Keratan sulfate

Keratan sulfate (KS), also called keratosulfate, is any of several sulfated glycosaminoglycans (structural carbohydrates) that have been found especially in the cornea, cartilage, and bone. Keratan sulfates are large, highly hydrated molecules which in joints can act as a cushion to absorb mechanical shock.

Keratan sulfate structure

Like other glycosaminoglycans keratan sulfate is a linear polymer that consists of a repeating disaccharide unit. Keratan sulfate occurs as a proteoglycan (PG) in which KS chains are attached to cell-surface or extracellular matrix proteins, termed core proteins. KS core proteins include Lumican, Keratocan, Mimecan, Fibromodulin, PRELP, Osteoadherin and Aggrecan.

The basic repeating disaccharide unit within keratan sulfate is -3Galβ1-4GlcNAcβ1-. This can be sulfated at carbon position 6 (C6) of either or both the Gal or GlcNAc monosaccharides. However, the detailed primary structure of specific KS types are best considered to be composed of three regions:cite journal | author = Tai GH, Huckerby TN and Nieduszynski IA. | title = Multiple non-reducing chain termini isolated from bovine corneal keratan sulfates | journal = J. Biol. Chem. | year=1996 | volume=271 | issue =38 | pages=23535–23546 | url=http://www.jbc.org/cgi/reprint/271/38/23535.pdf | pmid = 8798563 | doi = 10.1074/jbc.271.38.23535]

*A linkage region, at one end of which the KS chain is linked to the core protein.
*A repeat region, composed of the -3Galβ1-4GlcNAcβ1- repeating disaccharide unit and
*A chain capping region, occurring at the opposite end of the KS chain to the protein linkage region.

The monosaccharide mannose is found within the linkage region of keratan sulfate type I (KSI). Disaccharides within the repeating region of KSII may be fucosylated and N-Acetylneuraminic acid caps the end of all keratan sulfate type II (KSII) chains and up to 70% of KSI type chains.cite journal | author = Funderburgh JL. | title = Keratan sulfate: structure, biosynthesis, and function | journal = Glycobiology | year=2000 | volume=10 | issue =10 | pages=951–958 | pmid = 11030741 | doi = 10.1093/glycob/10.10.951]

KS classes

The designations KSI and KSII were originally assigned on the basis of the tissue type from which the keratan sulfate was isolated. KSI was isolated from corneal tissue and KSII from skeletal tissue. [cite journal | author = Meyer K, Linker A. "et al." | title = The mucopolysaccharides of bovine cornea | journal = J. Biol. Chem. | year=1953 | volume=205 | issue=2 | pages=611–616 | url= http://www.jbc.org/cgi/reprint/205/2/611 | pmid = 13129238] [cite journal | author = Meyer K, Hoffman P. and Linker A. | title = Mucopolysaccharides of Costal Cartilage | journal = Science | year=1958 | volume=128 | issue=3329 | pages=896 | url= http://www.sciencemag.org/cgi/reprint/128/3329/896| pmid = 13592269 | doi = 10.1126/science.128.3329.896] Minor monosaccharide compositional differences exist between KS extracted from both sources and even KS extracted from the same source. However, major differences occur in the way each KS type is joined to its core protein. [cite journal | author = Seno N, Meyer K. "et al." | title = Variations in Keratosulfates | journal = J. Biol. Chem. | year=1965 | volume=240 | pages=1005–1019 | url= http://www.jbc.org/cgi/reprint/240/3/1005 | pmid = 14284693] The designations KSI and KSII are now based upon these protein linkage differences. KSI is N-linked to specific asparagine amino acids via "N"-acetylglucosamine and KSII is O-linked to specific Serine or Threonine amino acids via N-acetyl galactosamine. [cite journal | author = Nieduszynski IA, Huckerby TN. "et al" | title = There are two major types of skeletal keratan sulphates | journal = Biochem. J. | year=1990 | volume=271 | issue=1 | pages=243–245 | url= http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1149539&blobtype=pdf | pmid = 2222415] The tissue based classification of KS no longer exists as KS types have been shown to be non tissue specific. A third type of KS (KSIII) has also been isolated from brain tissue that is O-linked to specific serine or threonine amino acids via mannose. [cite journal | author = Krusius T, Finne J. "et al" | title = Identification of an O-glycosidic mannose-linked sialylated tetrasaccharide and keratan sulfate oligosaccharides in the chondroitin sulfate proteoglycan of brain | journal = J. Biol. Chem. | year=1986 | volume=261 | issue=18 | pages=8237–8242 | url= http://www.jbc.org/cgi/reprint/261/18/8237 | pmid = 2941416]

Corneal KSI

The amount of KS found in the cornea is 10 fold higher than it is in cartilage and 2-4 times higher than it is in other tissues. [cite journal | author = Funderburgh JL, Caterson B. and Conrad GW. | title = Distribution of proteoglycans antigenically related to corneal keratan sulfate proteoglycan | journal = J. Biol. Chem. | year=1987 | volume=262 | issue =24 | pages=11634–11640 | pmid = 2957372]

Non-corneal KSI

Osteoadherin, Fibromodulin and PRELP are core proteins found in bone and cartilage, that are modified by N-linked KS chains.

KSII

KSIII

ee also

* N-Acetylglucosamine
* galactose

References

External links

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