Olinked

Olinked refers to a class of protein modifications in which a sugar molecule is attached to the oxygen atom of serine or threonine residues, a process known as O-linked glycosylation. This post-translational modification is common in secreted and membrane-bound proteins and is distinct from N-linked glycosylation, which attaches sugars to asparagine residues. In vertebrates, most O-linked glycosylation occurs in the Golgi apparatus after a protein has been translated and processed in the endoplasmic reticulum.

The most prevalent form is mucin-type O-glycosylation, initiated by a family of enzymes called GalNAc transferases (GALNTs), which add N-acetylgalactosamine (GalNAc) to serine or threonine. This serves as a starting point for diverse elongation pathways, producing various core and extended O-glycan structures such as core 1 and core 2. Other types of O-linked glycosylation include O-mylosylation, O-fucosylation, and O-glucosylation, each with specific donor sugars and transferases. A distinct intracellular modification is O-GlcNAc, where N-acetylglucosamine is added to serine or threonine within nuclear and cytoplasmic proteins by O-GlcNAc transferase (OGT); this modification is dynamic and often related to signaling and metabolism, and it is removed by O-GlcNAcase (OGA).

Functions of O-linked glycosylation include influencing protein folding and stability, modulating receptor signaling and cell adhesion, shaping the properties of mucus-forming mucins, and impacting immune interactions. Abnormal O-linked glycosylation is associated with diseases such as cancer and inflammatory disorders, and defects in related enzymes can underlie congenital disorders of glycosylation.