Biosynthesis Of Gallotannins And Ellagitannins

Gallotannins and ellagitannins make up the hydrolysable tannins and are derived from 1,2,3,4,6-penta-O-galloyl-P-D-glucopyranose (3.102). In oak leaves, and presumably many other plants that synthesize hydrolysable tannins, this compound is synthesized from P-glucogallin (1-O-galloyl-P-D-glucopyranose; 3.98). The subsequent esterification of gallic acid residues (3.47; see Section 8) occurs in a specific sequence: C6-C2-C3-C4 (Figure 3-16). When enzyme preparations obtained from oak leaves were provided with UDP-glucose, P-glucogallin was formed, but also di- and trigalloyl-glucoses (Gross, 1983). This suggested that P-glucogallin (3.98) was both an acceptor and a donor of gallic acid residues. The different steps towards the biosynthesis of pentagalloylglucose are catalyzed by different enzymes. With the exception of the enzyme catalyzing the formation of P-glucogallin, these enzymes are very large, with molecular weights between 260 and

Figure 3-15. Biosynthesis of stilbene from p-coumaroyl-CoA with three molecules malonyl-CoA, catalyzed by the enzyme stilbene synthase (a; E.C. 2.3.1.95).

450 kDa.

Gallotannins contain an additional 10-12 gallic acid moieties per molecule. This is effectively a continuation of the esterification reactions that resulted in the formation of pentagalloylglucose (3.99). The major difference between the formation of gallotannins and pentagalloylglucose -in terms of the chemistry - is that the additional gallic acid moieties have to react with phenolic hydroxyl groups, as opposed to the aliphatic hydroxyl groups of the glucose molecule. This process results in the formation of characteristic meta-depside groups (1.91).

PGG Glc u

PGG Glc u

H

PGG Glc

Figure 3-16. Biosynthesis of 1,2,3,4,6-penta-O-galloyl-P-D-glucopyranose (3.102) from gallic acid (G; 3.47) and UDP-Glucose. The most recently added gallic acid residue is indicated by a G in bold face. The intermediates are P-glucogallin (PGG; 3.98), 1,6-di-O-galloyl-P-D-glucopyranose (3.99), 1,2,6-tri-O-galloyl-P-D-glucopyranose (3.100), and 1,2,3,6-tetra-O-galloyl-P-D-glucopyranose (3.101).

Figure 3-16. Biosynthesis of 1,2,3,4,6-penta-O-galloyl-P-D-glucopyranose (3.102) from gallic acid (G; 3.47) and UDP-Glucose. The most recently added gallic acid residue is indicated by a G in bold face. The intermediates are P-glucogallin (PGG; 3.98), 1,6-di-O-galloyl-P-D-glucopyranose (3.99), 1,2,6-tri-O-galloyl-P-D-glucopyranose (3.100), and 1,2,3,6-tetra-O-galloyl-P-D-glucopyranose (3.101).

Using cell-free extracts from sumac (Rhus typhina), Hofmann and Gross (1990) provided evidence that the addition of the gallic acid residues occurred in a manner similar to the acylation of pentagalloylglucose, with P-glucogallin (3.95) serving as a donor of gallic acid residues. The biosynthesis of hexa, hepta, and octa galloylated gallotannins appears to be catalyzed by several gallotannin synthesizing P-glucogallin-dependent galloyltransferases that have a preferred but not unique substrate when it comes to the degree of substitution (penta-, hexa- or hepta- galloylglucose molecules) and the substitution pattern, i.e. the location of the meta-depside residues. As a consequence, a particular gallotannin molecule can have several biosynthetic origins (Niemetz and Gross, 2005).

Ellagitannins are formed from the oxidative coupling between gallic acid residues in pentagalloylglucose molecules leading to the formation of C-C coupled 3,4,5,3',4',5'-hexahydroxydiphenoyl (HHDP) residues (1.97; 1.98). Tellimagrandin II (3.103) is a monomeric ellagitannin in which the 4C1 conformation can be observed. The two galloyl residues are coupled via a 4,6-linkage.

Figure 3-17. Biosynthesis of Tellimagrandin II (3.103) and Cornusiin E (3.104) from pentagalloylglucose (3.102) by polyphenol oxidases of the laccase class.

Other linkages, namely 1,6-, 3,6- and 2,4-O HHDP linkages, are possible, but require the less stable 1C4 conformation of the glucose molecule. Linkages, both C-C and C-O, can also be formed between galloyl residues of different ellagitannin monomers, thereby giving rise to dimers, trimers, and tetramers. A wide range of compounds can thus be synthesized.

The biosynthesis of ellagitannins is not well understood. This is due to the many different compounds that exist in this class, and due to their complex chemical structure, which requires sophisticated analytical tools for identification. Niemetz and Gross (2003) isolated two enzymes that catalyze the formation of Tellimagrandin II (3.103) and Cornusiin E (3.104) from pentagalloylglucose. Both enzymes, pentagalloylglucose: O2 oxidoreductase and Tellimagrandin II: O2 oxidoreductase, require oxygen and were shown to belong to the laccase class polyphenol oxidases.

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