\5 IT4 OR2
OH O Figure 4 Basic structure of flavonoids.
tremely well the complex issues that need to be considered in focusing on specific metabolites in achieving the aim of improving food quality.
All flavonoids are derived from a chalcone precursor, the product of condensation of 4-coumaryl coenzyme A (CoA) (derived from phenylalanine produced in the shikimate pathway), and three molecules of malonyl-CoA (derived from acetyl CoA through the action of a carboxylase) involving the enzyme chalcone synthase (CHS) (Fig. 5). The CHS enzymes form a family of closely related multigene enzymes that produce a variety of secondary metabolites (29). This is achieved through variations in the number of rounds of condensation and differences in the folding pattern of the intermediate polyketide. Stilbene synthases from a variety of sources have exactly the same substrate specificity as CHS but the intermediate polyketide is folded differently, resulting in decarboxylation to form the stilbene (30). The stilbene resveratrol, which is found in wine, has been shown to possess anticancer activities in various cancer models (31). Resveratrol can be engineered successfully by genetic engineering (32).
The stereospecific cyclization of the chalcone, catalyzed by chalcone isomerase (CHI), provides a 25-flavanone with the typical flavonoid skeleton. Chalcone reductase (CHR) isomerizes both 6'-hydroxy and 6'-deoxy-chalcone to 5-hydroxy and 5-deoxyflavanone, respectively. CHR and CHS are coinduced in elicited or infected cells, whereas CHS alone is expressed in the arial portions of uninfected plants. Consequently, it is likely to be of greater importance in terms of its ability to influence food quality parameters. 2',4',4'-Trihydroxychalcone (the product of CHS and CHR) is a branch point for the synthesis in legumes of different flavonoids. Thus, chalcone isomerase (CHI) converts the chalcone to 4,7-dihydroxyflavanone and then to the isoflavones. Another enzyme, chalcone 0-methyltransferase (CHOMT), forms a chalcone that is a potent inducer of Rhizobium modulation genes. Over 50 regiospecific O-methy [transferase sequences are known, which adds to the diversity of the flavonoids, isoflavanoids, and hydroxycinnamates found in plants (33). The possibility exists of protein-
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