Biosynthesis of Flavan-3-ols

Pfeiffer J, Kiihnel C, Brandt J, Duy D, Punyasiri PA, Forkmann G, Fischer TC. 2006. Biosynthesis of flavan 3-ols by leucoanthocyanidin 4-reductases and anthocyanidin reductases in leaves of grape (Vitis vinifera L.), apple (Malus x domestica Borkh.) and other crops. Plant Physiol Biochem 44 323-334. 

Biosynthesis of Flavan-3-ols (40) and Condensed Tannins (Proanthocyanidins, 44) from Leucoanthocyanidins (Flavan-3,4-diols, 37)... 

Chemler JA, Lock LT, Koffas MAG, Tzanakakis ES (2007) Standardized biosynthesis of flavan-3-ols with effects on pancreatic beta-cell insulin secretion. Appl Microbiol Biotechnol 77 797-807... 

Platt R V, Opie C T, Haslam E 1984 Plant proanthocyanidins. Part 8. Biosynthesis of flavan-3-ols and other secondary plant products from 25-phenylalanine. Phytochemistry 23 2211-2217... 

Dihydroflavonol 4-reductase (DFR EC 1.1.1.219) is a member of the short-chain dehydrogenase/reductase family and catalyzes the stereospecific conversion of (+)-(2R,3R)-dihydroflavonols to the corresponding (2R,3S,4S) flavan-3,4-cw-diols (leucoanthocyanidins), with NADPH as a required cofactor. The enzyme activity was first identified in cell suspension cultures of Douglas fir (Pseudotsuga menziesii) and was shown to be related to the accumulation of flavan-3-ols and proanthocyanidins [96]. Leucoanthocyanidins and DFR were later shown to be required for anthocyanidin formation by complementation of Matthiola incana mutants blocked between dihydroflavonol and anthocyanidin biosynthesis [97, 98], DFR has been purified to apparent homogeneity and biochemically analyzed from flower buds of Dahlia variabilis [99]. DFR was shown to accept different substrates depending on the plant species from which it was isolated (reviewed in 100). 

Leucoanthocyanidin reductase (EAR), which is related to the isoflavoue reductase group of plaut enzymes, catalyzes the reduction of leu-coanthocyanidins to (+)-afzelechin, (+)-cate-chin, and (+)-gallocatechin, building blocks for PA biosynthesis [77] (Fig. 5). The above cate-chin series of flavan-3-ols possess 2,3-trans stereochemistry. The corresponding (ep/)-cate-chin series with 2,3-cis stereochemistry is formed by a different mechanism involving an unrelated reductase (ANR) that acts at the level of anthocyanidin [90] (Fig. 5). 

Formation of intermediates in the biosynthesis offlavonols, flavan-3-ols, anthocyanins and proanthocyanidins... 

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. 

Owing to the purported role of the flavans and flavan-3-ols as nucleophilic chain-terminating units, and of the flavan-4-ols and flavan-3,4-diols (leucoanthocyanidins) as electrophilic chain-extension units in the biosynthesis of the proanthocyanidins," the chemistry of these four classes of compounds is intimately linked to that of the proanthocyanidins. 

Earlier proposals (12, 13, U) and the results of biosynthetic experiments (18) have been adumbrated into a scheme of biosynthesis for the procyanidins (Figure k) in which it is suggested that they are formed as byproducts during the final stage of the synthesis of the parent flavan-3-ol structures, (+)-catechin and (-)-epicatechin (11, 18). A two step reduction of the f1av-3-en-3-ol... 

Stafford, H., Lester, H. (1984). Flavan-3-ol biosynthesis the conversion of (-l-)-dihydroquercetin and flavan-3,4-cis-diol (leucocyanidin) to (-t) catechin by reductases extracted from cell suspension cultures of Douglas fir. Plant Physiol., 76, 184-186. 

The biosynthesis of the proanthocyanidins is believed to proceed by addition of an electrophilic extension unit derived from a flavan-3,4-dioP or a flavan-3-oP to a nucleophilic starter unit, most likely a flavan-3-ol, with sequential addition of further chain-extension units. Although the genetics of interflavanyl bond formation in the proanthocyanidin polymerization process are not yet defmed, " the search for the elusive condensing enzyme continues unabated. ... 

Notable contributions regarding the biosynthesis of 3-deoxyanthocyanins, flavan-3-ols, and 5-deoxyflavonoids were also recently described." ... 

There is no final consensus on whether procyanidin biosynthesis is controlled thermodynamically or enzymatically. In either case proanthocyanidins are synthesized through sequential addition of flavan-3,4-diol units (in their reactive forms as carbocations or quinone methides) to a flavan-3-ol monomer [218]. Based on the latest findings there is some evidence that different condensation enzymes might exist which are specific for each type of flavan-3,4-diol [64] and that polymer synthesis would be subject to a very complex regulatory mechanism [63]. But so far, no enzyme synthetase systems have been isolated and enzymatic conversion of flavanols to proanthocyanidins could not be demonstrated in vitro [219]. If biosynthesis was thermodynamically controlled, the variation in proanthocyanidin composition could be explained by synthesis at different times or in different compartments [64], The hypothesis of a thermodynamically controlled biosynthesis is based on the fact that naturally and chemically synthesized procyanidin dimers occur as a mixture of 4—>8 and 4—>6 linked isomers in approximate ratios of 3-4 1 [220]. Porter [164] found analogous ratios of 4—>8 and 4—>6 linkages in proanthocyanidin polymers. 

The conversion of flavan-3,4-diols to flavan-3-ols and to their oligomeric proanthocyanidin forms is the beginning of the pathway unique to the biosynthesis of these secondary products (Stafford, 1989). Two basic types of hydroxylases... 

Catechins flavan-3-ols. (-r)-Catechin and (-)-epi-catechin are widespread in plants. Other C. have a more limited distribution (Fig.). C have been implicated in the biosynthesis of condensed Tannins (see). 

Havanols are a wide group of polyphenols that include flavan-3-ols (e.g., catechin and proanthocyanidins), flavan-4-ols, and flavan-3,4-diols. They arise from plant secondary metabolism through condensation of phenylalanine derived from the shikimate pathway with malonyl-CoA obtained from citrate that is produced by the tricarboxylic acid cycle, leading to the formation of the key precursor in the flavonoids biosynthesis the naringenin chalcone. The exact nature of the molecular species that undergo polymerization and the mechanism of assembly in proanthocyanidins are still unknown. From a structural point of view, flavanols... 

The amount of flavonoids and flavanols in cocoa and chocolate might be highly variable and this variability is multifactorial. For example, plant s genetic predisposition dictates the biosynthesis of the primary (sugars, amino acids, etc.) and secondary metabolites (proanthocyanidins, saponins, alkaloids, etc.) while genetics can cause as much as a fourfold difference in flavan-3-ol content of fresh cocoa beans [22, 23]. Variability with respect to secondary metabolites exists between varieties (or cultivars) of the same species. However, various environmental factors determine the extent to which genetic potentialities are... 

Stafford H A 1984 Flavan-3-ol biosynthesis, the conversion of (+)-dihydroquercetin and flavan-... 

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