3.4 Biosynthesis of Flavonoid Glycosides
Recently, both extensive and intensive research at the enzymatic level has more or less confirmed the original hypothetical steps postulated for the incorporation of acetate and phenylalanine into flavonoids. In fact, studies related to enzymology and regulation of flavone and flavonol glycoside biosynthesis has unfolded many further details of the individual reactions. It has been reported that more than 20 different flavonoid glycosides occurrig in irradiated parsley cells are based on only three flavone and three flavonol aglycones all of which have essentially very similar substitution modes (Kreuzaler and Hahlbrock*, 1973). The chemcial structures of the aglycones and their probable reactions are as shown here under.
It may be observed that except for the characteristic C-3 hydroxyl moiety of flavonols the six aglycones essentially differ only with respect to substitution at C-3' position.
Another school of thought suggests that the flavonoid glycoside aglycones may be obtained from the major pathways ultimately leading to the synthesis of aromatic compounds in the biological systems, namely:
(a) Acetate Pathway, and
(b) Shikimic Acid Pathway
It has been observed that one 6-carbon fragment of the C6-C3-C6 compounds derived from the acetate pathways gets combined with the remaining 9 carbon fragment ontained from the shikimic (phenyl propanoid) pathway as stated below:
Explanation
1. The C6–C3segment, perhaps in the oxidation form of a cinnamic acid molecule, gets combined with three molecules of acetate to yield first a C15 chalcone moiety an intermediate and subsequently the flavanone residue.
2. The simultaneous introduction of removal of OH moieties from the aromatic rings B and A gives rise to the production of a good number of derivatives,
3. Flavonoids are first formaed by the introduction of the hydroxy group at position 3, whereas dehydrogenation at positions 2 and 3 results in the formation of flavonols, and
4. Evidently, the simultaneous occurance of a variety of glycosides having the same aglycone in a specific plant species strongly supports the well established hypothesis that glycosylation usually takes place at a late stage of flavonoid biosynthesis.
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* Reinhard, E., In Tissue Culture and Plant Science—1974, (H.E. Street Ed.), Academic Press, New York, pp.
433-459, 1974.
