Carbohydrate Antigens
285
Fig. 4
Torsion angles of
α
(1
4) and
α
(1
6)linkages. The glycosidic bond
with the greatest conformational
flexibility is the
α
(1
6) glycosidic bond.
Its flexibility is attributable to the fact
that three torsion angles are required to
deFne the conformation of this
glycosidic bond.
O
O
CH
OH
HO
HO
O
O
CH
OH
HO
HO
f
y
O
HO
CH
2
HO
HO
O
O
CH
2
HO
HO
f
y
O
w
a
(1
4)
a
(1
6)
HO
Thetypeofl
inkagea
lsodeterm
inesthe
chain conformation or the 3D structure
of the intact polysaccharide. In polysac-
charides, all component monosaccharides
exist in the ring conFguration, except for
the terminal-reducing residue, in which
there is equilibrium between the ring and
its open-chain form. The geometry of the
individual sugar rings in a polysaccha-
ride is essentially rigid. However, adjacent
monosaccharides are potentially rotatable
around their glycosidic bonds. The range
of rotation varies for different glycosidic
linkages and is limited by steric hindrance
between adjacent rings and by hydrogen-
bonding patterns between groups in adja-
cent residues. Depending on sugar chain
linkage types, homopolysaccharides may
exist in four different conformations: the
extended ribbon (Type A), the flexible he-
lix (Type B), the crumpled ribbon (Type
C), and the flexible coil (Type D). The
fourth type has the most flexibility, as seen
in
α
(1
6) dextran. Differing from other
glycosidic bonds, three torsion (rotational)
angles,
φ
,
ψ
and
ω
are required to de-
Fne the conformation of a
α
(1
6)linkage.
Irregularities in composition and the ex-
istence of large branched structures can
inhibit regular conformational patterns.
Oligosaccharides with terminal branches
are relatively rigid in conformation. Such
structures are frequently seen in alloanti-
gens and serve as the key carbohydrate
structures
for
immunorecognition.
As
shown in ±ig. 2, addition of
α
(1
2)linked
fucosyl residue in the linear sugar chains
generates the terminal branch-type struc-
tures of human blood group A, B, and
H. Terminal branches are seen also in
other blood group substrances, such as the
Lewis (Le) sugar series, which are a fam-
ily of fucosylated glycans (±ig. 5). Sialyl
Le
x
is a ligand for E-selectin. Its solution
conformation was studied using proton
NMR to assign each proton and advanced
NOE (Nuclear Overhauser Effect) mea-
surements to determine inter-proton dis-
tances. Unlike other oligosaccharides with
multiple conformers in solution, Le
x
is pre-
dominantly in a single rigid conformation.
The conformation of sulphated Le
a
approx-
imates also that of the unsulfated analog
(Le
a
). The enhanced binding of sulfated
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