Agriculture Reference
In-Depth Information
Fig. 3.4 Schematic representation of glucosinolate hydrolysis (
middle
) including examples of
aliphatic, aromatic and indolic glucosinolates (
left
) and most common degradation products (
right
)
et al.
2003
). In general, glucosinolates are synthesised in a three-block pathway.
The elongation of certain amino acids, which are the precursors of aliphatic or
aromatic glucosinolates, by sequential insertions of few (up to nine) methylene
groups into the side chain is the first phase of the process. Subsequently, the amino
acid moiety, elongated or not, is converted to form the core structure, which is
common to all glucosinolates. The final block concerns various modifications, such
as hydroxylation,
O
-methylation, desaturation, acylation and others, which leads to
the formation of a broad range of structures (Halkier and Gershenzon
2006
). To
date, more than 140 different glucosinolates have been described (Fahey
et al.
2001
). In
Arabidopsis
nearly 30 different glucosinolates have been found in
most organs, at various developmental stages (Brown et al.
2003
). The chemical
structure of most of them consists of a
-D-thioglucose group linked to a (Z)-N-
hydroximinosulfate ester via a single sulfur atom (Halkier and Gershenzon
2006
).
Although knowledge about the glucosinolate biosynthesis is important, it is the
degradation products that are responsible for all their biological functions (Fig.
3.4
).
The process of hydrolysis begins with breakdown of the thioglucoside bond, which
leads to the formation of glucose and unstable aglycone. The latter may then
isomerise to different products depending on the structure of side chain and
availability of various cofactors. The initiation process is catalysed by myrosinase
(EC: 3.2.3.1) and has been investigated in a number of biochemical and molecular
studies. Myrosinase is separated from glucosinolates in idioblast cells to avoid
unnecessary glucosinolate hydrolysis. These two components, however, mix very
quickly after the loss of cellular integrity as a result of wounding or insect or
pathogen attack, to activate the binary glucosinolate - myrosinase system leading to
ʲ
