Glycosylation is the process or result of addition of saccharides to proteins and lipids. The process is one of four principal co-translational and post-translational modification steps in the synthesis of membrane and secreted proteins and the majority of proteins synthesized in the rough endoplasmic reticulum (RER) undergo glycosylation. It is an enzyme-directed site-specific process, as opposed to the non-ezymatic chemical reaction of glycation.
There exists N-linked glycosylation to the amide nitrogen of asparagine side chains and O-linked glycosylation to the hydroxy oxygen of serine and threonine side chains.
The polysaccharide chains attached to the target proteins serve various functions. For instance, some proteins do not fold correctly unless they are glycosylated first.
Also, polysaccharides linked at the amide nitrogen of asparagine in the protein confer stability on some secreted glycoproteins. Experiments have shown that glycosylation in this case is not a strict requirement for proper folding, but the unglycosylated protein degrades quickly.
Glycosylation may play a role in cell-cell adhesion (a mechanism employed by cells of the immune system) as well.
N-linked glycosylation is an essential posttranslational modification of proteins in eukaryotes. The substrate of N-linked glycosylation, dolichol pyrophosphate (DolPP)-GlcNAc(2)Man(9)Glc(3), is assembled through a complex series of ordered reactions requiring the translocation of the intermediate DolPP-GlcNAc(2)Man(5) structure across the endoplasmic-reticulum membrane.
Types
protein glycosylation
lipid glycosylation
DNA glycosylation
Pathology
congenital disorders of glycosylation (CDG syndromes)
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Reviews
Martin PT. Congenital muscular dystrophies involving the O-mannose pathway. Curr Mol Med. 2007 Jun;7(4):417-25. PMID: 17584082
Rampal R, Luther KB, Haltiwanger RS. Notch signaling in normal and disease States: possible therapies related to glycosylation. Curr Mol Med. 2007 Jun;7(4):427-45. PMID: 17584081
Schulz BL, Laroy W, Callewaert N. Clinical laboratory testing in human medicine based on the detection of glycoconjugates. Curr Mol Med. 2007 Jun;7(4):397-416. PMID: 17584080
Freeze HH. Congenital Disorders of Glycosylation: CDG-I, CDG-II, and beyond. Curr Mol Med. 2007 Jun;7(4):389-96. PMID: 17584079
Freeze HH.Novel perspectives on glycosylation and human disease. Curr Mol Med. 2007 Jun;7(4):387. PMID: 17584078
Hebert DN, Garman SC, Molinari M. The glycan code of the endoplasmic reticulum: asparagine-linked carbohydrates as protein maturation and quality-control tags. Trends Cell Biol. 2005 Jun 3; PMID: 15939591
Grewal PK, Hewitt JE. Glycosylation defects: a new mechanism for muscular dystrophy? Hum Mol Genet. 2003 Oct 15;12 Spec No 2:R259-64. PMID: 12925572
References
Haeuptle MA, Pujol FM, Neupert C, Winchester B, Kastaniotis AJ, Aebi M, Hennet T. Human RFT1 deficiency leads to a disorder of N-linked glycosylation. Am J Hum Genet. 2008 Mar;82(3):600-6. PMID: 18313027