Biomedical Engineering Reference
In-Depth Information
[74]. Similarly, viral spike glycoproteins depend on N -glycosylation for proper
folding and transport to cell surface. Those with an absolute requirement for
glycosylation include the D -glycoprotein of herpes simplex virus, the sendai
virus glycoprotein,the hemagglutinin of influenza X31 and the vesicular stoma-
titis virus-Indiana glycoprotein [75]. The initial step in human immuno-defi-
ciency virus (HIV) infection involves the binding of gp120 to the cell surface
molecule CD4, and N -glycans are found to be essential for generation of proper
conformation of gp120 to provide a CD4 binding site [76].
There are two potential N -glycosylation sites in human IFN
.Proper glycosy-
lation is found to be essential for dimerization,efficient secretion and biological
activity of IFN
g
exhibited only 50% of the antiviral
activity of the native molecule. It has been found that glycosylation at Asn 25 is
essential in the folding and dimerization of newly synthesized molecules, which
also provide resistance against common cellular proteases [78].
Again, Matzuk and Boime have shown that the assembly of N -glycosylation
mutants of hCG
g
[77]. Nonglycosylated IFN
g
-subunit with its counter subunit is decreased due to an altera-
tion in folding [79]. Out of two N -glycans (Asn 30 and Asn 13 ), Asn 30 glycan is
found to be more important for efficient folding of hCG
b
b
. However, once hCG
b
folds correctly, the N -glycans are no longer involved in its assembly with
a
-sub-
unit [80]. Furthermore, the composition of N -linked glycans of hCG
did not
change protein folding,as substrates with high-Man glycans fold as efficiently as
substrates containing complex glycans. Inefficient folding of hCG
b
lacking both
N -glycans correlated with the slow formation of last three disulfide bonds in the
hCG
b
b
folding pathway. However, coexpression of hCG
a
gene enhanced folding
and formation of disulfide bonds of hCG
a
subunit,the Gn residue at Asn 78 seems to be crucial for folding and maintenance
of stability. In an interesting observation it has been found that minor modi-
fications in N -glycans of placental hCG
b
lacking in N -glycans [81]. For hCG
-subunits.
These modifications include a higher degree of glycan branching, as evidenced
by larger amounts of Fuc, SA, Gal and Gn in hCG
a
prevent combining with
b
subunit [82]. In a compara-
tive study of in vitro folding of glycosylated and nonglycosylated hCG
a
,it re-
veals that most of the nonglycosylated protein folded into biologically inac-
tive form [unpublished findings].
b
3.2
Protein Stabilization and Structural Integrity
Glycans stabilize glycoprotein structure, decrease global dynamic fluctuations,
and prevent degradation and protect proteins from the unfolding state. The
terminal sugar residues (antennae) often serve as recognition markers and
modulate biological functions such as cell-adhesion, cell-extracellular matrix
interactions or protein clearance from circulation through surface interactions.
The core sugar residues function primarily as supporting structures between
the polypeptide and the outer sugar residues.The core sugar residues are neces-
sary and sufficient for structural integrity and in maintaining a functional poly-
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