Carbohydrate Antigens
283
a
(1
6)
a
(1
6)
a
(1
3)
a
(1
6)
a
(1
4)
a
(1
)
a
(1
4)
O
CH
2
OH
O
O
CH
2
Dextrans in microbes
O
CH
2
OH
O
CH
2
OH
Glycogen in mammal
O
Fig. 3
Schematics of microbe’s dextran and mammalian glycogen: linkages make the
difference. The only source of structural diversity in homopolysaccharides is the glycosidic
bonds linking the monoaccharides. This Fgure illustrates two commonly found
polysaccharides – one in microbes and another in mammals. The polysaccharide with
α
(1
6)linkage is dextran, which is found in many bacteria including
Lactobacillaceae
.The
polysaccharide with the
α
(1
4) is glycogen and it is found in mammals as one of the
energy storage biological molecules.
a class of polysaccharide called
dextrans
(Fig. 3). These macromolecules are com-
posed entirely of glucose residues and are
produced by numerous bacteria of
Lac-
tobacillaceae
. However, dextran molecules
derived from different strains differ sig-
ni±cantly in their glycosidic linkage com-
positions. Some dextran preparations are
predominantly or solely
α
(1
6)linked,
forming molecules with dominantly lin-
ear chain structures; others are composed
of multiple glycosidic linkages, including
α
(1
6)-,
α
(1
3)-,
α
(1
2)- and others,
generating heavily branched molecules.
These bacterial products and mammalian
glycogen differ from each other only in
linkage. Mammalian enzymes cannot di-
gest dextrans as well as most microbial
polysaccharides, and thus they may remain
in vivo
for a long period of time after an
immunization. Linkage largely determines
sensitivity or accessibility of polysaccha-
rides to enzymes and, frequently, their
antigenic reactivities. Branches occur in a
polysaccharide when two or more C-atoms
of one residue are involved in glycosidic
linkage with other residues. Polysaccha-
rides with more branches present more
terminal structures.
Most heteropolysaccharides are com-
posed of two or three different sugar
residues and termed as
diheteroglycans
and
triheteroglycans
, respectively. The het-
eropolymers that consist of more than
six distinct monosaccharides are rarely
seen in living organisms. On the ba-
sis of the organization of sugar residues
in heteropolysaccharides, they are further
previous page 957 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online next page 959 Encyclopedia of Molecular Cell Biology and Molecular Medicine read online Home Toggle text on/off