Bacteria rarely have their proteins glycosylated

to directory mode

The N-glycosidic connection (Type I connection)

While sugar residues are relatively rare in cytoplasmic proteins, N-glycosylation is one of the most common chemical modifications of secreted proteins, lumen proteins and membrane proteins. The carbohydrate content can make up more than 90% of the molecular mass of a protein and mostly consists of covalently bound oligosaccharide side chains with 4 to 15 sugars. These side chains can also be branched.

The N-glycosylation takes place in the endoplasmic reticulum (ER) and takes place as soon as the newly synthesized peptide chain arrives on the lumen side of the ER, i.e. during translation and before the final folding of the protein (co-translational). In some cases, glycosylation is essential for the correct folding or correct assembly of multi-subunit proteins. The large hydrophilic carbohydrate chains position the protein segment to which they are attached on the surface of the protein.

The N-glycosylation of lumen, membrane and secreted proteins takes place in two steps: the so-called core or core-Carbohydrates are attached as a whole to a large N-linked oligosaccharide. This reaction is catalyzed by glycosyl transferase during protein synthesis in the ER. In a second step, during the passage through the Golgi apparatus, further sugars are attached to this core unit.

N-Acetyl-glucosamine (NAG) is linked to an asparagine residue of the protein.

N-Acetyl-glucosamine is linked to the amide nitrogen of asparagine at the reducing end of the carbohydrate chain. Asparagine must appear in the sequence Asn-X-Thr (or Ser); X can be any amino acid except proline. The Asn-linked oligosaccharides occur considerably more often than O-linked glycoproteins.