Glycosylation

N Linked Glycans

Protein glycosylation by 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 tranferases on the cytoplasmic side of the endoplasmic recticulum (ER) while linked to dolichol. The oligosaccharide is then transferred from the dolichol precursor to the Asn of the target protein by oligosaccharyltransferase (OST) in the lumen of the ER. Further processing includes trimming of residues such as glucose and mannose, and addition of new residues via transferases in the ER and Golgi.1,2

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. Complex glycans commonly terminate with sialic acid residues. 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 tetrantennary forms.


References:

  1. Brooks, S.A., et . al., Functional and Molecular Glycobiology, pp. 73-88, Bios Scientific Pub. (Oxford, UK, 2002).
  2. Marth, J.D., in Essentials of Glycobiology, pp. 69-100, Cold Spring Harbor Press (Cold Spring Harbor, NY, 1999).

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