Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 164

Table 2. Factors regulating Asn-linked glycosylation

1. Array and activities of Golgi GlycTs, including GnT, GalT, FucT, ST
2. Polypeptide structure and physical accessibility of oligosaccharides to enzymes
3. Oligomerization of glycoprotein subunits
4. Order in which the glycoprotein encounters the processing glycosidases and GlycTs
5. Availability of dolichol and dolichol-linked donors
6. Transit time of glycoprotein in ER and Golgi
7. Competition between two or more GlycTs for a common substrate

several different glycan structures (site microheterogeneity). Some factors that
contribute to the regulation for the biosynthesis of Asn-linked glycans are given
in Table 2. The polypeptide structure and overall conformation strongly in-
fluence the type of modification that glycan chains undergo. Glycosylation sites
committed to becoming complex type structures are relatively more exposed.
The nature of glycoforms found in a glycoprotein is species and tissue specific,
and cell development and differentiation are accompanied by alteration in
glycosylation patterns [51]. Since this structural variation is confined to the
terminal glycosylation sequences, the synthesis must be highly regulated at the
processing level by Golgi GlycTs [52]. The number of glycosidases and GlycTs
involved in the synthesis of both the N - and O -linked glycans is estimated to be
over 100 [53]. In general, each enzyme is specific for the structure of an accep-
tor oligosaccharide and adds a monosaccharide in a particular linkage at a pre-
cise location. The high level of specificity displayed by GlycTs allows them to
synthesize complex structures with high degree of fidelity.

2.4
The Biosynthesis of Ser/Thr-Linked Glycans

The O- linked glycoproteins contain an -glycosidic linkage between a GalNAc

and the hydroxyl group of a Ser or Thr residue of peptide chains.The chain elon-
gation requires the sequential addition of monosaccharide residues. There are
eight core structures that have been identified in O -linked glycan [54], side
chains to which may be branched and have varying degrees of complexity. A
common feature of all O -linked glycans is the presence of nonreducing terminal
a

a

-linked sugars (SA and L -Fuc) and the absence of Man and Glc residues. The
structure of mucine-type oligosaccharide is shown in Fig. 5. O -Glycosylation is
a post-translational event, taking place in the Golgi after N -glycosylation,
folding and oligomerization, and is limited to residues present at protein sur-
face. Rules that govern placement and structure of O -glycans on glycoproteins
remain unclear and little is known about factors that start O -glycosylation steps.
A cumulative specificity model, deduced from the amino acid sequences
surrounding 90 Ser and 106 Thr O -glycosylation sites, has been inferred for the
acceptor substrate specificity of GalNAcT, catalyzing the first committed step of
O -glycosylation. The specificity is consistent with the existence of an extended