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Some Important Oligosaccharides
Oligomers of sugars frequently occur as disaccharides, formed by linking two monosaccharide units by glycosidic bonds. Three of the most important examples of oligosaccharides are disaccharides. They are sucrose, lactose, and maltose (Figure 16.19). Two other disaccharides, isomaltose and cellobiose, are shown for comparison.
Sucrose is common table sugar, which is extracted from sugarcane and sugarbeets. The monosaccharide units that make up sucrose are α-D-glucose and β-D-fructose. Glucose (an aldohexose) is a pyranose, and fructose (a ketohexose) is a furanose. The α C-1 carbon of the glucose is linked to the β C-2 carbon of the fructose (Figure 16.19) in a glycosidic linkage that has the notation α,β(1 - > 2). Sucrose is not a reducing sugar because both anomeric groups are involved in the glycosidic linkage. Free glucose is a reducing sugar, and free fructose can also give a positive test, even though it is a ketone rather than an aldehyde in the open-chain form. Fructose and ketoses in general can act as reducing sugars because they can isomerize to aldoses in a rather complex rearrangement reaction. (We need not concern ourselves with the details of this isomerization.)
When animals consume sucrose, it is hydrolyzed to glucose and fructose, which are then degraded by metabolic processes to provide energy. Humans consume large quantities of sucrose, and excess consumption can contribute to health problems; this fact has led to a search for other sweetening agents.
One that has been proposed is fructose itself. It is sweeter than sucrose; therefore, a smaller amount (by weight) of fructose than sucrose can produce the same sweetening effect with fewer calories. Consequently, high-fructose corn syrup is frequently used in food processing. The presence of fructose changes the texture of food, and the reaction to the change tends to depend on the preference of the consumer. Artificial sweeteners have been produced in the laboratory and have frequently been suspected of having harmful side effects; the ensuing controversies bear eloquent testimony to the human craving for sweets. Saccharin, for example, has been found to cause cancer in laboratory animals, as have cyclamates, but the applicability of these results to human carcinogenesis has been questioned by some. Aspartame has been suspected of causing neu-rological problems, especially in individuals whose metabolisms cannot tolerate phenylalanine.
Another artificial sweetener is a derivative of sucrose. This substance, sucra-lose, which is marketed under the trade name Splenda, differs from sucrose in two ways (Figure 16.20). The first difference is that three of the hydroxyl groups have been replaced with three chlorine atoms. The second is that the configuration at carbon atom 4 of the six-membered pyranose ring of glucose has been inverted, producing a galactose derivative. The three hydroxyl groups that have been replaced by chlorine atoms are those bonded to carbon atoms 1 and 6 of the fructose moiety and to carbon atom 4 of the galactose moiety. Sucralose is not metabolized by the body, and, consequently, it does not pro-vide calories. Tests conducted so far, as well as anecdotal evidence, indicate that it is a safe sugar substitute. It is likely to find wide use in the near future. We can safely predict that the search for nonfattening sweeteners will continue and that it will be accompanied by controversy.
Lactose is a disaccharidemade up of β-D-galactose and D-glucose. Galactose is the C-4 epimer of glucose. In other words, the only difference between glucose and galactose is inversion of configuration at C-4. The glycosidic linkage is β(1 - > 4), between the anomeric carbon C-1 of the β form of galactose and the C-4 carbon of glucose (Figure 16.19). Since the anomeric carbon of glucose is not involved in the glycosidic linkage, it can be in either the α or the β form. The two anomeric forms of lactose can be specified, and the designation refers to the glucose residue; galactose must be present as the β-anomer, since the β form of galactose is required by the structure of lactose. Lactose is a reducing sugar because the group at the anomeric carbon of the glucose portion is not involved in a glycosidic linkage, so it is free to react with oxidizing agents.
Maltose is a disaccharide obtained from the hydrolysis of starch. It consistsof two residues of D-glucose in an α(1 - > 4) linkage. Maltose differs from cel-lobiose, a disaccharide that is obtained from the hydrolysis of cellulose, only inthe glycosidic linkage. In cellobiose, the two residues of D-glucose are bonded together in a β(1 - > 4) linkage (Figure 16.19). Mammals can digest maltose, but not cellobiose. Yeast, specifically brewer’s yeast, contains enzymes that hydrolyze the starch in sprouted barley (barley malt) first to maltose and then to glucose, which is fermented in the brewing of beer. Maltose is also used in other beverages, such as malted milk.
The disaccharide sucrose is common table sugar. It consists of glucose and fructose linked by a glycosidic bond.
Lactose, found in milk, and maltose, obtained from starch, are two other common disaccharides.
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