Glycosaminoglycans keratan-sulfate

Six major classes of glycosaminoglycans There are six major classes of glycosaminoglycans. These include chondroitin 4- and 6-sulfates, keratan sulfate, dermatan sulfate, heparin, heparan sulfate, and hyaluronic acid. 

Glycosaminoglycans (GAGs) are unbranched chains having repeating disaccharide units, which, with the exception of keratan sulfate, contain an acid and a base. The KS disaccharide unit consists of a hexose and a base. The structures of the different classes of GAG disaccharide units are given in Fig. 1, and summarized in Table I. 

Most of the polysaccharides of interest in this text are termed glycosaminoglycans, polymers that contain an amino sugar in the repeat unit. Glycosaminoglycans that are abundant in mammalian tissues include hyaluronan, chondroitin sulfate, dermatan sulfate, keratan sulfate, and heparin-heparan sulfate (see Table 2.4). Most of these glycosaminoglycans,... 

All glycosaminoglycans and keratan sulfate, except hyaluronic add, are covalently linked to protein. Such linkages, shown in Figure 9.13, may be of the O- or the N-glycoside type. Also, at the link region, all glycosaminoglycans contain the -Gal-Gal-Xyl-sequence, in which the xylose molecule, a pentose, is... 

Figure 9.13 Structures of repeating units of glycosaminoglycans (A) hyaluronic acid (B) chondroitin sulfate (C) dermatan sulfate (D) two types of keratan sulfate type I is N linked and type II is O linked (E) heparan sulfate. Sulfate residues have been omitted in all cases.

As well as chondroitin sulfate, other glycosaminoglycans, such as keratan sulfate, with shorter chains (Mr = 2,500-5,000) are found in cartilage (Fig. 5-22). N-linked oligosaccharides similar to those found in glycoproteins are also present. Note that keratan sulfate in other tissues, e.g., intervertebral disk or cornea, has a longer chain (Mr= 10,000-25,000) and may have a different oligosaccharide link to the core protein. 

Keratan sulfate consists of the disaccharide, iV-acetyl-lactosamine, linked /3-(1 3) to the D-galactopyranose residue of the next iV-acetyl-lactosamine unit. Keratan sulfate is the most heterogeneous of the glycosaminoglycans, with variable sulfate content linked to C-4 or C-6 of the D-galactop)Tanose residue in lactosamine, and small amounts of L-fucose (6-deoxy-L-galactose), D-mannose, and iV-acetyl-neuraminic acid residues [93] (also see Sects. 10.2 and O 14 on the occurrence of iV-acetyl-D-neuraminic acid and L-fucose in other systems). Keratan sulfate is found in the cornea, on the surfaces of erythrocytes, in cartilage, and in bone. 

Proteo-GlycosAminoGlycans (Proteo-GAGs) contain chondroitin-, dermatan-, keratan-, or heparin-sulfate and have the properties of proteins called mucins (Sect. 12.3.1). The names refer to the tissues where they were first identified and are usually most prominent. Dermatan and keratan sulfate are named for the dermis of the skin and gingiva, chondroitin for cartilage and heparin for liver. 

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