Biosynthesis of Tannins

BIOSYNTHESIS OF TANNINS

Tannins belong to the phenolics class of secondary metabolites. All phenolic compounds; either primary or secondary are in one way or another formed through shikirnic acid pathway (phenylpropanoid pathway). Other phenolics such as isoflavones, coumarins, lignins, and aromatic amino acids (tryptophan, phenylalanine, and tyrosine) are also formed by the same pathway. Hydrolysable tannins (Hts) and condensed tannins (proanthocyanidins) are the two main categories of tannins that impact animal nutrition.

Common tannins are formed as follows:

1.     Gallic acid is derived from quinic acid.

2.     Ellagotannins are formed from hexahydroxydiphenic acid esters by the oxidative coupling of neighbouring gallic acid units attached to a D-glucose core.

3.     Further oxidative coupling forms the hydrolysable tannin polymers.

4.     Proanthocyanidin (PA) biosynthetic precursors are the leucocyanidins (flavan-3,4-diol and flavan-4-ol) which on autoxidation, in the absence of heat, form antho-cyanidin and 3-deoxyanthocianidin, which, in turn, polymerize to form PAs.