Biology Reference
In-Depth Information
provided the basis for the interpretation of isotopomer patterns of these
metabolites in a “retrobiosynthetic” approach. It was concluded that
gallic acid was derived from an early intermediate of the shikimate
pathway, most likely, 5-dehydroshikimic acid (
12
; Werner
et al.
, 1997).
Recently, this interpretation was corroborated by determination of δ
18
O-
values of gallic acid from
R. typhina
leaves that indicated its formation
by dehydrogenation of 5-dehydroshikimic acid and also excluded
alternative routes
via
phenylpropanoid C
6
-C
3
intermediates (Werner
et
al.
, 2004).
Supporting evidence for these results was published by Ossipov
et
al.
(2003), who reported the
in vitro
reduction of 5-dehydroshikimic acid
to gallic acid with enzyme preparations from birch (
Betula pubescens
)
leaves.
3.4 Biosynthesis of β-Glucogallin
β-Glucogallin (1-
O
-galloyl-β-
D
-glucopyranose,
2
) was first isolated from
Chinese rhubarb (
Rheum officinale
) in 1903 (Haslam, 1998) and was
long ago proposed as the first specific metabolite in the biosynthesis of
hydrolyzable tannins (Haddock
et al.
, 1982). For thermodynamic
reasons, esterification of gallic acid (
1
) and glucose must be expected to
involve the participation of an “activated” intermediate with a high
group-transfer potential. By analogy to the well-known caffeoyl-CoA
dependent formation of chlorogenic acid and related depsides (Stöckigt
and Zenk, 1974; Ulbrich and Zenk, 1980), galloyl-CoA was considered
the most likely candidate for this reaction. This unknown thioester was
synthesized
via
the
N
-hydroxysuccinimidyl derivative of 4-
O
-β-
D
-
glucosidogallic acid (Gross, 1982a), but it was soon recognized that this
compound was not involved in the biosynthesis of β-glucogallin (Gross,
1983a). It was found instead that a glucosyltransferase from oak leaves
catalyzed the efficient esterification of free gallic acid (
1
) and
“activated” glucose, uridine-5'-diphosphate glucose, affording β-
glucogallin (
2
) and related 1-
O
-acyl-β-
D
-glucopyranoses (Fig. 3.4;
Gross, 1982b, 1983b; Weisemann
et al.
, 1988). Numerous analogous
enzymes catalyzing the formation of phenolic 1-
O
-acylglucoses have
