Chorismate Supplement

Arogenate

Figure 7.9 Chorismate mutase (CM) transforms chorismate to prephenate, which is aminated to arogenate. PA, prephenate aminotransferase.

Figure 7.10 Formation of aromatic amino acids tyrosine (by the action of the enzyme arogenate dehydrogenase, ArDH) and phenylalanine (by the action of the enzyme arogenate dehydratase, ArD).

Figure 7.11 The building block molecule trans-cinnamic acid is biosynthesized by deamination of phenylalanine, a reaction catalysed by phenylalanine ammonia lyase (PAL).

Figure 7.11 The building block molecule trans-cinnamic acid is biosynthesized by deamination of phenylalanine, a reaction catalysed by phenylalanine ammonia lyase (PAL).

verted to benzoic acid, which in turn is hydroxylated to salicylic acid (SA). The latter is then conjugated to glucose to form the glycoside that accumulates after tissue infection (Figure 7.12) (Crozier et al., 2000). Recent findings have suggested that the rate-limiting step in SA biosynthesis is the conversion of cinnamate to benzoate, and that this involves a ^-oxidation pathway. Endogenous benzaldehyde can also be converted to benzoate and SA, but it is not the main endogenous precursor.

Figure 7.12 Biosynthesis of salicylic acid and related compounds. See text for details. (From Crozier et al, 2000.)

The hydroxylation of cinnamate to form SA is a crucial step in the formation of a group of LMWP lactone derivatives, the coumarins. These compounds are widely distributed in plants, particularly in the Umbelliferae and Rutaceae families (Torssell, 1997), and they make up the sweet-smelling volatile material that is released from newly mown hay. The biosynthesis of these bioactive compounds requires some hydroxylation steps to yield 2,4-dihydroxycinnamic acid. Glycolysation of the latter compound triggers lactonization, giving rise to the coumarin umbelliferone (Figure 7.13).

Cinnamate

2,4-Dihydroxycinnamic O-Coumaroyl glucoside Umbel life rone acid

Figure 7.13 Biosynthesis of the coumarin umbelliferone. Glu, glucose.

2,4-Dihydroxycinnamic O-Coumaroyl glucoside Umbel life rone acid

Figure 7.13 Biosynthesis of the coumarin umbelliferone. Glu, glucose.

Celery contains bergapten, a calcium antagonist compound. This compound belongs to the group of LMWP-defined psoralens. These are linear furanocoumarins which are widely distributed in plants and, when used internally and externally, they promote skin pigmentation and sun-tanning. The psoralens, because of their extended chro-mophore, absorb in the near-UV and allow this radiation to stimulate formation of melanin (Dewick, 1998). The biosynthesis of bergapten starts from the prenylation of umbelliferone to yield 7-demethylsuberosin. The latter compound goes towards cyclization into the furane ring of marmesin by the action of a cytochrome P450-dependent mono-oxygenase. Marmesin is converted to psoralen by cleavage of the hydroxyisopropyl fragment. Hydroxylation of psoralen yields bergaptol, the precursor of the methylated derivative bergapten (Figure 7.14). Psoralen can act as a precursor for other substituted furanocoumarins such as xanthoxin and isopimpinellin. Some fura-nocoumarins are troublesome for humans since they lead to photosensitization towards UV light, resulting in sunburn or serious blistering (Dewick, 1998).

Complex phenolic compounds of relatively high molecular weight

Cinnamate derivatives and their CoA (co-enzyme A) esters may function as building blocks for the construction of higher-complexity phenols. In general, three units of malonyl CoA are added, giving a polyketide which is transformed by the action of two enzmes, chalcone synthase (CHS) and chalcone isomerase (CHI), to chalcone and the flavanone naringenin, respectively (Figure 7.15). Compounds consisting of two substituted benzene rings joined by a heterocyclic ring containing oxygen are called flavonoids. Flavonoids are a diverse group of plant natural products synthesized from phenylpropanoids and acetate-derived precursors, and play important roles in growth and development, and in defence against microorganisms and pests. These compounds often possess antioxidant activity, and the potential health benefits of fruit, vegetables, green tea and red wine might partly be because of this property of flavonoids and other phytochemicals (Rice-Evans et al., 1997; Hollman and Katan, 1998; Khalsa, 1999). In addition, the isoflavonoids, which are limited primarily to the Leguminosae, exhibit oestrogenic and anticancer activity (Adlercreutz and Mazur, 1997; Dixon, 1999b), and, in common with the flavonoids, are receiving considerable attention as health-promoting 'vitalins' or nutraceuticals. Some isoflavonoids may exert oestrogenic activity. Different phyto-oestrogens have different mechanisms of action based on oestrogen-receptor subtypes, endogenous oestrogen concentrations, and cellular genetic make-up. Effects of phyto-oestrogens on the reproductive system have been known for decades, following several in vitro and animal studies; however, their roles

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