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chapter 10
Heteropolysaccharides !: Glycoproteins and Glycolipids
A (3-N-glycosidic linkage b e tw e e n N -acety lg lu co sam in e a n d th e
a m id e n itro g en of a n a s p a ra g in e re sid u e of th e protein (G IcN A c-A sn).
L inkage of w id e o c cu rre n ce .
A n a -O -glycosidic linkage b e tw e e n N -acety lg !u co sam in e
a n d th e hydroxyl g ro u p of serin e (R =H ) o r th re o n in e (R =C H 3) re sid u e
of th e p ro tein (G alN A c-S er/T hr). L inkage fo u n d in g ly co p ro tein s of
m u c u s se c re tio n s a n d blood g ro u p s u b s ta n c e s .
A p-O -g ly co sid ic linkage b e tw e e n g a la c to s e a n d a hydroxylysine
re sid u e of th e p ro tein (Gai-Hyl). L inkage fo u n d in co llag en .
C O O H
A p -O -g ly co sid ic linkage b e tw ee n x y lo p y ran o se a n d th e hydroxyl
g ro u p of a s e rin e re sid u e of th e protein (X yl-Ser). L inkage fo u n d in
thyroglobulin a n d p ro teo g ly can s.
H<
Hi
o—c—c
C H — C O O H
I
NHS
F I G U R E 1 0-1
Carbohydrate-peptide linkages in glycoproteins.
to an oligosaccharide chain, known as the outer domain.
The outer domain is made up of either oligosaccharides
consisting of mannose residues (oligomannosidic types) or
N-acetyllactosamine units (complex types, Figure 10-3).
The latter are disaccharides consisting of galactosyl-
—>■
4)-N-acetylglucosamine units.
Glycoproteins with O-glycosidic linkages (Figure 10-4)
do not show the common features of glycoproteins with
N-glycosidic linkages. The number of sugar residues may
vary from one (e.g., collagen) to many (e.g., blood group
substances). Many glycoproteins, however, do show the
presence of a common disaccharide constituent, namely,
galactosyl-/J-fl —> 3)-N-acetylgalactosamine,
which is
TABLE 10-1
Some Functions o f Glycoproteins in the Body
Function
Examples
Structure
Collagen
Lubrication and
Epithelial mucins, synovial fluid
protection
glycoproteins
Transport
Ceruloplasmin (copper carrier),
transferrin (iron carrier)
Endocrine
Thyrotropin, chorionic
regulation
gonadotropin, erythropoietin
Catalysis
Proteases, nucleases,
glycosidases, hydrolases
Defense against
Immunoglobulins, complement
infection
proteins, interferon
Membrane
Receptors for hormones (e.g.,
receptors
insulin), acetylcholine,
cholera toxin, electromagnetic
radiation (e.g., rhodopsin)
Antigens
Blood group substances
Cell-cell recognition
Fibronectin, laminin,
and adhesion
chondronectin
Miscellaneous
Glycophorin (an intrinsic
red blood cell membrane
constituent), intrinsic factor
(essential for absorption of
dietary vitamin B)2), clotting
factors (e.g., fibrinogen)
linked to either serine or threonine. In collagens, the
O-glycosidic
linkages
occur
via
hydroxyproline
or
hydroxylysine residues, which are unique to collagens.
A given glycoprotein may contain oligosaccharide chains
of both N- and O-glycosidic types. If more than one
N-glycosidic carbohydrate linkage site are present in a gly-
coprotein, they are usually separated by several amino acid
residues. In contrast, O-glycosidic carbohydrate linkages
M an « ( 1 - 6 )
M an « ( 1 - 3 )
M an ß(1—4) GIcNAc ß(1—4) GIcNAc ß— A sn
o r sch em atically ,
▼n
F I G U R E 1 0 -2
Structure of the common branched inner domain of oligosaccharides
linked in N-glycosidic linkages with asparagine. Man = Mannose T,
GIcNAc = N-acetylglucosamine • , Asn = Asparagine. [Adapted with
permission from N. Sharon and H. Lis,
C hem . E ng. N ew s,
p. 28 (March 30,
1981) ©1981 by the American Chemical Society.]
