Chemistry Reference
In-Depth Information
3.3
Glucansucrases
Another very valuable glycosyltransferase for the construction of well-defined
polysaccharides is glucansucrase (dextransucrase; systematic name: sucrose:(1
→
6)-
α
-
D
-glucosyltransferase; EC 2.4.1.5).
Recently Dijkhuizen and coworkers reported on a family of glucansucrases
found in
Lactobacillus reuteri
, which convert sucrose into large, heavily branched
α
-
D
-glucan 6-
α
-glucans
One
of
these
glucansucrases
(GTF180)
produces
an
(
α
→
)
(
α
→
)
α
-glucan with
1
3
and
1
6
glycosidic linkages. Kamerling and
(
→
,
→
)
coworkers could show that the
-
D
-glucan of
L. reuteri
strain
180 has a heterogeneous structure with no repeating units present [
203
]. It con-
tains only
1
3
1
6
-
α
3) units
were shown to be 6-substituted, and the polysaccharide is built-up from different
lengths of isomalto-oligosaccharides, interconnected by single
α
-
D
-Glcp-(1
→
6)-units in terminal position. All
α
-
D
-Glcp-(1
→
bridges.
The GTF180 enzyme shows large similarity with other glucansucrase enzymes,
but has a relatively large N-terminal variable region. Truncation of the enzyme,
by deletion of the variable region, had no effect on the linkage distribution of the
α
(
α
1
→
3
)
Seibel and coworkers succeeded in constructing various new complex glycocon-
jugates containing thioglycosidic linkages to different glycopyranosides (galactose,
glucose, neuraminic acid) (producing branched thiooligosaccharides) by changing
the chemoselectivity of the various glucansucrases from
α
-1,6- to
α
-1,2-,
α
-1,3- or
α
Seibel and coworkers also showed that the mutagenesis is an effective tool for al-
tering the regioselectivity and acceptor-substrate specificity of glucansucrase GTFR
of
Streptococcus oralis
, a dextran-producing enzyme. By random mutagenesis, they
were able to switch the regioselectivity and acceptor specificity of GTFR of
S. oralis
towards synthesis of (a) various short chain oligosaccharides or (b) novel (mutan)
polymers with completely altered linkages, without compromising its high transg-
3.4
Levansucrase
Dijkhuizen and coworkers identified and characterized a Lactobacillus levansucrase
(systematic
name:
sucrose:[6)-
β
-
D
-fructofuranosyl-(2
→
]n
α
-
D
-glucopyranoside
6-
-
D
-fructosyltransferase; EC 2.4.1.10) from
L. reuteri
strain 121, which could
β
3.5
Amylomaltase
Glycosyltransferases are also used extensively to modify natural polysaccha-
rides. Thermoreversible gels that retrograded reversibly - comparable to gelatin
