N Linked Structure

All eukaryotic cells express N-linked glycoproteins. Protein glycosylation of N-linked glycans is actually a co-translational event, occurring during protein synthesis. N-linked glycosylation requires the consensus sequence Asn-X-Ser/Thr. Glycosylation occurs most often when this consensus sequence occurs in a loop in the peptide. Oligosaccharide intermediates destined for protein incorporation are synthesized by a series of transferases on the cytoplasmic side of the endoplasmic recticulum (ER) while linked to the dolichol lipid. Following the addition of a specific number of mannose and glucose molecules, the orientation of the dolichol precursor and its attached glycan shifts to the lumen of the ER where further enzymatic modification occurs. The completed oligosaccharide is then transferred from the dolichol precursor to the Asn of the target glycoprotein by oligosaccharyltransferase (OST). Further processing includes trimming of residues such as glucose and mannose, and addition of new residues via transferases in the ER and, to a great extent, in the Golgi. In the Golgi, high mannose N-glycans can be converted to a variety of complex and hydrid forms which are unique to vertibrates.

Inhibition or elimination of glycosylation in the study of N-linked glycans can be brought about by a number of compounds. In the presence of compactin, coenzyme Q, and exogenous cholesterol, N-glycosylation is greatly inhibited. Treatment with tunicamycin completely blocks deglycosylation in that it inhibits GlcNAc C-1-phosphotransferase, which is critical in the formation of the dolichol precursor necessary for synthesizing of N-glycans.

The diverse assortment of N-linked glycans are based on the common core pentasaccharide, Man3GlcNAc2. Further processing in the Golgi results in three main classes of N-linked glycan sub-types; High-mannose, Hybrid, and Complex. Complex glycans contain the common trimannosyl core. Additional monosaccharides may occur in repeating lactosamine units. Additional modifications may include a bisecting GlcNAc at the mannosyl core and/or a fucosyl residue on the innermost GlcNAc. Complex glycans exist in bi-, tri- and tetraantennary forms.

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