Chemistry Reference
In-Depth Information
Figure 11.1
Structure of the repeating disac-
charide units and the connecting region linking
GAGs covalently to their protein core.
differs from chondroitin 4/6-sulfate in that its predominant uronic acid is
L
- IdoA.
In heparan sulfate (HS) the GlcNAc is
- glycosidically linked to
D
- GlcA or
L
- IdoA
to form characteristic disaccharide repeat units, but HS has a heterogeneous
macromolecular structure. HS contains a few sulfate-rich oligosaccharide units
(heparin-like clusters) that are separated by longer sulfate-free or sulfate- poor
sequences (for the structure of heparin pentasaccharide, see Figure 1.7d). The
sulfate-rich clusters of HS contain
N
- sulfate and
O
-sulfate groups on the C6 of
glucosamine, but also C2 sulfate groups of the
L
-IdoA and C3 sulfate groups - a
distinctive feature of heparin (for clinical applications, see Chapter 28). Keratan
sulfate (KS) is characterized by molecular heterogeneity. The polysaccharide is
composed principally of a repeating disaccharide unit of GlcNAc and Gal with no
uronic acid in the molecule. The sulfate content is variable with ester sulfate
present on C6 of both Gal and GlcNAc. Two types of KS have been distinguished
that differ in their overall carbohydrate content and tissue distribution. They
contain as additional monosaccharides Man, Fuc, sialic acid and GalNAc. KS I
isolated from cornea is linked to protein by a GlcNAc- asparaginyl bond, typical
of
N
-glycoproteins (for further details on
N
-glycosylation, please see Chapter 6 ).
KS II isolated from cartilage is attached to protein through GalNAc in
O
- glycosidic
linkage to either serine or threonine (for further details on
O
- glycosylation, please
see Chapter 7 ).
α
