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Membrane proteins extending from a cell surface often carry covalently attached carbohydrate chains or glycans.

The addition of glycans to proteins, or glycosylation, is catalyzed by glycosyltransferases that attach sugar units to amino acid side chains using different mechanisms.

Sugars are added to the hydroxyl groups of selected serines or threonines in O-type glycosylation or the amide groups of asparagine in N-type glycosylation.

The synthesis of glycoproteins begins in the rough ER with a preformed 14-sugar precursor glycan containing three glucose, nine mannose, and two N-acetylglucosamines. Five of the fourteen sugars form a conserved core in all N-linked oligosaccharides, while others vary.

The core is pre-assembled on an ER membrane-bound lipid carrier, Dolichol phosphate.

As a newly synthesized polypeptide emerges in the ER lumen, the membrane-bound enzyme oligosaccharyltransferase helps the precursor attach to selected asparagines.

The precursor is then further modified by glycosidases that add or remove monosaccharides to form the glycoprotein.