Keratan sulfate

Overview
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:


 * 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 KSII chains and up to 70% of KSI type chains.

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. 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. 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. 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.

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.

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