| GLYCOPROTEINS: STRUCTURES AND LINKAGES
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Sugars are attached to specific amino acids in proteins
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| Sugars may be attached to protein in either N-glycosidic or O-glycosidic linkage. N-linked oligosaccharides, characteristic of plasma and membrane proteins, are always attached by a glycosylamine linkage of N-acetylglucosamine (GlcNAc) to the amide nitrogen of an asparagine residue (Fig. 25.2A). The asparagine at the site of glycosylation must be in an Asn-X-Ser (Thr)-consensus sequence in order to undergo glycosylation. However, not all asparagine residues present in that consensus sequence are glycosylated, indicating that other factors, such as the conformation of the protein around the glycosylation site, are also important.
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| Mucins, collagens, and cytoplasmic proteins may be glycosylated
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| Many membrane proteins and proteins in mucous secretions (mucins) contain oligosaccharides linked by a glycosidic linkage between N-acetylgalactosamine (GalNAc) and the hydroxyl group of serine or threonine residues (Fig. 25.2B). There is no known consensus sequence that indicates which serine residues are to be glycosylated.
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| Figure 25.1 Generalized model of the structure of glycoproteins and proteoglycans. |
| A glucosyl-galactose disaccharide may be linked to the hydroxyl group of hydroxylysine residues in collagen (Fig. 25.2C).
Hydroxylysine is an unusual amino acid that occurs in mammalian collagens; it is formed by hydroxylation of lysine after this amino acid has been incorporated into the collagen chain. Collagen is first synthesized in the cell in a precursor form called procollagen. Procollagens are usually N-glycosylated glycoproteins, but peptide segments containing N-linked oligosaccharides are removed during maturation of the protein, and only the O-linked glycans remain in mature collagen. The less glycosylated collagens tend to form ordered, fibrous structures, such as occur in tendons; the more heavily glycosylated collagens are found in meshwork structures, such as basement membranes (Chapter 27).
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| Finally, a recently discovered structure found in many cytoplasmic proteins involves a single GlcNAc that is O-glycosidically linked to serine residues in protein (Fig. 25.2D). This GlcNAc appears to be attached to the same serine residues that become phosphorylated as part of regulatory and signal transduction processes. The GlcNAc may represent a means by which the cell blocks or controls the phosphorylation of certain proteins, while allowing others to be phosphorylated.
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