Protein glycosylation and glycation

Glycosylation is a post-translational modification, which means a chemical change made to a protein after it has been built. An enzyme joins a carbohydrate to the protein by a covalent bond, a strong bond in which atoms share electrons, so the sugar becomes a lasting part of the protein.

The sugar comes from a glycosyl group, which is a single sugar or a short chain of sugars (a mono- or oligosaccharide). It forms a glycosidic bond, a bond that links a sugar to another molecule, with the protein. So many enzyme-dependent steps are involved that glycosylation is one of the most complicated post-translational modifications.

There are two main types. In N-linked glycosylation, the sugar goes onto the amino acid asparagine in the endoplasmic reticulum, and this usually happens to proteins bound for the membrane or for secretion. In O-linked glycosylation, the sugar goes onto the amino acids serine or threonine in the Golgi apparatus.

Sugar chains help a protein to fold correctly, make it more stable and change how it interacts with other molecules, for example how a receptor recognises its ligand (the molecule that binds to it). When key sugar chains are missing, a receptor can accumulate inside the cell, often in the endoplasmic reticulum, instead of reaching the plasma membrane. Glycosyltransferase enzymes also add the final sugars to the H antigen on the red blood cell surface, which helps to determine blood group.

Do not confuse glycosylation with glycation, in which a sugar attaches to a protein without an enzyme, as when glucose attaches to haemoglobin to form glycated haemoglobin (HbA1c).