Because sugars contain hydroxyl groups, glycosidic bonds can join one monosaccharide to another. Oligosaccharides are built by the linkage of two or more monosaccharides by O-glycosidic bonds (Figure 11.11). In the disaccharide maltose, for example, two D-glucose residues are joined by a glycosidic linkage between the α -anomeric form of C-1 on one sugar and the hydroxyl oxygen atom on C-4 of the adjacent sugar. Such a linkage is called an α -1,4-glycosidic bond. Just as proteins have a directionality defined by the amino and carboxyl termini, oligosaccharides have a directionality defined by their reducing and nonreducing ends. The carbohydrate unit at the reducing end has a free anomeric carbon atom that has reducing activity because it can form the open-chain form, as discussed earlier. By convention, this end of the oligosaccharide is still called the reducing end even when it is bound to another molecule such as a protein and thus no longer has reducing properties.

The fact that monosaccharides have multiple hydroxyl groups means that many different glycosidic linkages are possible. For example, consider three monosaccharides: glucose, mannose, and galactose. These molecules can be linked together in the laboratory to form more than 12,000 structures differing in the order of the monosaccharides and the hydroxyl groups participating in the glycosidic linkages. In this section, we will look at some of the most common oligosaccharides found in nature.

A disaccharide consists of two sugars joined by an O-glycosidic bond. Three abundant disaccharides that we encounter frequently are sucrose, lactose, and maltose (Figure 11.12). Sucrose (common table sugar) is obtained commercially from sugar cane or sugar beets. The anomeric carbon atoms of a glucose unit and a fructose unit are joined in this disaccharide; the configuration of this glycosidic linkage is α for glucose and β for fructose. Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase. Lactose, the disaccharide of milk, consists of galactose joined to glucose by a β-1,4-glycosidic linkage. Lactose is hydrolyzed to these monosaccharides by lactase in human beings and by β-galactosidase in bacteria. In maltose, two glucose units are joined by an α-1,4-glycosidic linkage. Maltose comes from the hydrolysis of large polymeric oligosaccharides such as starch and glycogen and is in turn hydrolyzed to glucose by maltase. Sucrase, lactase, and maltase are located on the outer surfaces of epithelial cells lining the small intestine. The cleavage products of sucrose, lactose, and maltose can be further processed to provide energy in the form of ATP.

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Glucose is an important energy source in virtually all life forms. However, free glucose molecules cannot be stored because in high concentrations, glucose will disturb the osmotic balance of the cell, potentially resulting in cell death. The solution is to store glucose as units in a large polymer, which is not osmotically active.

Large polymeric oligosaccharides, formed by the linkage of multiple monosaccharides, are called polysaccharides and play vital roles in energy storage and in maintaining the structural integrity of an organism. If all of the monosaccharide units in a polysaccharide are the same, the polymer is called a homopolymer. The most common homopolymer in animal cells is glycogen, the storage form of glucose. Glycogen is present in most of our tissues but is most abundant in muscle and liver. As will be considered in detail in Chapter 21, glycogen is a large, branched polymer of glucose residues. Most of the glucose units in glycogen are linked by α-1,4-glycosidic bonds. The branches are formed by α-1,6-glycosidic bonds, present about once in 10 units (Figure 11.13).

The nutritional reservoir in plants is the homopolymer starch, of which there are two forms. Amylose, the unbranched type of starch, consists of glucose residues in α-1,4 linkage. Amylopectin, the branched form, has about 1 α -1,6 linkage per 30 α -1,4 linkages, in similar fashion to glycogen except for its lower degree of branching. More than half the carbohydrate ingested by human beings is starch found in wheat, potatoes, and rice, to name just a few sources. Amylopectin, amylose, and glycogen are rapidly hydrolyzed by α-amylase, an enzyme secreted by the salivary glands and the pancreas.

Cellulose, the other major polysaccharide of glucose found in plants, serves a structural rather than a nutritional role as an important component of the plant cell wall. Cellulose is among the most abundant organic compounds in the biosphere. Some 10¹⁵ kg of cellulose is synthesized and degraded on Earth each year, an amount 1000 times as great as the combined weight of the human race. Cellulose is an unbranched polymer of glucose residues joined by β -1,4 linkages, in contrast with the α-1,4 linkage seen in starch and glycogen. This simple difference in stereochemistry yields two molecules with very different properties and biological functions. The β configuration allows cellulose to form very long, straight chains. Fibrils are formed by parallel chains that interact with one another through hydrogen bonds, generating a rigid, supportive structure. The straight chains formed by β linkages are optimal for the construction of fibers having a high tensile strength. The α-1,4 linkages in glycogen and starch produce a very different molecular architecture: a hollow helix is formed instead of a straight chain (Figure 11.14). The hollow helix formed by α linkages is well suited to the formation of a more-compact, accessible store of sugar. Although mammals lack cellulases and therefore cannot digest wood and vegetable fibers, cellulose and other plant fibers are still an important constituent of the mammalian diet as a component of dietary fiber. Soluble fiber such as pectin (polygalacturonic acid) slows the movement of food through the gastrointestinal tract, allowing improved digestion and the absorption of nutrients. Insoluble fibers, such as cellulose, increase the rate at which digestion products pass through the large intestine. This increase in rate can minimize exposure to toxins in the diet.

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We have considered only homopolymers of glucose. However, given the variety of different monosaccharides that can be put together in any number of arrangements, the number of possible polysaccharides is huge. We will consider some of these polysaccharides shortly.