Carbohydrate Antigens
289
X-ray crystallographic characterization of
its 3D structure.
A model system,
α
(1
6) dextran and
antidextran antibodies, was applied to ad-
dress the question regarding sizes of
a carbohydrate-based antigenic determi-
nant. Native dextran N279 is a near-linear
macromolecule of glucose with predom-
inant
α
(1
6)linkage (90%). The relative
structural simplicity of this carbohydrate
antigen and the availability of oligosac-
charides of
α
(1
6)linked glucose made
it suitable for such studies (See Fig. 6).
Sizes of antibody-combining sites were
measured by competitively displacing the
dextran from dextran–antibody complexes
by various isomaltose oligosaccharides.
The combining site is considered to be
complementary in size to the smallest
oligosaccharide giving maximum compe-
tition. Such an oligosaccharide is de±ned
as the epitope or antigenic determinant. It
may vary in size from a lower limit of one
to two glucoses to an upper limit of six
to seven.
3.2
Types of Carbohydrate Epitopes: Terminal
and Internal Structures
A carbohydrate antigen may display both
the terminal sugar moieties and internal
chain structures on its solvent accessible
surface. This is owing to the hydrophilic
property of carbohydrates, making them
strikingly different from proteins. In aque-
ous solution, proteins tend to fold to bring
their hydrophobic side chains together,
forming an oily core with polar side chains
exposed. Surface moieties of a protein
antigen may serve as antigenic determi-
nants interacting with B-cell Ig-receptors;
Oligosaccharide as inhibitors
W3129
Nonreducing end
2
5 sugars
Cavity-type site
QUPC52
Internal chain structure
6
7 sugars
Immunodominant
Groove-type site
Internal
Terminal
a
(1
6)linkage
a
(1
3) linkage
Fig. 6
Isomaltose oligosaccharides as ‘‘rulers’’
to measure the sizes of antigenic determinants.
The diagram illustrates the two different
speciFcities of anti-
α
(1
6) dextran antibodies.
Antibodies bound to polysaccharide-
α
(1
6)
dextran can be either cavity-type for terminal
epitopes or groove-type for internal epitopes.
The size of the antibody binding site can be
determined by using oliogsaccharides of
different number of sugar residues as
competitive inhibitors that displace the
dextran–antibody complexes.
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