glucosides. • Metabolon formation and metabolic channeling in natural-product synthesis facilitate attempts to genetically engineer new pathways into plants to improve their content of valuable natural products. They also offer the opportunity to introduce new traits by genetic engineering to produce plant cultivars that adhere to the principle of substantial equivalence. (Jorgensen et al., Curr Opinion in Plant Biol 2005, 8:280–291) 8.0 Shikimates & phenylpropanoids (Dayrit) 57 Metabolic channeling of biosynthesis: flavonoids isoflavonoids phytoalexins Biosynthesis of O-methylated isoflavonoids in licorice, and conversion to phytoalexins. (Jorgensen et al., Curr Opinion in Plant Biol 2005, 8:280–291) IFS: 2-hydroxyisoflavanone synthase HI4’OMT: 2,7,4’-trihydroxyisoflavone 4’-Omethyltransferase D7OMT: daidzein 7-O-methyltransferase HID: 2-hydroxyisoflavanone dehydratase 8.0 Shikimates & phenylpropanoids (Dayrit) 58 Summary Many of the aromatic compounds found in higher plants belong to the shikimates. The shikimates have for a diverse range of properties and functions in plants, ranging from structural (lignins), flower and leaf coloration (anthocyanins and flavonoids), scent (some aryl propanes), and plant defense (flavonoids and isoflavonoids). The shikimates can be grouped by C6-Cn classification: C6: phenols and quinones C6-C1: benzoic acids; benzaldehydes; benzyl alcohols C6-C2: phenylethyl amines; styrenes; acetophenones C6-C3: phenyl propanoids: cinnamaldehydes; aryl propanes; lignins; lignans C6-C3-C6: flavonoids; isoflavonoids; catechins 8.0 Shikimates & phenylpropanoids (Dayrit) 59 glucose shikimic acid Summary of the shikimates C6-C1: p-hydroxybenzoic acid chorismic acid C6-C1: o-hydroxybenzoic acid (salicylic acid) C6-C1: p-aminobenzoic acid prephenic acid phenylalanine, tyrosine C6-C2: phenylethylamines C6-C3: cinnamic acids C6-C3: cinnamaldehydes C6-C3: cinnamic alcohols C6-C1: o-aminobenzoic acid (anthranilic acid) indole, tryptophan +3 x (C2) C6: phenols, quinones C6-C3-C6: flavonoids isoflavonoids catechins C6-C3: coumarins C6-C3: arylpropanes C6-C1: benzoic acids, benzaldehydes, benzyl alcohols acetophenones C6-C2: styrenes, condensed tannins