Biology Reference
In-Depth Information
pentagalloylglucopyranose oxidizing enzyme has been found to promote
the formation of the 4,6-HHBP-containing tellimagrandin II and a
different laccase-type phenolase has been found to dimerize
tellimagrandin II into cornussiin E by promoting the formation of a
diaryl ether valoneoyl bridge between the two monomers (Niemetz and
Gross, 2003a/b). This is all we know to date about enzymatic
implications in the construction of ellagitannins. One have to admit that
the task is enormous, considering the formidable structural diversity
expressed by ellagitannins simply derived for the most part from glucose
and gallic acid. No enzyme study has yet been undertaken on the steps
further downhill the ellagitannin biosynthetic pathway and leading to the
C
-glycosidic ellagitannins. However, the identification of several
structurally connected members of this ellagitannin subclass certainly
allows for sound working hypotheses. Whether or not the elaboration of
the
C
-glycosidic ellagitannin diversity is under a strict enzymatic control
or is attributable to a combinatorial-type synthesis solely relying on the
inherent chemical reactivity of these natural products, one have to keep
in mind that the occurrence of ellagitannins harboring the characteristic
C
-glycosidic bond appears to be limited to species from a rather short
selection of plant families (
vide supra
) borne out by a long evolutionary
process since the colonization of land by plants (around 400 million
years ago). So the truth probably lies in between the two aforementioned
extremes; some plant species might have learnt to develop enzymes to
ensure the formation of key bonds leading to specific structural subtypes,
while still letting chemistry play its role in expanding diversity further
within a given structural subtype.
The biochemical event that mediates the passage from the
glucopyranosic ellagitannin class to the open-chain
C
-glycosidic
ellagitannin class admitedly remains a matter of speculation, but
interesting observations have nevertheless been made from the study of
a plant species in which members of the two classes co-exist (Hatano
et al.
, 1986). Early spring leaves of
Liquidambar formosana
(
Hamamelidaceae
), a Chinese medicinal plant, contain high amounts of
tellimagrandin II (
20
). Analysis of the same leaf material in summer
indicated negligible amounts of
20
, whereas two other ellagitannins,
pedunculagin (
23
) and the
C
-glycosidic casuarinin (
4
) became
