Chemistry Reference
In-Depth Information
provides mechanical stability during peristalsis, regulates food movement through
the tract, and yields protection against gastric acid, proteolytic digestion and bile
salts [12] .
Due to their high density of glycosylation the mucins have proved to be valuable
affi nity ligands for the purifi cation of lectins (see Chapters 15.3 and 18.3 for
examples) and other carbohydrate- binding proteins.
Finally, a number of proteins are known to interact with secreted mucins.
However, the precise nature of
O
-GalNAc glycosylation and glycan recognition on
these interactions has not yet been elucidated. A member of the trefoil factor
family of peptides can bind to human MUC5AC so that both molecules are
secreted together into the stomach, forming a gel that has enhanced mucous vis-
cosity and protective capacity. Also, it is known that a number of interleukins have
high affi nity for glycolipids and glycopeptides, and they can bind to mucins. It is
suggested that the formation of cytokine reservoirs at mucosal surfaces could
enable their rapid release affecting local immune reactions.
7.6
Conclusions
This chapter reviews glycans attached to proteins through
O
- glycosidic linkages.
Five different types of
O
-glycan links are considered, and the sequence of glycans
found in the fi ve groups relates to their distribution in different proteins and their
expression at cellular and tissue levels. Elucidating the different glycan structures
and their modes of synthesis and function is important to evaluate their biological
signifi cance. The range of structures found among the fi ve groups varies consider-
ably. The
O
-GalNAc (mucin-type) constitutes a considerable database of struc-
tures, while the
O
- Man,
O
- Fuc/
O
-Glc and, particularly, the
-
O
- GlcNAc, as a single
monosaccharide, establish far fewer structures. These variations correlate well
with the biological roles of the proteins they are linked to and also refl ect the wider
functions ascribed to the
O
- GalNAc, mucin - type group.
The continuing accumulation of
O
-glycans in established databases has served
to allow a better comparison of occurrence and assessment of possible functional
roles.
β
Summary Box
Glycans attached to proteins through
O
-glycosidic linkages cover a number of
discrete types and account for a wide variety of structures that play many roles
in the function of the individual glycoproteins in Nature. The metabolic path-
ways leading to the structures show tight regulation and the potential to process
the glycan sequence in relation to biological function.
