section 16.1
Biosynthesis of Glycoproteins
317
castanospermine inhibit glucosidase I, which catalyzes
removal of the outermost glucose residue. Deoxynojir-
imycin is also a competitive inhibitor of sucrase in the
intestinal brush border, while castanospermine causes gas-
trointestinal irritation when consumed as a constituent of
the Australian legume
Castanospermum australe.
Bromo-
conduritol, a derivate of myo-inositol, is an irreversible
inhibitor of glucosidase II, which acts after glucosidase I
to remove both inner glucose residues. However, it inter-
feres with removal of only the innermost sugar, the second
step catalyzed by this enzyme. Swainsonine, a mannosi-
dase inhibitor, is a toxic constituent of locoweed; it inhibits
lysosomal a-mannosidase, causing accumulation within
lysosomes of mannose-rich oligosaccharides. In oligosac-
charide processing,
swainsonine inhibits
a-mannosi-
dase II, which acts late in the trimming process and leads
to side chains with hybrid (partly high-mannose, partly
complex) structures.
Finally, drugs that disrupt the integrity of the Golgi
apparatus (monensin, brefeldin A, bafilomycin, okadaic
acid, nocodazole) will alter glycoconjugate synthesis. The
molecular target of some of these drugs remains unknown;
however, some of these drugs disrupt vesicle transport
(brefeldin A) and others mimic the dispersion of Golgi
that takes place during mitosis (okadaic acid).
O-Glycan Ser(Thr)-Linked Oligosaccharides
Oligosaccharide chains linked by O-glycosidic bonds to
seryl or threonyl residues show more structural variability
than do the asparagine-linked oligosaccharides; the serine
(threonine)-linked oligosaccharides range in size from
1 to 20 or more monosaccharide residues. The residues
are added one at a time, directly on the protein, rather
than preassembled on a lipid carrier. Assembly is not
random, however, and the glycosyltransferases involved
have acceptor specificities that render certain structures
preferable.
The O-linked oligosaccharides are associated with
mucins (Chapter 10), glycoproteins in which carbohydrate
accounts for at least half the molecular weight and which
are produced by mucus-secreting organs and glands (e.g.,
submaxillary glands and the respiratory, urogenital, and
intestinal tracts). Antifreeze glycoproteins (Chapter 10),
which are O-linked glycoproteins, in the circulation of
certain Arctic fish allow the fish to survive freezing tem-
peratures. Elongation depends on the glycosyltransferases
available and on their acceptor specificities.
Polypeptide a-GalNAc transferase initiates O-glycan
synthesis by attachment of GalNAc to Ser/Thr resides of
proteins. There does not appear to be a specific amino acid
sequence signal for GalNAc transfer; however, Pro is often
S A <x6G alN A c-
G Ic N A c
core 6
1
136
G a lN A c
G Ic N A c
|
(36
core 4
G lc N A c |5 3 G a IN A c -
* -G a lN A c a 3 G a lN A c -
core 5
G Ic N A c
|P6
G a lc |3 3 G a IN A c -
core 1
core 2
FIGURE 16-10
Synthesis of O-glycan cores: (a) core 1 /53-Gal-T; (b) core 3
/S3-GlcNAc-T; (c) core 2 j
86
-GlcNAc-T; (d) core 4 /36-GlcNAc-T;
(e) core 5 a3-GalNAc-T; (f) a
6
-SA-T I blocks further O-glycan synthesis;
(h) possibly a human-specific pathway. [Reproduced with permission from
I. Brockhausen, Clinical aspects of glycoprotein biosynthesis.
C rit. Rev.
C lin. Lab. Sci.
30:81 (1993).]
found near glycosylated Ser/Thr and may provide a three-
dimensional structure that favorably exposes Ser/Thr for
glycosylation. Initiation and most O-glycosylation takes
place in the cis-Golgi. Six O-glycan core types have
been described and their synthetic pathways are shown
in Figure 16-10. The elongation of core O-glycans de-
pends upon the types, amounts, and localization of glyco-
syltransferases and proceeds in a fashion similar to that of
N-glycans. The elongation reactions of core 1 O-glycan
are shown in Figure 16-11; because of the specificity of
transferases for their substrates only certain pathways are
possible. Sialylation and sometimes fucosylation of sub-
strates may act as a STOP signal and terminate further
branch elongation of oligosaccharide chains. The products
of
a
2-fucosyltransferase (g in Figure 6-11) can be glyco-
sylated either by A-dependent a3-GalNAc-tranferase or
B-dependent a3-Gal-transferase, or by a combination of
both to give rise to the blood group antigens A, B, or AB,
respectively.
The A and B transferases have been mapped to
human chromosome 9q34 and only differ by four amino
acids. In the absence of both transferases, the O (or H)
blood group will contain the
a2
fucose. Individuals miss-
ing the blood group H a2-fucosyltransferase are said
to contain the “Bombay” type blood group and are se-
riously at risk from transfusion reactions from any of
the common blood types. Terminal sequences contain-
ing blood group antigens also occur on other glycoprotein
and glycolipid oligosaccharides. Sulfate may also occur
