Glycosylation and Glycosylphosphatidylinositol Membrane Anchors

Glycosylation is one of the most common covalent modifications of proteins. It is not normally a reversible phenomenon and does not play a role in regulation of protein activity. Both glycoproteins and glycolipids are especially abundant in the plasma membrane of eukaryotic cells where the oligosaccharide chains face the extracellular space. In general, the carbohydrate moieties of glycoproteins play little or no role in the biological functions of the proteins and, in the case of enzymes, generally do not participate in catalytic activity. However, because of the hydrophilicity of the sugars, covalently attached carbohydrates maintain the solubility of glycoproteins and ensure the correct folding of the extracellular domains. Glycans also can protect peptide chains from proteolysis, and provide for intercellular recognition and adhesion. Many proteins of importance to neural function, e.g., enzymes, receptors, ion channels, cell adhesion, and axonal guidance molecules, are glycoproteins in nature.