Glykosylaatioon

Glycosylation is the enzymatic process by which carbohydrate chains (glycans) are covalently attached to proteins or lipids. This modification is widespread in biology and influences protein folding, stability, trafficking, and cell–cell communication. In eukaryotes, glycosylation primarily occurs in the endoplasmic reticulum (ER) and the Golgi apparatus, where complex enzymatic steps shape the final glycan structures.

The two major forms are N-linked and O-linked glycosylation. N-linked glycosylation attaches a preassembled oligosaccharide to the amide nitrogen of asparagine within the consensus sequence Asn-X-Ser/Thr (X not proline). Transfer is catalyzed by the oligosaccharyltransferase complex in the ER, and the glycan is subsequently trimmed and remodeled in the ER and Golgi to yield high-mannose, complex, or hybrid structures. O-linked glycosylation attaches sugars to the hydroxyl groups of serine or threonine residues, mostly in the Golgi. It commonly begins with the addition of N-acetylgalactosamine (GalNAc) and is extended by other sugars, producing diverse O-glycans.

Other glycosylation types exist, including cytosolic/nuclear O-GlcNAc modifications and various lipid- and GPI-anchor glycosylations, but N- and O-glycosylation are the best characterized in proteins.

Biological significance is broad: glycosylation influences protein folding quality control in the ER, receptor and ligand interactions, immune recognition, and protein stability. Defects in glycosylation pathways cause congenital disorders of glycosylation (CDG) and are associated with diseases such as cancer and inflammatory conditions. In biotechnology, glycosylation patterns affect the efficacy and pharmacokinetics of therapeutic proteins, prompting efforts in glycoengineering to optimize function.