282
Carbohydrate Antigens
glycosidic linkages so as to extensively di-
versify their structures. Two amino acid
residues, such as two alanines, can pro-
duce only one possible dipeptide; however,
two molecules of glucose have the poten-
tial to generate 11 different disaccharides.
A trimer of any of the 9 common sugar
residues of the human body theoretically
can give rise to 119 736 different structural
isomers. This is strikingly in contrast to the
maximal construction of 8000 tripeptides
using 20 different amino acid residues.
Theoretically, carbohydrate structures can
have unlimited variation. This potential
makes them well suited to provide various
recognition signals.
Oligosaccharides
and
polysaccharides
are
composed
of
monosaccharides.
A
chain of 2 to 10 monosaccharide residues
is called an
oligosaccharide
.Apo
lyme
ro
f
monosaccharides that is longer than the
oligosaccharide chains of 10 to 20 sugar
un
i
t
sinl
eng
thi
st
e
rm
edapo
l
y
s
a
c
ch
a
-
ride. Usually, an oligosaccharide means
also a structurally deFned carbohydrate
molecule. Polysaccharides are, however,
large polymers of unspeciFed length with
repeated structures. Oligosaccharides may
be simply the shorter stretches or frag-
ments of a polysaccharide. They could also
be the sugar components of glycocojugates
of diverse sources. The oligosaccharides
attached to the glycocojugates of mam-
malian cells frequently consist of multiple
distinct monosaccharide residues linked
in complex structures and associated with
different biological molecules. ±or exam-
ple, the sugar chain structure of human
blood group A substance differs in com-
position from that of group B or group
H by only one monosaccharide residue
(±ig. 2). These sugar structures may link
to either a protein or a lipid molecule,
forming a glycoprotein and a glycolipid,
respectively.
Polysaccharide chains composed of a
single type of sugar residue are termed
ho-
mopolysaccharides
. Those formed by more
than one type of monosaccharides are
named
heteropolysaccharides
.I
nah
o
-
mopolysaccharide, the glycosidic bonds
linking the monosaccharides are the only
determining factor of their structural di-
versity. A well-documented example is
GalNAc
Gal
Gal
Gal
Fuc
Gal
Gal
GlcNAc
GlcNAc
GlcNAc
Gal
Gal
R
A
Fuc
Fuc
a
1-3
a
1-3
a
1-2
a
1-2
a
1-2
b
1-4 or
b
1-3
b
1-4 or
b
1-3
b
1-4 or
b
1-3
R
B
R
H
Fig. 2
Structures of human blood group A, B, and H. This diagram
illustrates three different human blood group antigens A, B, and H. The
addition of
α
(1
2)linked fucosyl residue to these oligosaccharides
provides relatively rigid terminal branches. Within the A, B, or H blood
group, the R group may be either lipid or protein. This kind of terminal
branch is also found in other blood group substances such as the lewis (Le)
sugar series, which are a family of fucosylated glycans (see also Fig. 5).
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