Hydroxylysine residues, glycosylation

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

Crosslinkages reducible with borohydride are characteristic of newly formed collagen but these disappear and are replaced by more stable crosslinks as collagen matures. For example, a 3-hydroxypyridine that joins three triple helices may be formed from the reaction of two ketoamine groupings (with elimination of one glycosylated hydroxylysine residue).636 639 Similar chemistry can also produce pyrrole crosslinks.639... 

Collagen contains carbohydrate, predominantly in the form of glycosylated hydroxylysine residues, as O-galactosyl- -glucosyl side chains. The intermolecular crosslinks in collagen occur when the enzymatic oxidation of lysine residues by lysine oxidase spontaneously crosslink via aldimine and aldol bonds (10). 

Currently about 20 proteins have been identified as collagens. Criteria for this classification include the presence of a triple helical domain ( collagen helix ) and the presence of hydroxyproline and hydroxylysine residues. The latter may also be glycosylated with either a single galactose residue or a disaccharide (glucopyranosyl... 

Protein glycosylation occurs mainly on serine and asparagine residues,but can also occur on hydroxylysine and hydroxyproline (Scheme 15). Glycosylation is very important in the endoplasmic reticulum and Golgi apparatus and can be involved in cell signaling. Many of the membrane-bound proteins and excreted proteins are glycosylated. Protein glycosylation is important in all forms of eukaryotes. ... 

The hydroxyproline residues stabilize the triple helix by forming hydrogen bonds between the a-chains, while the hydroxyl groups of hydroxylysine are partly glycosylated with a disaccharide (-Glc-Gal). 

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