Oglycosidic

O-glycosidic bonds are covalent linkages formed between the anomeric carbon of a sugar and an oxygen atom on another molecule, typically a hydroxyl group. This type of bond is contrasted with N-glycosidic bonds, which connect sugars to nitrogen atoms. O-glycosidic bonds are the defining linkage in many glycosides and glycoproteins, and they can be alpha or beta depending on the sugar's configuration.

In biology, O-glycosylation commonly occurs on serine or threonine residues of proteins, producing mucin-type O-glycosylation, as well as in cytoplasmic or nuclear proteins in the form of O-GlcNAc. The sugar attached in mucin-type glycosylation is often N-acetylgalactosamine (GalNAc), which can be extended with other sugars to form more complex O-glycans; in O-GlcNAc, the sugar is N-acetylglucosamine attached directly to Ser or Thr.

Biosynthesis involves glycosyltransferases that transfer activated sugar donors, such as UDP-GalNAc or UDP-GlcNAc, to serine or threonine residues. O-GlcNAc modification is dynamic and reversible, installed by O-GlcNAc transferase (OGT) and removed by O-GlcNAcase (OGA). Other forms include O-mannosylation, where mannose is linked to Ser/Thr in specific proteins, and the broader class of O-glycosides formed when sugars attach to alcohols or phenols in small molecules.

Functions of O-glycosylation include influences on protein folding, stability, signaling, and cell interactions. Abnormal O-glycosylation is associated with diseases such as cancer and inflammatory disorders. Analytical approaches to study O-glycosidic linkages include mass spectrometry and nuclear magnetic resonance spectroscopy.