Anthocyanin Modification Enzymes

There are many branches to the flavonoid biosynthetic pathways, with the best characterized being those leading to the colored anthocyanins and proanthocyanidins (PAs) and the generally colorless flavones, flavonols, and isoflavonoids. Genes or cDNAs have now been identified for all the core steps leading to anthocyanin, flavone, and flavonol formation, as well as many steps of the isoflavonoid branch, allowing extensive analysis of the encoded enzymes (Table 3.1). In addition, several DNA sequences are available for the modification enzymes that produce the variety of structures known within each class of compound. 

While the CHS-CHI-F3H-DFR-AS enzymes form the core flavonoid biosynthetic pathway (Fig. 3.2), every intermediate compound in the pathway can be the subject of complex modifications that include hydroxylations, methylations, esterifications, and decorations with a number of sugar moieties. In addition, many of the core enzymes can utilize various substrates resulting in a pathway that is not linear, but rather a complex grid (Fig. 3.2).2 The diverse forms of flavonoids or anthocyanins that accumulate in any plant under any given condition are the result of a combination of the biosynthetic enzymes being expressed together with their substrate specificity. Over the past few years, the structures of several flavonoid biosynthetic enzymes have been elucidated,1 -20 which opens up unlimited opportunities to understand structure-function relationships and to manipulate the pathway. 

Fig. 2 Bioengineering of floral scent through modification of the flux to anthocyanin production in carnation flowers. The main enzymes of the pathway are marked by red. Dashed arrows represent multiple biosynthetic steps. The left image is a flower of transgenic carnation plant in which expression of flavanone...

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