Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 155

connective tissues, and in cell membranes. Glycans make up 10-60% of the
molecular weight of glycoproteins. The diverse biological functions that these
macromolecules perform include acting as energy sources, as structural
components, as key elements in enzymatic catalysis, hormonal control, immun-
ological protection, ion-transport, blood clotting and lubrication, in various
molecular recognition processes including bacterial and viral infections, cell
adhesion in inflammation and metastasis, differentiation, development and
many other intercellular interactions and signal transduction events [3-5].
Oligosaccharides were regarded as compounds completely lacking biological
specificity. However, the discovery of the role of protein bound saccharides in
biological recognition changed this view. The large structural diversity of pro-
tein conjugated glycans,complexity of the biosynthetic pathways,tissue specific
and developmentally controlled expression, and plasticity of the structure of
complex sugars of glycoproteins in response to pathological conditions lends
further support to their importance. Glycans in glycoproteins have diverse bio-
logical roles,therefore becoming one of the main topics for discussion.Since the
majority of the candidate therapeutic proteins are glycosylated and produced by
recombinant DNA technology, alternate systems to express them and charac-
terization of the respective glycan structures have been discussed.
2
Site and Events of Glycosylation in Eukaryotic Cells
The majority of secretory as well as plasma membrane proteins and lysosomal
enzymes are glycosylated and have a common biosynthetic origin on the rough
ER [6].Nascent proteins are translocated into the cisternal space of the ER where
the signal peptide is cleaved, initial cotranslational folding and formation of
disulfide bonds occurs, addition and initial processing of high Man N -linked
14-saccharide core unit (Glc 3 Man 9 Gn 2 ) takes place, and finally, for multimeric
proteins, oligomerization or subunit assembly is attained before these become
competent to be transported out of the ER. The semiprocessed proteins are
translocated to the Golgi apparatus, further processed, and then either trans-
located to the cell surface or packaged into secretory vesicles for secretion [7,8].
2.1
Entry in the Secretory Route
The majority of proteins to be targeted to the secretory route are initially
synthesized as precursors containing a hydrophobic signal being either a cleav-
able N -terminal peptide [9] or a noncleavable internal sequence located near the
N- terminus [10]. Signal peptides of some proteins have been shown to mediate
efficient secretion in heterologous environments.
Protein translocation across the ER membrane can occur by two pathways.
Most secretory proteins are cotranslationally translocated in contrast to post-
translational translocation destined for other cellular compartments. Transport