Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 166

3
Functions
The observed large diversity in the structure of Asn-linked glycans is central to
the hypothesis that these are important in biological functions [3, 7, 28, 61, 62].
It is clear that there is no unifying single specific role of glycans and in some
cases they may not even have any function at all and be completely replaceable.
The effect of carbohydrate on the physicochemical properties of glycoproteins,
such as viscosity, solubility, isoelectric pH, degree of hydration, and other struc-
tural roles have been known for some time [2]. Studies on the native glycosyla-
ted, carbohydrate-depleted, and recombinant nonglycosylated proteins have
revealed such effects as stabilization of protein conformation, protection from
proteolysis and enhancement in solubility [63, 64].
The first clear demonstration of the functional significance of carbohydrate
was in the blood group substances, where the immunological specificity was
found to depend on monosaccharides or short glycan chains. A major break-
through occurred when it was discovered that the removal of SA resulted in
rapid clearance of glycoproteins from circulation [65]. Since then, the role of
glycans in a variety of other functions has been reported. Glycoconjugates play
roles in many cell-cell recognition processes, including metastasis and inflam-
mation. Glycan structures both mediate and modulate cell-cell and cell-matrix
interactions [66].
3.1
Protein Folding and Conformation
Bound carbohydrates can influence protein structure and the effects depend
upon type of sugar, linkage, stereochemistry, size of the bound saccharide and
characteristics of the protein. The conformational effects of protein glycosyla-
tion have been studied using various spectroscopic techniques [67-69]. In one
such study using time-resolved fluorescence energy transfer ( FET), it was sug-
gested that cotranslational glycosylation can trigger the timely formation of
structural nucleation elements, prevent aggregation of partially structured
chains by improving solubility, and generally assist in protein folding [69].
The role of N -glycosylation and disulfide bonds in the folding of proteins has
been studied extensively [70,71].Inhibition of core glycosylation with inhibitors
(e.g.tunicamycin) or site-directed mutagenesis leads to misfolding,aggregation
and degradation of proteins retained in ER [7].Absence of N -glycosylation leads
to impaired lipoprotein lipase secretion and accumulation of inactive protein
in ER [72]. Unglycosylated rabbit testicular angiotensin-converting enzyme
(ACE T) is inactive and rapidly degraded intracellularly. However, allowing gly-
cosylation only at the first or second site (out of five N -glycosylation sites) as
counted from the N -terminus was sufficient for normal synthesis and pro-
cessing of active ACE T [73].
Studies with simian viral hemagglutinin neuraminidase and yeast acid phos-
phatase suggest that N -glycans are needed for proper folding of glycoproteins