Flavonoids,derived from aromatic amino

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Some of the pathways of animal and bacterial metabolism of aromatic amino acids also are used in plants. However, quantitatively more important are the reactions of the phenylpropanoid pathway,173-1743 which is initiated by phenylalanine ammonia-lyase (Eq. 14-45).175 As is shown at the top of Fig. 25-8, the initial product from phenylalanine is trails-cinnam-ate. After hydroxylation to 4-hydroxycinnamate (p-coumarate) and conversion to a coenzyme A ester,1753 the resulting p-coumaryl-CoA is converted into mono-, di-, and trihydroxy derivatives including anthocyanins (Box 21-E) and other flavonoid compounds.176 The dihydroxy and trihydroxy methylated products are the starting materials for formation of lignins and for a large series of other plant products, many of which impart characteristic fragrances. Some of these are illustrated in Fig. 25-8. 

Phenylpropanoids and flavonoids found in plants are predominantly derived from the aromatic amino acid L-phenylalanine originated in the shikimic acid... 

A vast number of flavonoid compounds occur ii nature and these may well be derived from the aromatic amino acids. Birch and co-w orkers (66) have demonstrated the follow ing biosynthetic pathw ay in the green alga Chlam-ydomonas eugametos ... 

Flavonoids are a large class of plant natural products of low molecular weight. Over 3,000 different flavonoids have been chemically characterised and novel ones are still being reported. Flavonoids are aromatic molecules synthesised from the amino acid phenylalanine and an acetate-derived precursor as malonyl-coenzyme A (Fig. 11.1) (Winkel-Shirley 2001). This reaction is carried out by the enzyme chalcone synthase (CHS) to produce chalcone. The chalcone can subsequently be isomerised by the enzyme chalcone flavone isomerase (CHI) to yield a flavanone. From these intermediates the pathway diverges into several side branches yielding different subclasses of flavonoids, as summarised in Fig. 

A wide range of aromatic products in the plant kingdom originate from intermediates of the shikimic acid pathway. These include amino acids and ubiquinone among important primary metabolites and also many other compounds (in contrast fo fungi which possess many acetate-derived products), such as lignins, alkaloids, and phenolic acids. Flavonoids (and stilbenes) also arise in part by this route but additionally utilize acetate in the course of their biosynthesis, which will therefore be described in this chapter. 

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