CHI-chalcone isomerase. PKR-Polyketide reductase. CHS-chalcone synthase. CHR-chalcone isomerase. FNR-Flavanone 4-reductase. FHT-Flavanone 3-hydroxylase. FLS-Flavanol synthase.
Figure 5 Biosynthesis of chalcones.
protein reactions between all of these enzymes, the outcome dependent on many factors controlling their expression.
In addition, there are enzymes that are crucial for determining the broad class of flavonoids that are produced (chalcone isomerase for flava-none production, flavanone reductase for flavone production, and isoflavone synthase for the isoflavones). The highly species-specific nature of many of the biosynthetic steps makes it very difficult to predict the outcome of over-expressing any one enzyme (34). Instability has been observed in the expression of a transgene that can be exacerbated by environmental factors. Crosses between wild-type and transgenic plants in Petunia was exacerbated by environmental factors that caused methylation-mediated expression of the transgene. This result is in contrast to the manipulation of Arabidopsis and tobacco with the regulatory gene controlling pigmentation in maize. The gene controls the expression of several steps such as CHS and CHI. In all cases, there was a substantial increase in anthocyanin production (34,35). Nonetheless, the situation is not straightforward and severe oxidative stress symptoms have been observed with plants overexpressing a transcription factor gene.
Whereas very extensive information is available on the biosynthesis of flavonoids in terms of their induction, regulation, and tissue-specific expression in a wide range of species, very little is known about the post-translational regulation of flavonoid biosynthesis. The simple flavonoids with a single hydroxyl in the B-ring can be extensively modified by hydroxyla-tion, methylation, glycosylation, acylation, and a number of other modifications.
In terms of food quality characteristics, the enzymes that occur at the end of the biosynthetic pathway are likely to be of the greatest interest. The hydroxylases that are responsible for the conversion of kaempferol into quer-cetin and subsequently myricetin or pelargonidin into cyanidin and delphin-idin result in an increase in color, antioxidant activity, and potential nutritional value. Similarly the acyl- and methyltransferases increase pigment stability and may increase nutritional value. The glycosyltransferases have an effect on taste (bitterness). Similarly, astringency is determined by the concentration and type of tannins, hydrolyzable tannins, and flavan-3-ols.
Competing with these reactions are the enzymes that result in a loss of quality of the product, many of which involve some reaction with the phenolics present in the plant. Enzymatic browning is one of the main oxidative reactions leading to an undesirable loss of color, flavor, and nutrients in some products (e.g., browning of apples or cut salads) and an increase of quality in others (e.g., prunes, dates, and figs). The principal classes of enzymes involved are the peroxidases (PODs) and polyphenoloxidases (PPOs). PPO activity is found in all higher plant tissues and organs. A requirement of the processing of all frozen crops is that they be inactivated by blanching, before freezing, to prevent off-flavors developing as a result of their action and other quality deficits. No satisfactory genetic manipulation approach to reducing their effects has been devised, and this may prove difficult given the important role they play in the overall physiology of the plant (36).
Different plant families produce different chemical classes of PPs. Isoflavanoids are found predominantly in the Leguminosae, whereas the So-lanaceae produce sesquiterpenes. At present it is not possible to transfer the complete sesquiterpene phytoalexin of Solanaceae, such as tobacco, to a legume (or vice versa) as the biochemistry of the complex enzymatic pathways is not fully understood.
Isoflavones are less widely distributed and are found in highest concentrations in the soybean and food products manufactured from it. Genistein and daidzein are estrogenic and show a wide range of physiological responses as a consequence, including delaying the menstrual cycle in women and a series of beneficial effects on cardiovascular function and bone loss (37). Extensive screening of soy varieties is taking place to identify high-and low-isoflavone genotypes. Interestingly, the selection for agronomically beneficial allelles also appears to select for the varieties with higher isofla-vone levels, providing an example in which the goals of farmers and consumers may be congruent.
The branch pathway for the formation of the isoflavones is almost completely characterized and has several common links with that for antho-cyanins (Fig. 6). However, the first reaction specific for isoflavone synthesis is unique. The 2-hydroxyisoflavanone synthase (2-HIS), which causes a 2-hydroxylation, couples to an aryl migration of the B-ring of a flavanone. The cloning of 2-HIS offers the possibility of intoducing isoflavone synthesis into other food crops (38).
The IP pathway is responsible for the biosynthesis of a vast range of compounds that play a crucial role in maintaining membrane fluidity (sterols), electron transport (ubiquinone), glycosylation of proteins (dolichol), and the regulation of cellular development (gibberellins etc.). Terpenoids synthesized via the IP pathway are also used for more specialized purposes including defense (terpene-derived phytoalexins and volatile signals), pollinator at-tractants (monoterpenes), and phytoprotectants (carotenoids). As far as food quality is concerned, among the most important characteristics determined by the pathway are flavor and aroma as well as color. Antioxidant benefits are also likely from the carotenoid pathway as well as the capacity of ter-penes to protect against the adverse effects of procarcinogens by inducing phase II enzymes.
The key metabolite from which this diverse range of chemicals is synthesized is isopentenyldiphosphate (IPP). This is formed by two independent pathways (39) (Fig. 7). The cytoplasmic acetate-mevalonate pathway appears principally responsible for the synthesis of sterols and
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