O-N-acetylgalactosamine

O-linked glycosylation occurs at a later stage during protein processing, probably in the Golgi apparatus.

This is the addition of N-acetyl-galactosamine to serine or threonine residues by the enzyme UDP-N-acetyl-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase (EC 2.4.1.41), followed by other carbohydrates (such as galactose and sialic acid).

This process is important for certain types of proteins such as proteoglycans, which involves the addition of glycosaminoglycan chains to an initially unglycosylated "proteoglycan core protein."

These additions are usually serine O-linked glycoproteins, which seem to have one of two main functions.

One function involves secretion to form components of the extracellular matrix, adhering one cell to another by interactions between the large sugar complexes of proteoglycans.

The other main function is to act as a component of mucosal secretions, and it is the high concentration of carbohydrates that tends to give mucus its "slimy" feel.

Proteins that circulate in the blood are not normally O-glycosylated, with the exception of IgA1 and IgD (two types of antibody) and C1-inhibitor.

See also

 glycosylation