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–C3
segment, 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.
___________________________________________
*
Reinhard, E., In Tissue Culture and Plant Science—1974,
(H.E. Street Ed.), Academic Press, New York, pp.
433-459,
1974.
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
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