| General structures of glycoproteins
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A glycoprotein may contain a single N-linked oligosaccharide chain, or it may have several of these types of oligosaccharides. Furthermore, the N-linked oligosaccharides may all have identical structures, or they may be quite different in structure; such a glycoprotein may also contain O-linked oligosaccharide chains. Alternatively, a glycoprotein may contain only N-linked oligosaccharides, or only O-linked oligosaccharides. The total number of oligosaccharide chains may also vary considerably, depending on the protein and its function. For example, the low-density lipoprotein receptor that is found in plasma membranes of smooth muscle cells and fibroblasts contains two N-linked biantennary complex chains, in addition to a cluster of O-linked chains. This receptor has a membrane-spanning region of hydrophobic amino acids , an extended region on the external side of the plasma membrane that contains the O-linked, negatively charged oligosaccharides, and a functional domain that is involved in binding serum low-density lipoprotein (Fig. 25.5). The two N-linked oligosaccharides are near the functional domain. It remains unclear what role these N-linked chains play in the function of this receptor, but the O-linked oligosaccharides are believed to keep the molecule in an extended state.
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| Figure 25.3 Typical structures of high-mannose and complex, N-linked oligosaccharides. Asn, asparagine; Gal, galactose; Man, mannose. The core structure (shaded area) is common to both structures. |
| Figure 25.4 Typical structures of O-linked oligosaccharides. |
| Figure 25.5 Model of the low density lipoprotein (LDL) receptor. (See also Chapter 17.) |
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