Glykosylation

Glykosylation, also known as glycosylation, is the enzymatic process by which carbohydrate groups are covalently attached to proteins or lipids. This modification generates glycoproteins and glycolipids that contribute to protein folding, stability, trafficking, and interactions on the cell surface and in secreted fluids. The most common forms are N-linked glycosylation, in which sugars are attached to the amide nitrogen of asparagine, and O-linked glycosylation, attached to the hydroxyl groups of serine or threonine residues. A third class includes various less common linkages.

N-linked glycosylation begins in the endoplasmic reticulum with the en bloc transfer of a preassembled oligosaccharide from a dolichol carrier to the nascent polypeptide at a consensus sequence of Asn-X-Ser/Thr (X not proline). After transfer, glucose residues are trimmed as the polypeptide folds and engages ER quality-control factors. The glycoprotein then moves to the Golgi, where further trimming and remodeling yield diverse N-glycan types, including high-mannose, hybrid, and complex forms. These structures influence folding efficiency, stability, trafficking, and receptor recognition.

O-linked glycosylation occurs mainly in the Golgi apparatus. It attaches sugars to serine or threonine residues without a universal consensus sequence. Core structures are extended by a family of glycosyltransferases using nucleotide sugars, producing mucin-type or other O-glycans that decorate proteoglycans, mucins, and many cell-surface proteins.

Biological roles of glycosylation include protein folding assistance, protection from proteolysis, modulation of cell–cell and cell–matrix interactions, and immune recognition. Abnormal glycosylation is associated with congenital disorders of glycosylation, cancer, infections, and inflammatory diseases. Analytical approaches include mass spectrometry, chromatography, and lectin-based assays to characterize glycan compositions and structures.