Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 168

peptide structure.In factor-X activator of Russell's viper venom,the core glycans
are involved for maintaining integrity of secondary structure and not the peri-
pheral glycans [83].
The stabilizing effect can even be brought about by a single sugar residue [2].
The attached glycans can stabilize protein conformation by forming hydrogen
bonds or having other hydrophobic interactions with the polypeptide backbone.
Circular dichroism (CD) spectroscopy of model glycopeptides suggests that
bound glycans interact directly by hydrogen bonding with the peptide backbone
to stabilize a particular structure [84]. In human corticosteroid binding protein
it has been presumed that the glycan interacts with a tryptophan residue in the
protein to create a stable steroid binding site [85].
A high degree of glycosylation induces a well-defined saccharide conforma-
tion and an extended peptide backbone structure.The radius of gyration,a mea-
sure of the statistical average distance of the end of the chain to its centre, of
mucin chain is 2.5 to 3-fold larger than that of a denatured polypeptide chain of
equal number of amino acid residue [86]. The glycans tend to stiffen the poly-
peptide backbone and the structure of mucin approaches that of a rigid rod.
This effect of bound carbohydrate on conformational stability is important
for orientation and function of membrane bound glycoproteins. In an another
study with five model glycoproteins from various sources, differential scanning
microcalorimetry and CD spectroscopy indicated that glycans have an apparent
stabilizing effect on conformation and enhance thermal stability [87]. Thus,
glycosylated enzymes expressed in S.cerevisiae are more heat stable than their
unglycosylated forms expressed in E. coli [88].
The stereochemical features of the N -glycosidic linkage between the first Gn
and Asn, important for the orientation of glycan chains, have been statistically
analyzed employing 44 different glycosylation sites belonging to 26 glycopro-
teins [89]. It was found that N -glycosylation does not significantly change the
rotamer distribution for the Asn side chain as compared to nonglycosylated Asn.
Carbohydrates have a high hydrogen bonding potential, but bonding between
Gn and peptide is infrequent and Gn shows a general tendency to extend into the
solvent. Freedom of rotation about the glycosidic bonds and solvent exposed
nature accounts for the flexibility of attached glycans. However, steric limita-
tions often restrict the rotation about some of the linkages [90].
In most X-ray crystal structures of glycoproteins, only the first 1-4 sugar
residues most proximal to the glycosylation site are defined by proper electron
density, except in few cases where longer fragments of carbohydrates have been
resolved [91]. Hydrogen bonds and hydrophobic interactions between polypep-
tide and attached glycan have been observed. Study of crystal structure of gluco-
se oxidase showed that the N -linked Man residues form hydrogen bonds with the
backbone N and the carbonyl O of Glu [92]. Computer simulation of molecular
dynamics of ribonuclease B also revealed possible hydrogen bonding of N of Lys
side chain with the ring O and the hydroxymethyl group of glucosamine [93].
The partial NMR spectroscopic studies of intact glycoproteins so far report-
ed, indicates that the core Gn of N -glycan at amino-terminal adhesion domain