Biology Reference
In-Depth Information
reactions were not randomly distributed, but displayed high
regiospecificity to constitute the following metabolic sequence: β-
glucogallin (
2
) → 1,6-digalloylglucopyranose (
14
, Schmidt
et al.
, 1987)
→ 1,2,6-trigalloylglucopyranose (
15
, Gross and Denzel, 1991) →
1,2,3,6-tetragalloylglucopyranose (
16
, Hagenah and Gross, 1993) →
1,2,3,4,6-pentagalloylglucopyranose (PGG) (
3
, Cammann
et al.
, 1989).
Interestingly, an identical pattern has been reported for the chemical
substitution of glucose hydroxyls in experiments with 1-benzyl or 1-
methyl-β-
D
-glucopyranose that was explained as a combination of
reactivity differences in response to variations in chemical nature (
i.e.
,
primary
vs.
secondary hydroxyl), neighboring group activation and steric
hindrance (Williams and Richardson, 1967; Reinefeld and Ahrens,
1971).
The above discussed enzyme studies may give the impression of a
straightforward directed pathway that is depending on one simple and
uniform reaction mechanism. There is no doubt that this interpretation
applies to the route from β-glucogallin to pentagalloylglucopyranose,
however, observations on the existence of other enzyme activities,
leading to side-reactions and ramifications of the main pathway, should
be briefly mentioned for completeness. For instance, an acyltransferase
was found in oak leaves that catalyzed an unusual galloyl exchange
between β-glucogallin (
2
) and free glucose, a reaction that was
detectable only with labeled substrates,
i.e.
:
β-glucogallin + *glucose ↔ *β-glucogallin + glucose
(the asterisk symbolizes the radioactive label), the physiological
significance of which remaining obscure (Gross
et al
., 1986). Another
enzyme from sumac interfered with the acylation of 1,6-
digalloylglucopyranose (
14
) to 1,2,6-trigalloylglucopyranose (
15
) by
promoting this reaction in the absence of the established acyl donor, β-
glucogallin (
2
) (see
Fig. 3.6). It was recognized that this enzyme
catalyzed a “disproportionation” reaction, in which two molecules of 1,6-
digalloylglucopyranose (
14
) were converted into 1,2,6-
trigalloylglucopyranose (
15
) and anomeric 6-
O
-galloylglucose as a
partially deacylated by-product (Denzel and Gross, 1991). This finding
