Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 176

that the yield and the authenticity of glycosylation vary with the product and the
expression system.The choice of expression system for the production of glyco-
proteins is governed by the following factors:
(a) structural complexity (number of disulfide bonds, oligomerization, extent
and type of glycosylation, etc);
(b) product stability and refolding;
(c) desirability of secretion;
(d) intended use (therapeutic or diagnostic);
(e) yield.
In a strict sense, recombinant proteins should be comparable to their counter-
parts purified from natural sources in terms of biological, clinical and pharma-
cological properties.As the natures of glycosylation modulate these attributes of
a glycoprotein,and no single eukaryotic host cell system is capable of processing
every potential heterologous glycoprotein glycans similar to its ectopic tissues,
it is imperative to examine various expression systems to find the one that can
produce an authentic product. An excellent review on the major glycosylation
attributes of different expression systems and the role of culture conditions on
glycosylation has recently been published [162].
The acceptable limit of glycosylation differences between natural and recom-
binant proteins would depend on the type of product, the intended use and the
pharmacological attainment. For example, glycosylation in the Fc region rarely
affects the immunoreactivity of monoclonal antibodies (MAbs). However, this
does affect Fc receptor binding, antibody mediated cytotoxicity and is involved
in antibody elimination. While glycosylation of Fab regions has variable effects
on the binding activity of MAbs, glycosylation in the hinge region might effect
antibody sensitivity to proteases [163].
The most commonly used expression systems are insect cells with baculo-
virus vectors, yeast and mammalian cells. Filamentous fungi (e.g. Aspergillus
sp.) have also been developed as host cells for the production of heterologous
proteins, although there is need to improve the culture with respect to de-
crease fungal proteases and glycosylation patterns [164]. Unlike higher
eukaryotes, a number of late modification steps, such as Man core processing
and addition of Fuc, Gal, Gn and SA are absent in prokaryotes and simple
eukaryotes [165].
Expression of recombinant proteins in the milk of transgenic animals is
gaining popularity due to simple and cost effective production. Over the past
several years, the feasibility of this approach has been demonstrated by the pro-
duction, at mg ml -1 levels, of pharmaceutically relevant monomeric proteins
such as human 1-antitrypsin, human tPA, human protein C and hexameric

fibrinogen [166]. However there are limitations of mammary tissue in making
the meaningful post-translational modifications, which has been overcome by
coexpression of key processing enzymes [167].

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