Formation of Proanthocyanidins and Phlobaphenes

The PAs, or condensed tannins, are polymers synthesized from flavan-3-ol monomer units. The phlobaphenes are 3-deoxy-PAs formed from flavan-4-ol monomers. The biosynthesis of both types of PAs follows the biosynthetic route of anthocyanins from chalcones through to the branch points to flavan-3-ol and flavan-4-ol formation. In this section, the specific enzymes forming the monomers are discussed, along with a discussion on the polymerization process. Although the chemistry of tannins is described in detail elsewhere in this book, it is useful to briefly mention the nature of the monomer subunit types and the polymer forms. 

FIGURE 3.4 Biosynthetic route to proanthocyanidins from leucoanthocyanidins. The product of the ANS is given in the flavylium cation form. Enzyme abbreviations are defined in the text and in Table 3.1. 

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