Glycosylation of a protein entails the covalent attachment of carbohydrate groups (typically oligosaccharides) and the resulting modified protein is called a glycoprotein. Covalent attachment of sugar residues to proteins occurs in the endoplasmic reticulum (ER) and Golgi apparatus. The oligosaccharide chains usually contain less than 15 sugar residues but are very diverse and are often branched. They are linked to the protein component via either the –OH groups of serine and threonine, termed O-glycosylation, or the –NH2 group of asparagine, termed N-glycosylation.
Few proteins ultimately destined for the cytosol are glycosylated. However, many proteins that are either secreted into the extracellular space or localised to noncytosolic membrane surfaces (e.g. the extracellular surface of the plasma membrane or the lumenal surface of the Golgi or lysosomes) are glycosylated. Generally these proteins are synthesised in the rough ER. Glycosylation of newly synthesised polypeptides actually affects their folding.
How does glycosylation affect the protein and what is the function of this type of modification? Glycosylation may increase the protein's resistance to proteases.
The oligosaccharide chains do not fold up into compact structures in the way that proteins do; they tend, therefore, to occupy more space and restrict access of proteases. In the same way, at the cell surface, glycosylated proteins may offer some protection to the cell. They also mediate cell–cell adhesion and specific molecular interactions involved in cell–cell recognition. For example, the recognition of carbohydrate groups by a family of proteins called selectins is essential for cell–cell adhesion.