Keratan sulfate repeating disaccharide unit

Figure 6 Structures of the disaccharide repeating units of chondroitin sulfate (CS), hyaluronan (HA), and keratan sulfate (KS).

Variants of keratan sulfate in which other neutral sugars occur and in which there is more than 1 mole of sulfate ester per disaccharide repeating unit are known. Keratan sulfate has also been found to contain smaller amounts of neutral monosaccharides, such as D-mannose and L-fucose, and 5-amino-3,5-dideoxy-D-gZj/cero-D-gcZacto-2-nonulosonic acid. 

As shown in Figure 48-8, the keratan sulfates consist of repeating Gal-GlcNAc disaccharide units containing sulfate attached to the 6 position of GlcNAc or occasionally of Gal. Type 1 is abundant in cornea, and type II is found along with chondroitin sulfate attached to hyaluronic acid in loose connective tissue. Types I and II have different attachments to protein (Figure 48—8). 

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. 

GAGs are classified into several major types based on the nature of the repeating disaccharide unit heparan sulfate, chondroitin sulfate, dermatan sulfate, keratan sulfate, and hyaluronan. A hypersulfated form of heparan sulfate called heparin, produced mostly by mast cells, plays a key role In allergic reactions. It is also used medically as an anticlotting drug because of its ability to activate a natural clotting inhibitor called antlthrombln III. 

At least seven types of glycosaminoglycans exist, which differ in the monosaccharides present in their repeating disaccharide units—chondroitin sulfate, dermatan sulfate, heparin, heparin sulfate, hyaluronic acid, and keratan sulfates I and II. Except for hyaluronic acid, the glycosaminoglycans are linked to proteins, usually attached covalently to serine or threonine residues (Fig. 49.9). Keratan sulfate I is attached to asparagine. 

Keratan sulfate is very much the odd one out in the polysaccharides embraced by the term glycosaminoglycan since it contains no uronic acid. In place of the uronic acid residue in other glycosaminoglycans, there is a neutral hexose unit— n-galactose. The second component monosaccharide is normal being a 2-acetamido-2-deoxyhexose. The repeating disaccharide unit of keratan sulfate is -0-jS-D-galactopyranosyl-(l- 4)-O- (2-acetamido-2-deoxy- 3-D-glucopyranosyl 6-sulfate) - (1 3) - ( VIII)... 

Keratan sulfate consists of the repeating disaccharide Af-acetyl-lactosamine linked P-1 3 to the D-galactopyranose unit of the next disaccharide (see Fig. 6.14D). 

Like the other acid mucopolysaccharide.s, keratan sulfate contains a repeating disaccharide unit, but many of its features are more resemblant of glj coproteins. This compound, or rather group of compounds, may therefore be regarded as being on the borderline between the tw O classes of substances. 

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