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).
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.
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