Biosynthesis procyanidin polymers

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

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 above observations tend to suggest that more than one metabolic pool for the production of proanthocyanidins may exist in a plant and these in some cases are localized in different tissues. There is some evidence for this in Stafford s observations on procyanidin biosynthesis in Douglas-fir callus tissue (130, 131). Here quite high concentrations of dimers and trimers based on catechin-4 (5) units are found, but the associated polymer is based almost entirely on epi-catechin-4 (2) units, and evidence was obtained that the latter are formed much more rapidly than the catechin-4 oligomers (131). Thus the main biosynthetic process appears to be directed towards the synthesis of epicatechin-4 units that are diverted to polymers, and it seems likely that the two types of unit may be derived from distinct metabolic pools. 

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...

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