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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Explain what is meant by the glycome, glycobiology, and the science of glycomics.

  • Explain what is meant by the terms monosaccharide, disaccharide, oligosaccharide, and polysaccharide.

  • Explain the different ways in which the structures of glucose and other monosaccharides can be represented, and describe the various types of isomerism of sugars and the pyranose and furanose ring structures.

  • Describe the formation of glycosides and the structures of the important disaccharides and polysaccharides.

  • Explain what is meant by the glycemic index of a carbohydrate.

  • Describe the roles of carbohydrates in cell membranes and lipoproteins.

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BIOMEDICAL IMPORTANCE

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Carbohydrates are widely distributed in plants and animals; they have important structural and metabolic roles. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize the cellulose of the plant cell walls. Animals can synthesize carbohydrates from amino acids, but most are derived ultimately from plants. Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose formed by hydrolysis of dietary starch and disaccharides, and other sugars are converted to glucose in the liver. Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus. It is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage; ribose and deoxyribose in nucleic acids; galactose for synthesis of lactose in milk, in glycolipids, and in combination with protein in glycoproteins (see Chapter 46) and proteoglycans. Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance.

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Glycobiology is the study of the roles of sugars in health and disease. The glycome is the entire complement of sugars of an organism, whether free or present in more complex molecules. Glycomics, an analogous term to genomics and proteomics, is the comprehensive study of glycomes, including genetic, physiological, pathological, and other aspects.

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A very large number of glycoside links can be formed between sugars. For example, three different hexoses may be linked to each other to form over 1000 different trisaccharides. The conformations of the sugars in oligosaccharide chains vary depending on their linkages and proximity to other molecules with which the oligosaccharides may interact. Oligosaccharide chains encode biological information and that this depends upon their constituent sugars, their sequences, and their linkages.

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CARBOHYDRATES ARE ALDEHYDE OR KETONE DERIVATIVES OF POLYHYDRIC ALCOHOLS

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Carbohydrates are classified as follows:

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  1. Monosaccharides are those sugars that cannot be hydrolyzed into simpler carbohydrates. They may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms (3-7), and as aldoses or ketoses, depending on whether they have an aldehyde or ketone group. Examples are listed in Table 15–1. In addition to aldehydes and ketones, ...

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