Pathways anthocyanin

Stafford, H.A., Anthocyanins and betalains evolution of the mutually exclusive pathways. Plant ScL, 101, 91, 1994. 

Catechol-O-methyltransferase (COMT EC 2.1.1.6) is located in many tissues and catalyzes the methylation of polyphenols. The methylation is a well-established pathway in the metabolism of flavonoids such as those that undergo 3, 4 -dihydrox-ylation of ring B excreted as 3 -0-methyl ether metabohtes in rat bile. " Recently, the apparent methylation of both cyanidin-3-glucoside and cyanidin-3-sambubioside (cyanidin is an anthocyanin with a 3, 4 -dihydroxylation of ring B) to peonidin-3-glucoside and peonidin-3-sambubioside was reported in humans. In rats, this transformation occurred mainly in the liver and was catalyzed by COMT."°... 

In the second pathway, the (methylated or not) anthocyanin glucoside may be absorbed intact and serve as a snbstrate for the UDP-glncose dehydrogenase enzyme (GDH EC 1.1.1.22) that converts the glncose form into the corresponding glucu-ronide form, possibly in both hnman hver and small intestine. 

FIGURE 4.3.3 Interconversion pathways of various forms of anthocyanins in acidic aqueous medium. 

Schwarz, M., Wabnitz, T.C., and Winterhalter, R, Pathway leading to the formation of anthocyanin-vinylphenol adducts and related pigments in red wines, J. Agric. Food Chem., 51, 3682, 2003. 

Flavanones In some cases, flavanones produced by CHI will accumulate to sizeable amounts instead of being diverted away to form flavonols, anthocyanins, and flavanols (see Fig. 5.4). These flavanone products, hesperetin and naringenin being the most common, are frequently encountered in citrus fruits and juices (USDA Flavonoids Database Release 2.1,2007). In most of these cases, essentially no flavonols or anthocyanins are encountered the flavonoid pathway is essentially blocked at the F3H step. 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

RAUSHER, M.D., MILLER, R.E., TIFFIN, P., Patterns of evolutionary rate variation among genes of the anthocyanin biosynthetic pathway, Mol. Biol. Evol., 1999,16, 266-274. 

The riocus encodes the enzyme flavonoid 3 -hydroxylase (F3 H) [17, 18], and is an important controller of flux in the anthocyanin pathway in soybean seed coats (Fig. 4.1). F3 H diverts metabolic flux away from biosynthesis of orange (pelargoni-din) and blue (delphinidin) anthocyanins toward the red cyanidin-3-(9-glucoside, which is the main anthocyanin in the seed coats of black soybean [7, 8]. T increases the accumulation of delphidin-3-O-glucoside in black seed coats, even though it is not required for its biosynthesis [19]. Possible mechanisms for this include positive feedback, or the stabilization of the putative anthocyanin biosynthetic metabolon [20] by F3 Fl-derived membrane anchoring (Fig. 4.1). 

There are many branches to the flavonoid biosynthetic pathways, with the best characterized being those leading to the colored anthocyanins and proanthocyanidins (PAs) and the generally colorless flavones, flavonols, and isoflavonoids. Genes or cDNAs have now been identified for all the core steps leading to anthocyanin, flavone, and flavonol formation, as well as many steps of the isoflavonoid branch, allowing extensive analysis of the encoded enzymes (Table 3.1). In addition, several DNA sequences are available for the modification enzymes that produce the variety of structures known within each class of compound. 

The key enzymes involved in the formation of the hydroxycinnamic acids (HCAs) from phenylalanine and malonyl-CoA are now discussed in detail, while later sections address the branches of the flavonoid pathway leading to anthocyanins, aurones, flavones, flavonols, PAs, and isotlavonoids. This is followed by brief reviews of the regulation of flavonoid biosynthesis and the use of flavonoid genes in plant biotechnology. To assist the reader. Figure 3.1 presents the carbon numbering for the various flavonoid types discussed. 

FIGURE 3.2 General phenylpropanoid and flavonoid bios5mthetic pathways. The B-ring hydroxylation steps are not shown. For formation of anthocyanins from leucoanthocyanidins two routes are represented a simplified scheme via the anthocyanidin (pelargonidin) and the likely in vivo route via the pseudobase. Enzyme abbreviations are defined in the text and in Table 3.1. 

Regulation of anthocyanin production involves transcriptional activators of the R2R3 MYB and the basic helix-loop-helix (bHLH) (or MYC) types (Table 3.2). This was first revealed by studies of the monocot Z. mays. It was found that the anthocyanin pathway is turned on in this species through the combined action of one member of the COLORED ALEURONEl (C1)/PURPLE PLANT (PL) MYB family and one member of the REDl (R)/BOOSTERl (B) bHLH family. The members of the MYB and bHLH families are functionally redundant, and their specific expression patterns enable spatial and temporal control of anthocyanin biosynthesis. 

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