Figure 1.

An overview of the N-linked glycosylation reaction of proteins in higher eukaryotes: pyrophosphate and monosaccharides are added to the dolichol lipid on the cytosolic side of the endoplasmic reticulum. The lipid linked oligosaccharide (LLO) is inverted to the luminal side of the endoplasmic reticulum (ER). Additional monosaccharides are added to form the mature LLO. Oligosaccharyltransferase (OST) catalyzes the transfer of the oligosaccharide from the LLO to the side-chain of an asparagine residue in -N-X-T/S- consensus sequence within a protein. Protein folding occurs after N-linked glycosylation. The three terminal glucose residues are trimmed before translocating to the Golgi apparatus for sorting. Misfolded proteins are targeted for degradation by proteasomes.