lOglycosylation

lOglycosylation, commonly written as O-glycosylation, is a form of protein glycosylation in which carbohydrate moieties are covalently attached to the hydroxyl group of serine or threonine residues. This modification contrasts with N-glycosylation, which attaches glycans to asparagine, and most commonly occurs during maturation in the Golgi apparatus after peptide synthesis.

The initial step is transfer of N-acetylgalactosamine (GalNAc) to Ser/Thr by a family of GalNAc transferases, followed by extension to core structures such as Core 1 and Core 2 with additional sugars. O-glycans vary widely in length and composition and can be further modified by sulfation or sialylation.

O-glycans contribute to mucin properties, protease resistance, protein stability, and interactions with lectins, affecting processes from cell adhesion to signaling. They are often attached to secreted proteins and cell-surface receptors, influencing tissue architecture, receptor function, and immune recognition. The diversity of O-glycan structures is tissue- and cell-type specific, and alterations in their biosynthesis are associated with diseases, including congenital disorders of glycosylation and cancer progression.

Analytical approaches include mass spectrometry and specific lectin assays; advances in glycoproteomics improve mapping of O-glycosylation sites. In addition to mucin-type O-glycosylation, other forms such as O-GlcNAcylation, O-fucosylation, and O-mannosylation occur in different cellular contexts and compartments.