Flavan-3,4-diol biosynthesis

A similar dihydroquercetin reductase activity has been shown to be a key step in the biosynthesis of anthocyanidins (Stafford, 1989). The enzyme from Dahlia has a molecular weight of about 41,000. However, there is evidence that the specificity of 3-hydroxyflavanone reductases of anthocy-anidin biosynthesis are distinct from those of flavan-3,4-diol biosynthesis (Stafford, 1989) see Chapter 11). 

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). 

HeUer W, Forkmann G, Britsch L, Grisebach H (1985) Enzymatic reduction of (+)-dihydroflavonols to flavan-3,4-cu-diols with flower extracts from Matthiola incana and its role in anthocyanin biosynthesis. Planta 165(2) 284-287... 

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. 

Flavonoids can be classified according to their biosynthetic origins. Some flavonoids are both intermediates in biosynthesis and end-products, e.g. chalcones, flavanones, flavanon-3-ols and flavan-3,4-diols. Other classes are only known as the end-products of biosynthesis, e.g. anthocyanins, flavones and flavonols. Two further classes of flavonoids are those in which the 2-phenyl side-chain of flavonoid isomerizes to the 3-position (giving rise to isoflavones and related isoflavonoids) and then to the 4-position (giving rise to the neoflavonoids). The major classes of flavonoids, with specific examples, are summarized helow. 

The branch pathway for anthocyanin biosynthesis starts with the enzymatic reduction of dihydrofiavonols to their corresponding flavan 3,4-diols (leucoanthocyanidins) by substrate-specific dihydroflavonol 4-reductases (DFR). Flavan 3,4-diols are the immediate precursors for the synthesis of catechins and proanthocyanidins. Catechins are formed by enzymatic reduction of the flavan 3,4-diols in the presence of NADPH to leucoanthocyanidins, which are subsequently converted to anthocyanidins by the 2-oxoglutarate-dependant dioxygenase, anthocyanidin synthase. Further glycosylation, methylation, and/or acylation of the latter lead to the formation of the more stable, colored anthocyanins (Scheme 1.1). The details of the individual steps involved in flavonoid and isoflavonoid biosynthesis, including the biochemistry and molecular biology of the enzymes involved, have recently appeared in two excellent reviews.7,8... 

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. 

Fig. 8 Biosynthesis of anthocyanins (39) via leucoanthocyanidins (flavan-3,4-diols, 37) from dihydroflavonols (3-hydroxy-flavanones, 31, 32)...

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

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 biosynthesis of anthocyanidins has been elucidated more recently than most other flavonoids. These positively charged compounds are derived from flavan-3,4-diols via dihydroflavonols. The hydroxyl group of the 3-position of anthocyanidins is introduced by flavanone 3-hydroxylase which converts flavanones to dihydroflavonols (see above). (25)-Flavanone 3-hydroxylase from flowers of Petunia hy-brida catalyzes the conversion of (25)-naringenin (10) [but not (2/ )-naringenin] to (2/ ,3i )-dihydrokaempferol (13). The hydrolase is a soluble enzyme that requires oxygen, 2-oxoglutarate, Fe, and ascorbate as cofactors (Britsch and Grisebach, 1986). (2S)-Eriodictyol (17) is converted to (2/ ,3/ )-dihydroquercetin. 

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... 

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... 

Biosynthesis Chemical reactions Flavan-3,4-diols Flavan-3-ols Flavan-4-ols Flavanols Occurrence Proanthocyanidins Stmctural features... 

Fig. 7.7.6. Biosynthesis of procyanidins through a flavan-3,4-diol intermediate. The bottom four diol and flavan-3-ol units react together to form procyanidin oligomers and polymers...

Anthocyanidin synthase (ANS), the key enzyme in the biosynthesis of anthocyanins, catalyzes oxidation of leucoanthocyanidin (flavan-3,4-diol) to a 2-flaven-3,4-diol that spontaneously isomerizes to 3-flaven-2,3-diol (anthocyanidin) (Fig. 5). This is subsequently glycosylated at C-3, transported to the vacuole, and finally converted to the colored flavilium cation at the acidic... 

In flavonoid biosynthesis, flavan-3,4-diols can be converted to flavan-3-ols (cf. 18.1.2.5.7). The intermediate is assumed to be a carbocation which is reduced to flavan-3-ol (Formula 18.21). When the reducing agent, e. g., NADPH, is limited, the cation can react with flavan-3-ol to... 

Flavan-3-ols and Flavan-3,4-diols are the precursors of the second large group of tannins. They polymerize to the so-called condensed tannins. We have already mentioned the first group of tannins, the gallotannins. Flavan-3-ols are better known under the name catechins (Fig. 106). They are widely distributed in the plant kingdom. The same is true of the flavan-3,4-diols, which belong to the so-called leuco anthocyanins. This is because flavan-3,4-diols can be very easily converted into anthocyanins by boiling with alcoholic hydrochloric acid. However, this conversion does not play any role in the biosynthesis of the anthocyanins. 

The biosynthesis of llavonoids in the heartwood of trees is very difficult to study by tracer methods, so that the isolation of possible intermediates, and their conversion in vitro into various end-products, is still important. Roux and his co-workers have recently reported in the heartwood of the legume Trachylohium verrucosum the presumed precursors of the flavane-3,4-diols,... 

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