Carbohydrates are sometimes important for protein folding. They are negatively charged and can interact with amino acid side chains.
For example expression of functional human CD4 on a surface of T-lymphocytes requires glycosylation of either one or two N-linked sites within the third Ig-like domain. Otherwise the protein is improperly folded and cannot be exported from the ER (it is retained there).
Many enzymes are glycoproteins. For example the hydrolase sucrase - isomaltase from microvilli of intestinal epithelial cells have many sugar residues. Such hydrolases belong to the glycocalyx (the loose network built up of the carbohydrates of glycoproteins and glycolipids) which cover the outer membrane surface. Negatively charged sugar residues prevent the close contacts between different enzymes.
Other glycosylated enzymes are the chaperones calnexin and calreticulin.
Carbohydrates can modulate the activity of enzymes. Glycosylation also affects better water solubility of proteins (sugar residues are highly hydrophilic)
Some glycoproteins act as carriers. They can bind to certain molecules such as vitamins, hormones, cations. For example ceruloplasmin is a transport glycoprotein found in vertebrate plasma. It mobilizes iron into the serum.
Glycosylation of proteins can maintain their structure.
In human immunoglobulins the carbohydrate chain wraps around one of the protein domains preventing its contact with the adjacent domain. When the carbohydrate chain is removed, the domain could no longer perform its ordinary function. Because immunoglobulin function is determined to a large extent by its structure, the carbohydrate removing affected the structure of the molecule.
IgG is unique among the serum glycoproteins because it is associated
with at least 30 different biantennary N-linked oligosaccharides. This
extremely high microheterogenity is probably produced because human individuals
have series of B-cell clones equipped with different sets of glycosyltransferases.
Another example of structural (and functional) role of carbohydrates is the VIP receptor (VIP= Vasoactive Intestinal Peptide ; it is a neuropeptide ). It contains at least three tri- or tetraantennary sialylated N-linked oligosaccharides. Some of them have terminal galactose residues. Glycosylation of VIPR is required for stabilization of the receptor and for the proper transport of de novo synthesized receptors to the cell surface.
At least one carbohydrate chain may also be required for high affinity ligand binding (it is unusual among the neurotransmitter receptors).
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The role of glycoproteins