chapter 16
Carbohydrate Metabolism III: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans
309
Oligosaccharide
type
Oligosaccharide Structure
(a) Asn-linked
Complex
(bian ten n ary )
N e u N A c a 2 — 6 G a lß 1 — 4G lcN A c ß l— 4;M ana1 .
t
t
N euN A c a
2—6
G al p1— 4 GIcNAc p1 — A M a n a r
M a n ß l— 4 G IcNAc G IcN A c1— A sn
h
F u c
High-mannose
M a n a t
M a n a t
M a n a l-
Ktenalv
3 M an a 1
M a n ß l— 4 GIcNAc ß1— 4 GIcNAc — A sn
(b) Ser(Thr)-linked
N euN A ca2^_
6\
i
G alN A c a U
G alN A c—
S er(T h r)
3\
/ 3
1
Gal p1
F u c o l
I
(c) Glycosaminoglycan
(Id U A al— 4G lcN A c a1 — 4)
---------
G lcU A —
G a l
—
G al
—
X yl— S e r
' 2
6 t
•
I
l k
1
s o ,
s o ,
I
(d) Collagen
G lc a
1
— 2 G a l
—
O
—
H ydroxylysine
FIGURE 16-1
Structures of representative oligosaccharide groups that occur as parts of glycoconjugates. -S 0
3
= sulfate.
a) Asn-linked: The core structure is shown in the dashed box. The sugar-protein linkage is always an N-glycosidic
bond. In addition to the two types shown here, an intermediate, or mixed, structure has features of both.
b) Ser(Thr)-linked: The structure shown is for one of the oligosaccharides found in porcine submaxillary mucin. The
sugar-protein linkage is an O-glycosidic bond to serine or threonine.
c) Glycosaminoglycan: The example shown is the predominant structure of heparin. The presence of a link
oligosaccharide and the variability in the length of the chain of repeating disaccharide units are features common to
many proteoglycans.
d) Collagen: The carbohydrate groups are attached as a late posttranslational step via O-glycosidic linkages to
hydroxylysyl residues generated in an earlier posttranslational reaction.
The following abbreviations for sugars and related compounds are used throughout this chapter: Gal = D-galactose,
Man = D-mannose, Fuc = L-fucose (
6
-deoxy-L-galactose), IdUA = L-iduronic acid, GalNAc = N-acetyl-
D-galactosamine, NeuNAc = N-acetyl-D-neuraminic acid, NeuNGly N-glycolyl-D-neuraminic acid, Glc = D-glucose,
Xyl = D-xylose, GlcUA = D-glucuronic acid, GIcNAc = N-acetyl-D-glucosamine.
GDP-mannose that is essential for glycoprotein synthesis.
This defect is autosomal recessive and has been mapped
to human chromosome 16pl3.3-pl3.12. All infants with
CDGs have neurological abnormalities, failure to thrive,
developmental delay, and dysmorphic features. Female
patients with CDGs type I have hypergonadotropic
hypogonadism
and
do
not
attain
secondary
sexual
characteristics.
Gal-l-P uridyltransferase deficiency
is autosomal re-
cessive and results in galactosemia. Removal of all
galactose from the diet resolves acute symptoms. How-
ever, long-term outcome may still be poor with frequent
developmental delay, characteristic speech abnormalities,
and ovarian failure. The hypergonadotropic ovarian failure
associated with this deficiency may be related to improper
glycosylation of follicle-stimulating hormone (FSH) that
impairs binding to receptor.
Gal-4-epimerase deficiency
causes severe developmen-
tal delay with poor growth. Acute symptoms can be re-
solved with a galactose-free diet; however, small quanti-
ties of galactose are supplemented to allow for synthesis
of UDP-Gal in the absence of dietary galactose.
Leucocyte adhesion deficiency type II (LADII)
pa-
tients lack products of fucosyltransferases; and therefore,
they express the rare Bombay blood group phenotype and
are negative for the Lewis blood group. They show se-
vere mental retardation, are short in stature, and have
dysmorphic features (low hairline, hypertelorism, broad
