SECTION 16.1
Biosynthesis of Glycoproteins
323
syndecan-like proteoglycans. Chemical modification or
enzymatic removal of proteoglycan from the cell surface
prevented gonococcal attachment and subsequent inva-
sion. An understanding of the GAG fine structure involved
in such mechanisms may be useful medically to prevent
such infections.
The sugar residues in these polymers are often mod-
ified by N-deacetylation and N- or O-sulfation. The
sulfotransferases use phosphoadenosine-phosphosulfate
(PAPS; “active sulfate”; see Chapter 17) as the source
of sulfate groups. A modification unique to glycosamino-
glycans is epimerization of glucuronic acid residues to
iduronic acid, which occurs before polysaccharide poly-
merization. Because modification of the glycosaminogly-
can chains is often incomplete, extensive microhetero-
geneity in the oligosaccharides is not related to the
glycosyltransferase reactions.
The chondroitin sulfates, heparin, and heparan sulfate
are composed of
1. A core protein;
2. A carbohydrate-core protein link region that contains
xylose, galactose, and glucuronic acid;
3. A repeating disaccharide that undergoes extensive
postsynthetic modification.
Based on the “one enzyme-one linkage” concept, at
least six glycosyltransferases and four or more modi-
fying enzymes are necessary for their biosynthesis. Ex-
cept for keratin sulfate, which has both N- and O-linked
oligosaccharides, the biosynthesis of most proteoglycans
begins with synthesis of the core protein followed by xy-
losylation of specific Ser amino acids in the polypeptide.
The consensus sequence for xylose attachment is Ser-Gly
in regions predominantly containing acidic amino acids
and devoid of basic amino acids (using the example of
syndecan synthesis). Xylosylation probably begins in the
endoplasmic reticulum and continues in the Golgi. The
oligosaccharide linker is completed by the sequential at-
tachment of two Gal and one GlcUA residues. At this
point the attachment of either GalNAc or GlcNAc to this
linkage oligosaccharide will direct the synthesis toward ei-
ther CS/DS or heparin/HS type GAGs, respectively. The
hexosaminyltransferases that attach GalNAc and GlcNAc
to the link oligosaccharide are not the same transferases
that carry out the subsequent polymerization reaction. The
polymerization reaction alternates the addition of more
hexosamine residues and GlcUA or Gal to form large het-
eropolymer GAG chains. These chains are then sequen-
tially modified by variable N-deacetylation/N-sulfation,
and/or O-sulfation, and possible epimerization of GlcUA
to IdUA. Sulfation is catalyzed by sulfotransferases that
use PAPS to modify sugars at 2'-, 3'-,
A'-
or
6
'-positions
depending upon the sugars. These modifications result in
the fine structural differences in the GAGs that are im-
portant for their function. Heparin and heparan sulfate
use the enzymes UDP-GlcNAc: GlcUA oligosaccharide
GlcNActransferase and UDP-GlcUA:GlcNAc oligosac-
charide glucuronyltransferase, in that order. For chon-
droitin sulfates, the GlcNAc transferase is replaced by
GalNAc transferase. Epimerization of glucuronic acid
and O-sulfation of C
4
and C(, of galactosamine and C
2
iduronic acid complete the synthesis of chondroitin sul-
fates. There is usually one sulfate per disaccharide unit,
but the particular uronic acid present and the position of
the sulfate are variable and lead to microheterogeneity.
Regulation of the degree and position of sulfation may
be important for function. Indeed, two diseases of pro-
teoglycan synthesis, the
diastrophic dystrophy type of
osteochondrodysplasia
and
macular corneal dystrophy
type I,
both result from the absence of sulfate groups on
proteoglycans.
In heparan sulfate and heparin, deacetylation of the
GlcNAc residues in the repeating disaccharides is a prereq-
uisite for further modification, and the activity of deacety-
lase may regulate the extent of modification. For exam-
ple, a specific pentasaccharide sequence in heparin binds
to and activates antithrombin III (Chapter 36). This se-
quence, containing at least four sulfate groups, is required
for binding. Since N-deacetylation must precede addition
of these groups, the deacetylase partly regulates the anti-
coagulant activity of heparin. Heparin also has some sul-
fate groups at C
3
of the glucosamine residues within the
antithrombin Ill-binding sequence.
Other modification reactions include N-sulfation of the
glucosamine residue, formation of iduronate by epimer-
ization of glucuronate residues, and O-sulfation of
C&
on
glucosamine and C
2
on iduronic acid. Epimerization may
be linked to O-sulfation of iduronic acid, since experimen-
tal elimination of PAPS
blocks the epimerization reac-
tion. O-sulfation may stabilize the iduronic configuration
around C
5
, favoring its formation.
Heparin, but not heparan sulfate, is a good anticoagu-
lant, probably because of higher charge density. Heparan
sulfate is found in virtually all tissues, but heparin appears
to be produced only by mast cells. Their different distribu-
tions may reflect differences in activity of the deacetylase.
Hyaluronic acid (HA) is not sulfated and is the only
glycosaminoglycan that is not part of a proteoglycan. Con-
sequently, chain initiation does not require a core protein.
HA may be polymerized at the plasma membrane by se-
quential addition of GlcNAc and GlcUA from the corre-
sponding UDP-sugar to the reducing ends of the growing
polysaccharide. HA is a major constituent of the vitreous
humor and synovial fluid of the eye.
