Chemistry Reference
In-Depth Information
stages: (a) elongation of the aminoacid side chain (for Met and Phe);
(b) thioglucoside formation and (c) sidechain secondary modifications.
These aminoacid modifications give rise to a great diversity of glucosi-
nolates: to date more than 120 structures have been characterized in
plants.
28
Most of the enzymes responsible for the biosynthesis of gluco-
sinolates in the model plant A. thaliana have been identified. Among
them, the UDP-glucose:thiohydroximate S-glycosyltransferases (S-UGT,
GT 1 family) appears to be unique as it catalyzes the S-glycosidic bonding
between glucose and the acceptor thiohydroximate that leads to the
formation of desulfoglucosinolate (Fig. 3). Several representatives in
Brassicales order have been so far identified, isolated, or characterized.
S-UGT74B1 cloned from Brassica napus cDNA was partially characterized,
although its substrate specificity for sulfur-containing acceptor was not
demonstrated.
29
A few years later, the orthologous protein from
A. thaliana was cloned, expressed, and enzymatic characterization was
reported, that led to the conclusion that in vivo, S-glycosylation is the
biological activity of S-UGT74B1.
30
Further studies highlighted the
enzymatic mechanism of S-UGT74B1.
31
Genes coding for orthologs of
S-UGT74B1 were identified in other brassica genomes, such as B. rapa
32
and B. oleracea.
33
Analysis of A. thaliana metabolic enzymes transcrip-
tional co-regulation led to the identification of a second S-UGT, which
differs from S-UGT74B1 by the acceptor specificity.
34
Unlike S-UGT74B1,
which is thought to be involved in aromatic glucosinolate synthesis,
S-UGT74C1 is co-regulated with the enzymes that synthesize
aliphatic glucosinolates. Thus, S-UGT74C1 may be able to catalyze the S-
glucosylation of aliphatic thiohydroximates. However, since this study,
no molecular characterization of this enzyme has been reported to date.
1.3 Other S-Glycosyltransferases activities
In 2007, in the search for a dual N- and O-glucosyltransferase in
A. thaliana, Brazier-Hicks et al. reported the screening of 44 recombinant
GTs towards O-, N-, and S-glycosyltransferase activity.
35
6 of these 44
enzymes exhibited significant activity for S-glycosylation (all of them
belonging to the GT1 family). Except S-UGT74B1 that was previously
identified as an endogenous S-glucosyltransferase (see above), no other
studies on the remaining 5 enzymes confirmed the S-GT activity.
Another study that reported the screening of several acceptors by OleD
protein demonstrated that wild-type OleD could S-glycosylate acceptors
(see below).
36
1.4 Bioengineered S-GT
The use of GT in chemical synthesis has been widely studied, because the
application fields are numerous, mostly in the therapeutic domain.
Moreover, use of natural GTs that exhibit broad substrate specificity
could lead to new glycoforms of natural or artificial compounds, like
S-glycosylated products.
37
Another way to artificially improve the
substrate promiscuity of GT is the sequence- and structure-guided
engineering. Several examples of GT-engineering have been reported in
the literature so far (for a review, see Ref. 11). Among these reports, only
Search WWH ::
Custom Search