Agriculture Reference
In-Depth Information
methionine is a methyl-group donor and precursor of important secondary metab-
olites. It was shown that 80 % of methionine is used for SAM synthesis whereas
20 % is incorporated into proteins (Giovanelli et al. 1985 ). Synthesis of methionine
requires products of three metabolic pathways: the carbon skeleton originates from
aspartate, the sulfur atom from cysteine, and the methyl group from serine (Ravanel
et al. 1998 ). In higher plants methionine synthesis starts from a
ʳ
-replacement
ʳ
reaction catalysed by cystathionine
-synthase (CGS) leading to formation of
cystathionine from cysteine and O -phosphohomoserine (OPH; Fig. 3.3 ).
Cystathionine is converted to homocysteine by ʱ , ʲ -elimination catalysed by
cystathionine ʲ -lyase. The final step includes the transfer of the methyl group
from N 5 -methyl-tetrahydrofolate to homocysteine, which in plants is catalysed by
cobalamin-independent methionine synthase (MS; Hesse et al. 2004a ; Ravanel
et al. 1998 ). In Arabidopsis three MS isoforms are known: two of them are in
cytosol and a third one is localised to the plastids (Ravanel et al. 2004 ). Subse-
quently methionine is converted to SAM by SAM synthetase which requires ATP.
It was shown that accumulation of SAM inhibits the enzyme activity (Ravanel
et al. 1998 ). When SAM is used for synthesis of ethylene or polyamines,
methylthioadenosine (MTA) is produced as intermediate. MTA can be used for
synthesis of another methionine molecule increasing SAM availability as methyl-
group donor (Burstenbinder et al. 2007 ).
Methionine synthesis undergoes a complex regulation. The pathway is regulated
by feedback inhibition of aspartate kinase: the carbon flux inhibits the synthesis of
methionine (Hesse et al. 2004a ). It was also shown that CGS is involved in the
control of the pathway on the level of sulfur and carbon-nitrogen skeleton delivery
to the pathway (Hesse et al. 2004a ). OPH is a common substrate for CGS and
threonine synthase (TS). TS affinity for OPH is significantly higher than CGS
affinity. TS is also positively regulated by SAM. Sufficient concentration of SAM
enhances the activity of TS and affinity for OPH (Curien et al. 1996 ). Carbon
skeletons are thus delivered to methionine biosynthetic pathway only when the
SAM concentration decreases (see also “Control of methionine biosynthesis -
posttranscriptional regulation” in the following sections of this chapter).
Sulfur-Containing Secondary Metabolites: Glucosinolates
and Phytosulfokines
Sulfur is also present in plant metabolites as sulfo-group-modifying carbohydrates,
proteins, and many natural products. Many sulfated metabolites play distinct roles
in plant defence against biotic and abiotic stresses. Some of the best known groups
of sulfated compounds are glucosinolates, which play an important role in protec-
tion against herbivores. They are also responsible for taste and flavour of many
Brassica vegetables (e.g. cabbage, broccoli). Products of glucosinolate degradation,
isothiocyanates, possess an anticarcinogenic activity in mammalian cells (Mithen
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