Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 191

and terminal regions. Oligosaccharides of glycoproteins are far more complex
than the peptide chain. With regard to proteins, DNA serves as the template for
RNA synthesis which in turn serves as a template for protein synthesis. In con-
trast, the biosynthesis of oligosaccharides does not occur from a template or
blueprint but results from a complex series of reactions capable of generating
many diverse structures. The observation that a single pituitary cell type is cap-
able of synthesizing two distinct glycoprotein hormones which have, at least in
part,distinctly different oligosaccharide structures indeed suggests that the pro-
cessing of oligosaccharides is not a random event.
The glycan structures of recombinant glycoproteins differ with the host cell
type,despite having the same polypeptide structure.This is mainly as a result of
species-specific and tissue-specific glycosylation. Since the nature of glycosyla-
tion could affect the biological activities of proteins, it is preferable to use cell
lines for the expression of recombinant therapeutic glycoproteins of identical
properties to those of native proteins.There is a need to develop mutants of yeast
and insect cells which on one hand do not produce hyper-mannosylated pro-
ducts, and on the other are capable of terminal sialylation of the expressed
proteins. In this respect, co-expression of ST and GalNAcT with the protein of
interest is important. Concurrently it is also important to exploit and develop
mouse NSO cells for expression of glycoproteins. Biological half-life is the most
important pharmacokinetic property of a glycoprotein, increasing that will not
only reduce the clinical doses of therapeutic proteins, but will also reduce the
chances of autoimmune types of diseases. Potentially, by changing amino acids
at a maximum of three sites, it is possible to introduce a new glycosylation site
and hence to improve available bioactivity of a therapeutic glycoprotein.
Attempt have been made to make fusion protein with 37 amino acids from C -ter-
minus of hCG to improve the biological half-life and hence the available bio-

activities of other glycoproteins.
Controlled carbohydrate modeling is a recent exciting development in the
area of glycobiotechnology,but at present is too far from the point of utilization.
In vitro applications of GlycTs and glucosidases to modify oligosaccharide
structures of the purified recombinant glycoprotein is another option for rede-
signing glycan structure, although it still needs more work before it is technolo-
gically and economically feasible.

b

Acknowledgments. The authors are grateful to the Department of Biotechnology, Government
of India for funding the National Institute of Immunology to carry out the work. The authors
are also grateful to Professor Abdesh Surolia, Molecular Biophysics Unit, Indian Institute of
Sciences, Bangalore and Dr. Chitra Mandal, Indian Institute of Chemical Biology, Calcutta for
providing constructive suggestions while this article was being written.