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. It is also synthesized in the central nervous system where it participates both in development and in the glial scar formation following an injury. Keratan sulfates are large, highly hydrated molecules which in joints can act as a cushion to absorb mechanical shock. 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 -3Galβ1-4GlcNAc6Sβ1-. 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 keratan sulfate type II (KSII) chains and up to 70% of KSI type chains. 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.