Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 185

[238].The sulfated glycans found on LH do not affect its bioactivity at the recep-
tor level but do have a marked impact on its circulatory half-life and in vivo
potency. A receptor present on hepatic endothelial cells recognizes glycans
terminating with SO 4 -GalNAc and can account for rapid removal of glycopro-
teins bearing these structures [239].
In hTSH, N -glycans of b -subunit has a more pronounced role than that from

a -sub-

unit of hCG, highly sialylated glycans are important in prolonging plasma half-
life [241].The carboxy terminal peptide (CTP) of

-subunit in metabolic clearance and in vivo activity [240]. Similarly, in b

hCG is also important in pro-
longing plasma half-life of the hormone.This has been ligated to hFSH [242] and
hTSH [243] to generate analogs with increased plasma half-life and bioactivity.
Furthermore, in hTSH, there was a significant decrease in circulatory half-life
upon deletion of glycans at Asn a 78 compared to at Asn a 52 [244], which may be
related to surface-exposed location of Asn a 78 [134].These results are comparable
with the observed site-specific role of glycans in the in vivo activity of hFSH
[245]. The circulatory half-life, and not the in vitro activity, appears to be the
primary determinant of the in vivo activity of these hormones.
Glycoprotein glycans can also affect clearance rate by mechanisms which do
not involve high-affinity receptors. They can prolong glycoprotein circulatory
half-life by increasing both size and surface charge and affecting filtration rate
through the kidney glomerular tubules [229]. Furthermore, highly branched
glycans (tri- or tetraantennary) are less susceptible to renal clearance than bi-
antennary structures as in EPO [246].

b

7
Controlled Carbohydrate Remodeling

There is interest in the development of methods that will permit modification of
glycan structures on therapeutic glycoproteins. This might increase serum half-
life and solubility of drug, decrease antigenicity, promote uptake by target cells
and tissues, improve efficacy and reduce dosage [247]. The glycosyltransferases
as well as glycosidases have been exploited for the synthesis of oligosaccharides
and glycoconjugates. The in vivo function of glycosidases is to cleave glycosidic
bonds, although under appropriate conditions they can be useful as synthetic
catalysts. Glycosyltransferases are highly specific in the formation of glycosides
though, the availability is limited. Glycosidases have the advantage of wider
availability and lower cost, but they are not as specific or high yielding in syn-
thetic reactions.
Despite no PCR equivalent replication system being available for the amplifi-
cation of minute amounts of carbohydrates and no machine being available for
the solid phase synthesis of glycans, advances have been made in their chemical
synthesis. For example, synthesis of core N -glycan structure (Man 3 Gn 2 ) and
oligomannose glycans from monosaccharides have recently been accomplished
[248-250]. At present it is possible to synthesize carbohydrate chains 20 mers
long,to derivatize glycans and link them together,and to employ enzymatic and