Heteropolysaccharides I:
Glycoproteins and Glycolipids
Glycoproteins, a wide range of compounds of diverse
structure and function, are components of cell membranes,
intercellular matrices, and extracellular fluids, such as
plasma. They occur in soluble and membrane-bound forms
(Table 10-1).
The proportion of carbohydrate varies considerably in
glycoproteins derived from different tissues or from var-
ious sources. Collagen, for example, contains an amount
of carbohydrate that varies with the source: skin tissue
collagen, about 0.5%; cartilage collagen, about 4%; and
basement membrane collagen, more than 10%. Glycopro-
teins containing high amounts of carbohydrate include gly-
cophorin, a membrane constituent of human erythrocytes,
about 60%, and soluble blood group substances, as much
as 85%.
In glycoproteins, the protein and the carbohydrate
residues are bound in covalent linkage. There are five com-
mon types of carbohydrate, four of which are common to
human glycoproteins (Figure 10-1). All the linkages are ei-
ther N- or O-glycosidic bonds. The amino acid residue that
participates in the N-glycosidic linkage is asparagine, and
the amino acid residues that participate in O-glycosidic
linkage are serine, threonine, hydroxylysine, and hydrox-
yproline. The glycoproteins exhibit microheterogeneity,
i.e., glycoproteins with an identical polypeptide sequence
may vary in the structure of theire oligosaccharide chains.
Microheterogeneity arises from incomplete synthesis or
partial degradation and poses problems in the purification
and characterization of glycoproteins. For example, hu-
man serum a i-acid glycoprotein has five linkage sites for
carbohydrates and occurs in at least 19 different forms as
a result of differences in oligosaccharide structures.
The oligosaccharide side chains of glycoproteins con-
sist of only a limited number (about
1 1
) of different
monosaccharides. These monosaccharides are hexoses
and their derivatives (N-acetylhexosamine, uronic acid,
and deoxyhexose), pentoses, and sialic acids derived
from neuraminic acid, a nine-carbon sugar. The most
common of the many different types of sialic acids is
N-acetylneuraminic acid. (All of these sugars are in the
D-configuration, unless indicated otherwise.) The sugar
residues of the oligosaccharide chains are not present in
serial repeat units (unlike the proteoglycans, Chapter 11),
but they do exhibit common structural features. For exam-
ple, the oligosaccharide chains bound in N-glycosidic link-
ages may be envisaged as consisting of two domains. The
inner domain, common to all glycoproteins, is attached
to the protein via an N-/Tglycosidic linkage between an
asparaginyl residue and N-acetylglucosamine. The inner
domain consists of a branched polysaccharide, which is
trimannosyl-di-N-acetylglucosamine (Figure 10-2). The
peripheral mannose residue of the inner domain is linked
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