| 25 Complex Carbohydrates: Glycoproteins
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After reading this chapter you should be able to:
- Describe the general structures of the various types of N-linked oligosaccharides, i.e., high mannose, biantennary, triantennary and tetraantennary complex chains.
- Outline the sequence of reactions involved in biosynthesis and processing of N-linked oligosaccharides to produce the various types of oligosaccharide chains.
- Distinguish between co-translational and post-translational glycosylation and pruning reactions.
- Explain how N-linked oligosaccharides are involved in protein folding and in cell recognition, and why O-linked oligosaccharides are so important in mucin function.
- Describe how each of the monosaccharides involved in biosynthesis of N-linked and O-linked oligosaccharides are synthesized from glucose
 and activated for synthesis of glycoconjugates. - Distinguish lectins from other types of proteins and describe their role in cell function and the reasons for their toxicity.
- Describe several diseases that are associated with deficiencies in enzymes involved in synthesis, modification or degradation of complex carbohydrates.
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| Most mammalian proteins contain covalently attached sugars - i.e., they are glycoproteins. There are two distinct types of sugar-containing proteins that occur in animal cells: glycoproteins and proteoglycans. Along with glycolipids, which are discussed in the next chapter, they are part of the group of sugar-containing molecules that are called glycoconjugates. Figure 25.1 presents models of the structure of glycoproteins and proteoglycans in order to demonstrate the differences between them: glycoproteins have short oligosaccharide (glycan) chains (1-20 sugars in length); they are highly branched and generally do not have a repeating sequence. In contrast, proteoglycans are long, linear, unbranched glycans that have a disaccharide-repeating unit. This chapter will focus on the glycoproteins; Chapter 26 will address glycolipids, and Chapter 27 will consider the proteoglycans.
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| The class of glycoproteins includes most of the proteins that are integral components of the plasma membrane, that function as receptors for hormones or other molecules in the circulation, or that mediate interactions between cells. In addition, many of the proteins of the endoplasmic reticulum, Golgi apparatus and lysosome, and those that are secreted by cells, including serum and mucous proteins, are glycoproteins. Indeed, glycosylation is the major post-synthetic modification of proteins; it occurs either during the course of protein synthesis in the endoplasmic reti-culum or once the protein has been synthesized and transported to the Golgi apparatus. The functions of the carbohydrate chains of glycoproteins are diverse: they may stabilize the protein against denaturation, protect it from proteolytic degradation, enhance its solubility, or serve as recognition signals for transport or cell-cell interactions.
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