Biology Reference
In-Depth Information
Still in the 1980s, little experimental evidence was available on the
natural intermediates and biochemical events of these plausible,
nevertheless theoretical pathways as documented, for instance, by a
review published in 1985 on the biosynthesis of tannins (Hillis, 1985).
Intensive enzymatic investigations in my laboratory have significantly
contributed to considerably changing this unsatisfying situation.
Experiments on these questions were performed with cell-free extracts
from leaves of pedunculate oak (
Quercus robur
,
syn
.
Q. pedunculata
) or
red oak (
Q. rubra
), staghorn sumac (
Rhus typhina
) and fringe cups
(
Tellima grandiflora
). The results obtained by this laborious but also
highly evidential technique allowed us to describe the principles and
many details of the pathways leading to these important natural plant
products.
3.3 Gallic Acid, the Principal Phenolic Unit
It is now generally accepted that major anabolic routes to benzoic acids
(phenylcarboxylic acids) in plants proceed via cinnamoyl-coenzyme A
(CoA) thioesters, whose propenoic side-chain undergoes degradation,
either oxidatively by β-oxidation to afford benzoyl-CoA's (Löscher and
Heide, 1994) or in a non-oxidative sequence to
free benzaldehydes (Abd
El-Mawla and Berhues, 2002). Major problems, however, have always
been encountered in the case of gallic acid (
1
). A rather conventional but
unproven concept assuming the CoA-dependent β-oxidation of 3,4,5-
trihydroxycinnamic acid (
9
; see Fig. 3.3, route A) (Zenk, 1964) suffered
from the fact that this compound has never been identified as natural
product, occasionally being regarded as the “missing cinnamic acid”
(Haslam, 1982).
A variation of this proposal (see route B in Fig. 3.3) avoided this
problem by putting the side-chain degradation reaction one step forward
to caffeic acid (
10
), but remained speculative as well (El-Basyouni
et al
.,
1964). A significantly different approach was presented by others (Conn
and Swain, 1961; Cornthwaite and Haslam, 1965) that favored the direct
aromatization of shikimic acid or, more likely, 5-dehydroshikimic acid
(
12
), the
enol
-form (
13
) which should be dehydrogenated to gallic
