Galactose hydroxylysine

FIGURE 6.20 A disaccharide of galactose and glucose is covalently linked to the 5-hydroxyl group of hydroxylysines in collagen by the combined action of the enzymes galactosyl transferase and glucosyl transferase. 

Monosaccharide attachment to some of the hydroxylysine residues within the a chains occurs at this stage as galactose (gal) and glucose (glc) residues are added by specific galactosyl transferase and glucosyl transferases. The carbohydrates are attached as single monosaccharides or as gal-glc disaccharides. 

Glycosylation Some hydroxylysine residues are modified by gly-cosylation with glucose or glucosyl-galactose (see Figure 4.7). 

Hydroxyproline and hydroxylysine result from the hydroxylation by specific hydroxylases of proline and lysine residues after their incorporation into a-chains. The enzymes require ascorbic acid as a cofactor. [Note An ascorbic acid deficiency results in scurvy.] The hydroxyl group of the hydroxylysine residues of collagen may be enzymatically glycosy lated (most commonly, glucose and galactose are added sequentially to the triple helix). 

Hydroxylysine may be glycosylated in addition to providing the means for cross-linking tropocollagen molecules. It may contain D-galactose that is beta-... 

Fig. 7.5 Structure of the diglycan attached to hydroxylysine. The LH3 isoform of lysine hydroxylase attaches UDP- galactose to hydroxylysine in a fi-linkage, and then a-links UDP glucose to the C2 atom of the galactose. This reaction occurs in the endoplasmic reticulum and in the connective tissue matrix (From Fig. 11.4 in Biochemistry, L. Stryer, 3rd Ed. 1988. W.H. Freeman Co., New York)...

A (3-0-glycosidic linkage between galactose and a hydroxylysine residue of the protein (Gal-Hyl). Linkage found in collagen. 

Procollagen(I) is an example of a protein that undergoes extensive posttransla-tional modifications. Hydroxylation reactions produce hydroxyproline residues from proline residues and hydroxylysine from lysine residues. These reactions occur after the protein has been synthesized (Fig. 49.3) and require vitamin C (ascorbic acid) as a cofactor of the enzymes, for example, prolyl hydroxylases and lysyl hydroxylase. Hydroxyproline residues are involved in hydrogen bond formation that helps to stabilize the triple helix, whereas hydroxylysine residues are the sites of attachment of disaccharide moieties (galactose-glucose). 

Asn-X-Thr/Ser, is involved in the carbohydrate-peptide bond, and second, O-glycosidic linkage of galactosamine, where serine and/or threonine, as well as hydroxylysine in no apparent signal sequence, are involved in the carbohydrate-peptide bond. The constituents of carbohydrate chains include galactose, mannose, fucose, A -acetylglucosamine, A-acetylgalactosamine, and sialic acid. 

Most of the hydroxylysine residues of basement membranes are linked to the disaccharide 2-O-a-D-glucosyl-D-galactose by -glycosidic bonds... 

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