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Glycosylation

O Linked Glycans

O-Linked glycans are usually attached to the peptide chain through serine or threonine residues. O-Linked glycosylation is a true post-translational event and does not require a consensus sequence. The most common type of O-linked glycans contain an initial GalNAc residue (or Tn epitope): these are commonly referred to as mucin-type glycans. Other O-linked glycans include glucosamine, xylose, galactose, fucose or manose as the initial carbohydrate bound to the Ser/Thr residues. O-linked glycoproteins are usually large proteins (>200kDa). Glycosylation generally occurs in high-density clusters and may contribute as much as 50-80% to the overall mass. O-Linked glycans tend to be very heterogeneous, hence they are generally classified by their core structure.

Elongation and termination of O-linked glycans is carried out by several glycosyltransferases. One notable core structure is the Gal-b(1-3)-GalNAc (core 1) sequence that has antigenic properties. Termination of O-linked glycans usually includes Gal, GlcNAc, GalNAc, Fuc, or sialic acid.1,2 By far the most common modification of the core Gal-b(1-3)-GalNAc is mono-, di-, or trisialylation.3-7 A less common, but widely distributed O-linked hexasaccharide structure contains b(1-4)-linked galactose and b(1-6)-linked N-acetylglucosamine as well as sialic acid.8,9


References:

  1. Brooks, S.A., et . al., Functional and Molecular Glycobiology, pp. 89-115, Bios Scientific Pub. (Oxford UK 2002).
  2. Marth, J.D., in Essential of Glycobiology, pp. 101-113, Cold Spring Harbor Press (Cold Spring Harbor, NY 1999).
  3. Fukuda, M., et al., J. Biol. Chem., 262, 11952-11957 (1987).
  4. Pahlsson, P., et al., Glycoconj. J., 11, 43-50 (1994).
  5. Saito, S., et al., J. Biol. Chem., 269, 5644-5652 (1994).
  6. Spiro, R. G., and Bhoyroo, V. D., J. Biol. Chem., 249, 5704-5717 (1974)
  7. Uchida, Y., et al, J. Biochem., (Tokyo), 86, 573-585 (1979).
  8. Hard, K., et al., Eur. J. Biochem., 205, 785-798 (1992)
  9. Wilkins, P.P., et al., J. Biol. Chem., 271, 18732-18742 (1996).

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