enzymes present within these cellular compartments. However, in spite of the diversity, all N-glycans are synthesized through a common pathway with a common core glycan structure. The core glycan structure is essentially made up of two N-acetyl glucosamine and three mannose residues. This core glycan is then elaborated and modified further, resulting in a diverse range of N-glycan structures. Synthesis of precursor oligosaccharide The process of N-linked glycosylation starts with the formation of dolichol-linked GlcNAc sugar. Dolichol is a lipid molecule composed of repeating isoprene units. This molecule is found attached to the membrane of the ER. Sugar molecules are attached to the dolichol through a pyrophosphate linkage (one phosphate was originally linked to dolichol, and the second phosphate came from the nucleotide sugar). The oligosaccharide chain is then extended through the addition of various sugar molecules in a stepwise manner to form a precursor oligosaccharide. The assembly of this precursor oligosaccharide occurs in two phases: Phase I and II. Phase I takes place on the cytoplasmic side of the ER and Phase II takes place on the luminal side of the ER. The precursor molecule, ready to be transferred to a protein, consists of 2 GlcNAc, 9 mannose, and 3 glucose molecules. Transfer of glycan to protein Once the precursor oligosaccharide is formed, the completed glycan is then transferred to the nascent polypeptide in the lumen of the ER membrane. This reaction is driven by the energy released from the cleavage of the pyrophosphate bond between the dolichol-glycan molecule. There are three conditions to fulfill before a glycan is transferred to a nascent polypeptide: Asparagine must be located in a specific consensus sequence in the primary structure (Asn–X–Ser or Asn–X–Thr or in rare instances Asn–X–Cys). Asparagine must be located appropriately in the three-dimensional structure of the protein (Sugars are polar molecules and thus need to be attached to asparagine located on the surface of the protein and not buried within the protein) Asparagine must be found in the luminal side of the endoplasmic reticulum for N-linked glycosylation to be initiated. Target residues are either found in secretory proteins or in the regions of transmembrane protein that face the lumen. Oligosaccharyltransferase is the enzyme responsible for the recognition of the consensus sequence and the transfer of the precursor glycan to a polypeptide acceptor which is being translated in the endoplasmic reticulum lumen. N-linked glycosylation is, therefore, a co-translational event Processing