Anthocyanidin biosynthesis

Flavonol synthase (FLS E.C.l.14.11.23) catalyzes the committed step in the production of fiavonols by introduction of a double bond between C2 and C3 of the corresponding dihydroflavonols. Like E3H, ELS has been described as a 2-oxoglutatarate-dependent dioxygenase based on its cofactor requirements for 2-oxoglutarate, Fe, and ascorbate. FLS was initially identified in enzyme preparations from illuminated parsley cell suspension cultures [67]. Subsequently, FLS was characterized from the flower buds of Matthiola incana and carnation (Dianthus caryophyllus L.), and it was suggested that there was regulation between flavonol and anthocyanidin biosynthesis [83, 84]. 

Dihydroflavonol 4-reductase (DFR EC 1.1.1.219) is a member of the short-chain dehydrogenase/reductase family and catalyzes the stereospecific conversion of (+)-(2R,3R)-dihydroflavonols to the corresponding (2R,3S,4S) flavan-3,4-cw-diols (leucoanthocyanidins), with NADPH as a required cofactor. The enzyme activity was first identified in cell suspension cultures of Douglas fir (Pseudotsuga menziesii) and was shown to be related to the accumulation of flavan-3-ols and proanthocyanidins [96]. Leucoanthocyanidins and DFR were later shown to be required for anthocyanidin formation by complementation of Matthiola incana mutants blocked between dihydroflavonol and anthocyanidin biosynthesis [97, 98], DFR has been purified to apparent homogeneity and biochemically analyzed from flower buds of Dahlia variabilis [99]. DFR was shown to accept different substrates depending on the plant species from which it was isolated (reviewed in 100). 

Peel, G Pang, Y Modolo, LV, Dixon, RA. (2009) The LAPl MYB transcription factor orchestrates anthocyanidin biosynthesis and glycosylation in Medicago Plant J 59 136-149. 

Hhllkr. W.. L. Bri isch. G. Forkm.xnn. and H. Grishbach, Leu-coanthocyanidins as intermediates in anthocyanidin biosynthesis in flowers of Matthiola incana, Planta, 163, 191-196 (1985a). 

According to these, and earlier investigations, a specific role for dihydroflavones in flavonoid biosynthesis has been defined. Thus the biosynthetic pathway to quercetin (14) and cyanidin (19) in buckwheat has been formulat as shown in Figure 6.2. Nevertheless, the possibility that the oxygenation pattern of ring C may be determined by other kinds of control mechanism still exists, and it has been tentatively proposed " that these may represent some of the differences between the routes for anthocyanidin biosynthesis and that for the other fiavonoids and isoflavonoids. 

Figure 5.4. Abbreviated scheme for biosynthesis of major flavonoid subclasses, showing the primary enzymes and substrates leading to different subclasses. Bold-faced, uppercase abbreviations refer to enzyme names, whereas substrate names are presented in lowercase letters. PAL, phenylalanine ammonia lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CHI, chalcone isomerase CHR, chalcone reductase IPS, isoflavone synthase F3H, flavonone 3-hydroxylase F3 H, flavonoid 3 -hydroxylase F3 5 H, flavonoid 3 5 -hydroxylase FNSI/II, flavone synthase DFR, dihydroflavonol 4-reductase FLS, flavonol synthase ANS, anthocyanidin synthase LAR, leucoanthocyanidin reductase ANR, anthocyanidin reductase UFGT, UDP-glucose flavonoid 3-O-glucosyltransferase. R3 = H or OH. R5 = H or OH. Glc = glucose. Please refer to text for more information.

Turnbull JJ, Nakajima J, Welford RW, Yamazaki M, Saito K and Schofield CJ. 2004. Mechanistic studies on three 2-oxoglutarate-dependent oxygenases of flavonoid biosynthesis anthocyanidin synthase, flavonol synthase, and flavanone 33-hydroxylase. J Biol Chem 279 1206-1216. 

Nakajima, J.-I. et al.. Reaction mechanism from leucoanthocyanidin to anthocyanidin 3-glucoside, a key reaction for coloring in anthocyanin biosynthesis. J. Biol. Chem., Tib, 25797, 2001. 

Figure 3-8. Biosynthesis of anthocyanins and condensed tannins. The enzymes involved in this pathway are (a) anthocyanidin synthase (E.C. 1.14.11.19), (b) anthocyanin 3-glycosyl transferase, and (c) BANYULS.

