translocator (PEPTr), forms quinic acid. Under the catalytic action of gallate
1-b-glycosyl transferase enzyme, and via the formation of UDP from UDP-a-glucose,
gallic acid yields 1-O-galloyl-b-D-glucose. From 1-O-galloyl-b-D-glucose, one
molecule of D-glucopyranose is cleaved consecutively in each step and it results in
galloylation to form 1,6-digalloyl-b-D-glucose, 1,2,6-trigalloyl-b-D-glucose,
1,2,3,6-tetragalloyl-b-D-glucose and finally 1,2,3,4,6-pentagalloyl glucose under
the catalysis of enzymes like b-glucogallin-dikisgalloyl glucose-O-galloyl transferase, b-glucogallin-trikisgalloyl glucose-O-galloyl transferase, b-glucogallintetrakisgalloyl glucose-O-galloyl transferase respectively. 1,2,3,4,6-pentagalloylb-D-glucopyranose or simply 1,2,3,4,6-pentagalloyl glucose is the key intermediate
found in many plant families. After the intermediate 1,2,3,4,6-pentagalloyl glucose
formation, the galloylation of pentagalloylglucose continues and ester linkage
between two galloyl moieties forms gallotannins or depsidic metabolites. The enzymatic catalysis of 1,2,3,4,6-pentagalloyl glucose by different galloyltransferase
enzymes forms different linkages to form different gallotannins (Niemetz and Gross
et al. 2005).
After the formation of intermediate 1,2,3,4,6-pentagalloyl glucose, oxidation
takes place by the action of different oxido-reductase enzymes, leading to C–C
linkage formation between suitably orientated galloyl residues of glucogalloyl
molecules and it results in hexahydroxydiphenoyl (HHDP) units formation which is
the core moiety of ellagitannins (Grundhöfer et al. 2001). The biosynthetic pathway
of hydrolysable tannin and complex tannin is given in Fig. 21.5.
21.4.2 Biosynthesis of Condensed and Complex Tannins
The phosphoenolpyruvate obtained from glycolysis in plants, forms quinic acid by
the phosphoenolpyruvate translocator (PEPTr). Quinic acid in the next step, yields
shikimic acid and chorismate. Shikimic acid is found to be the main precursor of
different metabolite formation in plants. In the next step, chorismate gives phenylalanine when asparagine (Asn) forms aspartate (Asp) side by side. After this, the
enzyme called phenylalanine ammonia lyase (PAL) helps to convert phenylalanine
into cinnamic acid or cinnamate. Hydroxyl group insertion occurs to the para
position of cinnamic acid by the help of the enzyme called cinnamic acid
hydroxylase (CAH) to form 4-coumaroyl-CoA. P-coumaroyl-CoA gives chalcone,
when it is condensed with malonyl CoA. The enzymes involved in this step are
4-coumaroyl-CoA ligase (4CL) and chalcone synthase (CHS). Naringenin chalcone
isomerizes under the enzymatic condition produced by chalcone isomerase (CHI) to
form naringenin, which is a flavanone. Naringenin then forms a dihydroflavonol
called dihydrokaempferol via the attachment of a hydroxyl group in 3
rd position by
flavanone 3-hydroxylase (F3H). In the next step, another hydroxyl group insertion
occurs in dihydrokaempferol and gives dihydroquercetin by flavonoid 3′-hydroxylase (F3′H). Further, reduction takes place by dihydroflavonol reductase
(DFR) which results in the formation of leucocyanidin. Leucocyanidin cleaves the
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S. Mal and D. Pal
1-b-glycosyl transferase enzyme, and via the formation of UDP from UDP-a-glucose,
gallic acid yields 1-O-galloyl-b-D-glucose. From 1-O-galloyl-b-D-glucose, one
molecule of D-glucopyranose is cleaved consecutively in each step and it results in
galloylation to form 1,6-digalloyl-b-D-glucose, 1,2,6-trigalloyl-b-D-glucose,
1,2,3,6-tetragalloyl-b-D-glucose and finally 1,2,3,4,6-pentagalloyl glucose under
the catalysis of enzymes like b-glucogallin-dikisgalloyl glucose-O-galloyl transferase, b-glucogallin-trikisgalloyl glucose-O-galloyl transferase, b-glucogallintetrakisgalloyl glucose-O-galloyl transferase respectively. 1,2,3,4,6-pentagalloylb-D-glucopyranose or simply 1,2,3,4,6-pentagalloyl glucose is the key intermediate
found in many plant families. After the intermediate 1,2,3,4,6-pentagalloyl glucose
formation, the galloylation of pentagalloylglucose continues and ester linkage
between two galloyl moieties forms gallotannins or depsidic metabolites. The enzymatic catalysis of 1,2,3,4,6-pentagalloyl glucose by different galloyltransferase
enzymes forms different linkages to form different gallotannins (Niemetz and Gross
et al. 2005).
After the formation of intermediate 1,2,3,4,6-pentagalloyl glucose, oxidation
takes place by the action of different oxido-reductase enzymes, leading to C–C
linkage formation between suitably orientated galloyl residues of glucogalloyl
molecules and it results in hexahydroxydiphenoyl (HHDP) units formation which is
the core moiety of ellagitannins (Grundhöfer et al. 2001). The biosynthetic pathway
of hydrolysable tannin and complex tannin is given in Fig. 21.5.
21.4.2 Biosynthesis of Condensed and Complex Tannins
The phosphoenolpyruvate obtained from glycolysis in plants, forms quinic acid by
the phosphoenolpyruvate translocator (PEPTr). Quinic acid in the next step, yields
shikimic acid and chorismate. Shikimic acid is found to be the main precursor of
different metabolite formation in plants. In the next step, chorismate gives phenylalanine when asparagine (Asn) forms aspartate (Asp) side by side. After this, the
enzyme called phenylalanine ammonia lyase (PAL) helps to convert phenylalanine
into cinnamic acid or cinnamate. Hydroxyl group insertion occurs to the para
position of cinnamic acid by the help of the enzyme called cinnamic acid
hydroxylase (CAH) to form 4-coumaroyl-CoA. P-coumaroyl-CoA gives chalcone,
when it is condensed with malonyl CoA. The enzymes involved in this step are
4-coumaroyl-CoA ligase (4CL) and chalcone synthase (CHS). Naringenin chalcone
isomerizes under the enzymatic condition produced by chalcone isomerase (CHI) to
form naringenin, which is a flavanone. Naringenin then forms a dihydroflavonol
called dihydrokaempferol via the attachment of a hydroxyl group in 3
rd position by
flavanone 3-hydroxylase (F3H). In the next step, another hydroxyl group insertion
occurs in dihydrokaempferol and gives dihydroquercetin by flavonoid 3′-hydroxylase (F3′H). Further, reduction takes place by dihydroflavonol reductase
(DFR) which results in the formation of leucocyanidin. Leucocyanidin cleaves the
728
S. Mal and D. Pal