PathwayStep1049 PathwayStep1042 Keratan sulfate degradation Keratan sulfate biosynthesis Keratan sulfate/keratin metabolism Authored: Jassal, B, 2011-10-05 Edited: Jassal, B, 2011-10-05 GENE ONTOLOGYGO:0042339 Keratan sulfate (KS) (a glycosaminoglycan, GAG) is a linear polysaccharide that consists of the repeating disaccharide unit GlcNAc-Gal (N-acetylglucosamine-galactose). KS can perform a structural function and is found in bone, cartilage and the cornea. In joints, it also acts as a shock absorber due to its highly hydrated nature. There are several classes of KS, KSI, II and III. KSI is N-linked to asparagine (Asn) residues in the core protein and is predominantly found in the cornea. KSII is O-linked to serine (Ser) or Thr (threonine) residues in the core protein and is found predominantly in cartilage linked to the protein aggrecan, forming the most abundant proteoglycan in cartilage. A third class of KS, KSIII, are proteoglycans in the brain. KSIII chains are linked to Ser/Thr residues in the core protein via mannose (Funderburgh 2000, Funderburgh 2002).<br><br>Normally, the body degrades GAGs as a natural turnover. Defects in the degradative enzymes cause the autosomal recessive mucopolysaccharide storage disease Morquio's syndrome (also called mucopolysaccharidosis IV). This involves the build up of KS in lysosomes, manifesting clinically as skeletal, dental and corneal abnormalities (Tomatsu et al. 2005). Pubmed11030741 Pubmed12512857 Pubmed16287098 Reactome Database ID Release 431638074 Reactome, http://www.reactome.org ReactomeREACT_121288 Reviewed: D'Eustachio, P, 2012-03-28