Paolocci F, Robbins MP, Madeo L, Arcioni S, Martens S, Damiani F. 2007. Ectopic expression of a basic helix-loop-helix gene transactivates parallel pathways of proanthocyanidin biosynthesis. Structure, expression, analysis, and genetic control of leucoanthocyanidin 4-reductase and anthocyanidin reductase genes in Lotus corniculatus. Plant Physiol 143 504-516. 

Xie DY, Sharma SB, Paiva NL, Ferreira D, Dixon RA. 2003. Role of anthocyanidin reductase, encoded by BANYULS in plant flavonoid biosynthesis. Science 299 396-399. 

Pfeiffer J, Kiihnel C, Brandt J, Duy D, Punyasiri PA, Forkmann G, Fischer TC. 2006. Biosynthesis of flavan 3-ols by leucoanthocyanidin 4-reductases and anthocyanidin reductases in leaves of grape (Vitis vinifera L.), apple (Malus x domestica Borkh.) and other crops. Plant Physiol Biochem 44 323-334. 

The branch pathway for anthocyanin biosynthesis starts with the enzymatic reduction of dihydrofiavonols to their corresponding flavan 3,4-diols (leucoanthocyanidins) by substrate-specific dihydroflavonol 4-reductases (DFR). Flavan 3,4-diols are the immediate precursors for the synthesis of catechins and proanthocyanidins. Catechins are formed by enzymatic reduction of the flavan 3,4-diols in the presence of NADPH to leucoanthocyanidins, which are subsequently converted to anthocyanidins by the 2-oxoglutarate-dependant dioxygenase, anthocyanidin synthase. Further glycosylation, methylation, and/or acylation of the latter lead to the formation of the more stable, colored anthocyanins (Scheme 1.1). The details of the individual steps involved in flavonoid and isoflavonoid biosynthesis, including the biochemistry and molecular biology of the enzymes involved, have recently appeared in two excellent reviews.7,8... 

Scheme 1.1 Pathway for the biosynthesis of the major classes of flavonoids. 1, Chalcone synthase 2, chalcone isomerase 3, flavone synthase 4, flavanone 3-hydroxylase 5, flavonol synthase 6, dihydroflavonol reductase 7, anthocyanidin synthase 8, anthocyanidin glucosyltransferase 9, chalcone-ketide reductase 10, chalcone isomerase 11, isoflavone synthase 12, isoflavone 2 -hydroxylase 13, isoflavone reductase 14, pterocarpan synthase 15, pterocarpan 6a-hydroxylase 16, prenyltransferase 17, prenylcyclase.

The biosynthesis of the flavonoids and anthocyanidins involves a union of a C -C unit with a C mit. It is of interest that the Cr-Ca intermediate is derived from the S-deoxy-n-arobino-heptulosonic acid 7-phos-phate pathway, while the C6 intermediate is derived from the acetate pathwayIn the formation of quercetin (XXVI) by the buckwheat plant,... 

Biosynthesis of 3-Hydroxy-anthocyanidins (38), ep/ -Flavan-3-ols (52) and Condensed Tannins (Proanthocyanidins, 44) from Leucoanthocyanidins (Flavan-3,4-diols, 37)... 

The anthocyanidin reductase enzyme recently described in Arabidopsis and Medicago was shown to be present in tea with very high activity and could produce epicatechin as well as epigallocatechin from the respective anthocyanidins, thus explaining very high contents of llavan-3-ols. Especially, two enzymes, dihydroflavonol 4-reductase and leucoanthocyanidin 4-reductase selectively catalyze key steps in their biosynthesis of catechins (43,62) and gallocatechins (63,64), respectively (Fig. 14) [31]. 

The proanthocyanidins share a biosynthesis sequence with the anthocyanins, a class of flavanoids well understood in biochemical and genetic terms. Three enzymes, leucoanthocyanidin reductase (LAR), anthocyanidin synthase (ANS), and anthocyanidin reductase (ANR), function at branches between the anthocyanin and proanthocyanidin biosynthesis pathways (Scheme 1). LAR is a member of the reductase—eptmerase—dehydrogenase enzyme... 

Scheme 1 Schematic representation ofthe biosynthesis pathways of anthocyanins and proanthocyanidins. CHS, chalcone synthase FS, flavanone synthase F3H, flavanone-3-hydroxylase FLS, flavone synthase DFR, dihydroflavonol reductase LAR, leucoanthocyanidin reductase ANS, anthocyanidin synthase ANR, anthocyanidin reductase.

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