hydroxyl group in 4th position by the help of leucocyanidin deoxygenase (LDOX)
to produce cyanidin or anthocyanidin, which gives (–)-epicatechin by the reduction
of ring double bond. Leucocyanidin upon the enzymatic action of leucoanthocyanidin 4-reductase (LAR), gives (+)-catechin. Leucocyanidin polymerises in the
presence of anthocyanidin synthase (ANS) to produce dimer and trimer procyanidin
via C 4 –C 6 or C 4 –C 8 linkage. Procyanidin polymer finally gives condensed tannin
by different C–C linkage formation. Epicatechin units can go through glycosylation
to produce complex tannins (Harding et al. 2014). The biosynthetic pathway of
condensed tannin and complex tannin is given in Fig. 21.6.
Phosphoenolpyruvate
Quinic acid
1-O-galloyl- -D-glucose
1,6-digalloyl- -D-glucose
1,2,6-trigalloyl- -D-glucose
1,2,3,6-tetragalloyl- -D-glucose
1,2,3,4,6-pentagalloyl glucose
P
CH 2
OH
O
HO
HO O
OH
OH
HO
HO
OH
O
OH
HO
OH
O
OH
Gallic acid
OH
OH
OH
O
O
O
OH
HO
HO
OH
O
O
O
OH
OH
HO
O
O
O
OH
OH
OH
OH
OH
HO
O
O
O
OH
O
HO
O
O
O
OH
OH
OH
OH
OH
HO
O
OH
OH
OH
O
O
O
O
O
OH
O
O
O
OH
OH
HO
HO
OH
HO
O
OH
OH
OH
O
OH
OH
OH
O
O
O
O
O
O
O
O
O
OH
OH
OH
OH
OH
HO
O
OH
OH
OH
O
OH
OH
HO
O
HO
HO
OH
UDP- -glucose
gallate 1- -glycosyltransferase
UDP
PEPTr
D-glucopyranose
-glucogallin-dikisgalloyl
glucose-O-galloyl transferase
-glucogallin-trikisgalloyl
glucose-O-galloyl transferase
-glucogallin-Ogalloyl transferase
1-O-galloyl- -
D-glucose
D-glucopyranose
1-O-galloyl- -
D-glucose
D-glucopyranose
-glucogallin-tetrakisgalloyl
glucose-O-galloyl transferase
Gallotannin
O
O
O
O
O
O
O
O
O
OH
OH
OH
OH
OH
HO
O
OH
OH
OH
O
OH
OH
HO
O
O
HO
OH
O
OH
HO
HO
Ellagitannin
O
O
O
O
O
O
O
O
O
HO
HO
HO
OH
OH
OH
O
OH
HO
HO
O
HO
OH
OH
O
OH
OH
HO
Galloylation
C -C
li n k a g e b e tw
e e n g a ll o y l r e s id u e s
o f g lu c o g a ll o y l m o le c u le s
1-O-galloyl- -
D-glucose
D-glucopyranose
Fig. 21.5 Biosynthetic pathway of gallotannin and ellagitannin
21 Tannins and Polyphenols Extracted from Natural Plants …
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