Glycosylation

Glycosylation is the attachment of sugar chains, called glycans, to proteins and lipids. It is one of the most common modifications of proteins, and it affects how they fold, how long they last and how cells recognize one another.

In N-linked glycosylation, a preassembled sugar block is attached to an asparagine in the sequence Asn-X-Ser/Thr as the protein enters the endoplasmic reticulum. In O-linked glycosylation, sugars are added one at a time to serine or threonine, mostly in the Golgi apparatus. Both kinds are trimmed and extended as the protein moves along the secretory pathway.

In the endoplasmic reticulum, glycans act as tags for quality control: proteins that are not folded correctly are held back by the chaperones calnexin and calreticulin. Sugars protect proteins from degradation, help them dissolve and mark them for routing. At the cell surface, glycans form the "sugar coat" that mediates recognition, as in the carbohydrates that define the ABO blood groups, and they support the extracellular matrix.

Defects in glycosylation cause a group of rare congenital disorders. Antibodies carry glycans that tune their immune activity, and the glycans can be engineered to improve drugs. Viruses such as HIV and coronaviruses cover their surface proteins with host-made glycans, a "glycan shield" that helps them evade immune recognition. See carbohydrate.

• Proteins — the molecules modified
• Endoplasmic Reticulum — where N-linked glycans are added
• Carbohydrate — the class of molecules glycans belong to