Flavonoids biosynthetic routes

The F6H showed low activity against flavones and little action on the flavonol kaemp-ferol. Nevertheless, it is possible that a similar biosynthetic route through hydroxylation of precursors, perhaps by the same F6H, is involved in the production of other 6-hydroxyfla-vonoids. However, an alternative activity of the 20GD type has been characterized at the biochemical level from C. americanum that catalyzes the 6-hydroxylation of partially methylated flavonoids. ... 

Aromatic Amino Acid Biosynthesis. The shikimate pathway is the biosynthetic route to the aromatic amino acids tryptophan, tyrosine and phenylalanine as well as a large number of secondary metabolites such as flavonoids, anthocyanins, auxins and alkaloids. One enzyme in this pathway is 5-enolpyruvyl shikimate-3-phosphate synthase (EPSP synthase) (Figure 2.9). 

Stilbenoids are derived from cinnamic acid and three acetate units from mal-onyl coenzym A. The first part of the biosynthesis is in common to flavonoids. The two biosynthetic routes are diverging at the point of cyclization of a styryl-3,5,7-triketoheptanoic acid. A C-acylation produces a chalcone and subsequent modifications lead to the flavonoids. An aldol condensation of the same intermediate polyketide produces a stilbene-2-carboxylic acid that is unstable and constitutes a range of structures known as stilbenoids. Figure 9C.5 shows an overview of the biosynthetic pathway (Gorham 1995). 

Scheme 10. Possible biosynthetic routes leading from the shikimic acid pathway to betalains and the coexisting flavonoids (excluding anthocyanins) in betalain-bearing members of the Caryophyl-lales.

Chalcones are the major intermediates of flavonoid biosynthetic pathways they are produced by the condensation of three molecules of malonyl-CoA and a single molecule of 4-coumaryl-CoA. The major precursor malonyl-CoA is derived from citrate, an intermediate product of the TCA cycle. Acetyl-CoA is produced in mitochondria, plastids, peroxisomes, and cytosol via various routes. The cytosolic acetyl-CoA, produced by the multiple subunit enzyme ATP-citrate lyase, is used by acetyl-CoA carboxylase (ACC) to form malonyl-CoA for flavonoid biosynthesis. Another precursor, 4-coumaryl-CoA, is available via the polypropanoid pathway, in which phenylalanine generated via the shikimate and aerogenate pathway is... 

Fig. 51.1 Chemical structures of stilbenes (a) flavonoids biosynthetic pathway (b). The expression of a stilhene synthase gene in transgenic plants competes with substrates used in the first step of the complex route of flavonoids...

Fig. 2.5 Biosynthetic routes to flavonoids and 5-deoxyflavonoids in the Leguminosae. CHS, chalcone synthase CHR, chalcone reductase CHI, chalcone isomerase IFS, isoflavone synthase. The acetate-derived A-ring carbon atoms of chalcones and 6 -deoxychalcones are indicated in bold.

The biosynthesis of flavonoids, stilbenes, hydroxycinnamates, and phenolic acids involves a complex network of routes based principally on the shikimate, phenyl-propanoid, and flavonoid pathways (Figs. 1.35 and 1.36). These biosynthetic pathways constitute a complex biological regulatory network that has evolved in vascular plants during their successful transition on land and that ultimately is essential for their growth, development, and survival [Costa et al., 2003]. 

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. 

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