Carbohydrate Antigens
291
was seen in crystallized carbohydrate an-
tibody complexes, the crystal structure of
a monoclonal Fab (YsT9.1.) and a
Bru-
cella
A cell wall polysaccharide showed
binding between a groove-type site and
an internal chain epitope of microbial
polysaccharide. It discriminates the
Bru-
cella abortus
A antigen from the nearly
identical
Brucella melitensis
Man
t
igen
.By
forming a groove-type binding site, lined
with tyrosine residues, this antibody ac-
commodates the rodlike A antigen but
excludes the kinked structure of the M
antigen. This is consistent with immuno-
chemical data that the binding site of
this groove-type anti-Brucella A polysac-
charide antibody can be optimally ±lled by
a pentamer or hexamer of
α
(1
2)linked A
polysaccharide. The size of this epitope is
similar to that which accommodates to the
groove-type anti-
α
(1
6) dextran antibod-
ies, but much larger than the cavity-type
anti-
α
(1
6) dextrans.
4
Carbohydrate-protein Interactions
The biological signi±cance of the sugar
chain’s structural diversity depends on
whether cellular machineries can recog-
nize their signals. In fact, at least two
large classes of proteins, lectins and anti-
bodies, are present in living organisms and
serve as specialized ‘‘decoders’’ to decipher
the information content of sugar chains.
Lectins and antibodies represent two dis-
tinct models of protein–carbohydrate in-
teractions. The former is widely seen in
the events of cell–cell interactions and
cell signaling, but is also found in the
innate immune systems as antimicrobe
reactors. The latter is a specialized system
of immune recognition and anti-infection
response. Our main interest here is with
the carbohydrate binding characteristics
and their functional correlations.
4.1
Lectins
Lectins are a large class of proteins or
glycoproteins that can speci±cally or se-
lectively bind carbohydrates. They are,
however, neither enzymes nor antibod-
ies. A wide range of living organisms,
from microbes to mammals, produces
lectins. Apparently, they are conserved
in evolution. According to species origin,
they are classi±ed into at least four cat-
egories, which include microbial lectins,
plant
lectins,
invertebrate
lectins,
and
vertebrate
lectins. In
the
last
decade,
most attention has focused on vertebrate
lectins. On the basis of the similari-
ties in sequence homology and activity,
vertebrate lectins are grouped into ±ve
subfamilies, including C-type (Calcium-
dependent), P-type, S-type, I-type, and
pentraxins.
The carbohydrate binding activity of a
lectin is commonly described in terms
of the monosaccharide speci±city, as sug-
gested ±rst by Professor Y. C. Lee. Exper-
imentally, a type of inhibition assay, such
as agglutination inhibition, is performed
to identify the monosaccharide that most
effectively inhibits the reaction. Lately, dis-
accharides have been considered to be a
better way to de±ne a lectin’s speci±city.
Most lectins are able to cross-react with
a panel of sugar chains with a common
terminal sugar residue.
X-ray crystallographic studies identi±ed
some common characteristics in the bind-
ing pockets of some lectins and anticarbo-
hydrate antibodies. Aromatic amino acid
residues appear frequently in carbohydrate
binding sites of various ±ne speci±cities.
Such residues are large and participate in
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