Chemistry Reference
In-Depth Information
Figure 7.1
O
- Glycan biosynthetic pathways.
The diagram shows the formation of core 1 and
core 3 from the addition of the fi rst GalNAc
serine/threonine acceptors on the polypeptide
chain. The red arrows indicate the favored
pathway in gastrointestinal tissues for core 3
biosynthesis, relative to the alternative core 1
pathway in blue. Some examples of branch-end
additions are shown (see also Figure 27.4 for
the regulation between core 1 extension and
core 2 generation).
monosaccharides, further adding to
O
-glycan structural variety and identity (see
Info Box 1 ).
The addition of
O
-glycan chains to polypeptide serine and threonine residues
does not follow the tripeptide sequon model for
N
- linked oligosaccharides. Poten-
tially any serine or threonine may be a site and, in contrast, to
N
- glycan sites often
appear as clusters. No general consensus sequence has been identifi ed; however,
a series of general rules have been identifi ed and an algorithm has been developed
to predict
O
- glycosylation sites [4]. The family of UDP-
N
- acetylgalactosamine:
polypeptide
N
- acetylgalactosaminyltransferase s ( ppGaNTase s), which add the fi rst
GalNAc residues to serine and threonine acceptors, show selection which is regu-
lated at the enzymatic level (see Info Box 2). Common features which determine
