Biology Reference
In-Depth Information
diverge (see Fig. 3.2). Before discussing the biosynthesis of ellagitannins
in detail, it appears appropriate to insert a short detour for a brief
summary of the state of the art in the biochemistry of this important
branch of gallic acid-derived natural products.
As mentioned earlier in this chapter, these compounds originate from
the addition of
meta
-depsidically-bound galloyl groups to the
pentagalloylglucopyranose (
3
) core to yield fairly large molecules, the
proper gallotannins, which have eventually been described as “complex”
gallotannins to discriminate them from the “simple” galloylglucoses that
precede their biosynthesis. Experimental access to their formation proved
rather trivial when crude enzyme extracts from sumac (
Rhus typhina
)
leaves, a rich source of gallotannins, were found to catalyze the
galloylation of 1,2,3,4,6-pentagalloylglucopyranose (
3
) in a manner
already known for the biogenesis of “simple” galloylglucoses. Again, β-
glucogallin (
2
) served as the principal acyl donor in these
transformations, which, however, involved phenolic hydroxyl groups
that are chemically quite different from the aliphatic hydroxyls of
glucose (Hofmann and Gross, 1990). Several enzymes catalyzing the
galloylation of penta- and hexagalloylglucoses have been isolated and
characterized until today (for details, see the recent review article of
Niemetz and Gross, 2005). A summary of the biogenetic
interrelationships of gallotannins is given in Fig. 3.7.
3.7 Biosynthesis of Ellagitannins
3.7.1 Search for pentagalloylglucose oxidizing enzymes
In contrast to the limited occurrence of gallotannins in nature,
ellagitannins are typical constituents of many plant families, in which
they can be found in an enormous structural variety, a feature that is for
the most part based on their pronounced tendency to form dimeric and
oligomeric derivatives. The structural characteristics of this challenging
group of natural products have attracted chemists already many decades
ago and have consequently also stimulated considerations on their
biogenetic origin. It was observed already in the mid-1930s that free
