O-Linked glycans are usually attached to the peptide chain through serine or threonine residues. O-Linked glycosylation is a true post-translational event and does not require a consensus sequence and no oligosaccharide precursor is required for protein transfer. The most common type of O-linked glycans contain an initial GalNAc residue (or Tn epitope), these are commonly referred to as mucin-type glycans. Other O-linked glycans include glucosamine, xylose, galactose, fucose, or manose as the initial sugar bound to the Ser/Thr residues. O-Linked glycoproteins are usually large proteins (>200 kDa) that are commonly bianttennary with comparatively less branching than N-glycans. Glycosylation generally occurs in high-density clusters and may contribute as much as 50-80% to the overall mass. O-Linked glycans tend to be very heterogeneous, hence they are generally classified by their core structure. Nonelongated O-GlcNAc groups have been recently shown to be related to phosphorylation states and dynamic processing related to cell signaling events in the cell. O-Linked glycans are prevalent in most secretory cells and tissues. They are present in high concentrations in the zona pelucida surrounding mammalian eggs and may funtion as sperm receptors (ZP3 glycoprotein). O-Linked glycans are also involved in hematopoiesis, inflammation response mechanisms, and the formation of ABO blood antigens.
Elongation and termination of O-linked glycans is carried out by several glycosyltransferases. One notable core structure is the Gal-b(1-3)GalNAc (core 1) sequence that has antigenic properties. Termination of O-linked glycans usually includes Gal, GlcNAc, GalNAc, Fuc, or sialic acid. By far the most common modification of the core Gal-b(1-3)-GalNAc is mono-, di-, or trisialylation. A less common, but widely distributed O-linked hexasaccharide structure contains b(1-4)-linked Gal and b(1-6)-linked GlcNAc as well as sialic acid.
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