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6.6 Disaccharides

Learning Objectives

1. Identify the structures of sucrose, lactose, and maltose.
2. Identify the monosaccharides that are needed to form sucrose, lactose, and maltose.

In Section 6.four "Circadian Structures of Monosaccharides", you learned that monosaccharides tin form circadian structures by the reaction of the carbonyl grouping with an OH group. These cyclic molecules can in turn react with some other booze. Disaccharides (C₁₂H₂₂O_eleven) are sugars equanimous of two monosaccharide units that are joined by a carbon–oxygen-carbon linkage known every bit a glycosidic linkage. This linkage is formed from the reaction of the anomeric carbon of ane cyclic monosaccharide with the OH group of a second monosaccharide.

The disaccharides differ from one another in their monosaccharide constituents and in the specific type of glycosidic linkage connecting them. There are three common disaccharides: maltose, lactose, and sucrose. All 3 are white crystalline solids at room temperature and are soluble in water. We'll consider each sugar in more particular.

Maltose

Maltose occurs to a limited extent in sprouting grain. It is formed most oftentimes past the partial hydrolysis of starch and glycogen. In the manufacture of beer, maltose is liberated by the action of malt (germinating barley) on starch; for this reason, it is often referred to equally malt sugar. Maltose is about xxx% every bit sweet as sucrose. The human body is unable to metabolize maltose or any other disaccharide directly from the nutrition because the molecules are too large to laissez passer through the cell membranes of the intestinal wall. Therefore, an ingested disaccharide must first exist broken downwards by hydrolysis into its two constituent monosaccharide units. In the torso, such hydrolysis reactions are catalyzed past enzymes such equally maltase. The aforementioned reactions tin can exist carried out in the laboratory with dilute acid every bit a goad, although in that case the rate is much slower, and high temperatures are required. Whether information technology occurs in the torso or a glass chalice, the hydrolysis of maltose produces two molecules of D-glucose.

$$\text{maltose}\stackrel{{\text{H}}^{\text{+}}\text{ or maltase}}{\to }\text{ two D-glucose}$$

Maltose is a reducing saccharide. Thus, its two glucose molecules must exist linked in such a style equally to get out ane anomeric carbon that tin can open to form an aldehyde group. The glucose units in maltose are joined in a caput-to-tail manner through an α-linkage from the showtime carbon cantlet of 1 glucose molecule to the 4th carbon atom of the 2d glucose molecule (that is, an α-1,four-glycosidic linkage; run into Figure 6.6 "An Equilibrium Mixture of Maltose Isomers"). The bail from the anomeric carbon of the commencement monosaccharide unit is directed downward, which is why this is known every bit an α-glycosidic linkage. The OH group on the anomeric carbon of the second glucose can be in either the α or the β position, as shown in Figure half-dozen.6 "An Equilibrium Mixture of Maltose Isomers".

Effigy vi.six An Equilibrium Mixture of Maltose Isomers

Lactose

Lactose is known as milk sugar considering information technology occurs in the milk of humans, cows, and other mammals. In fact, the natural synthesis of lactose occurs only in mammary tissue, whereas about other carbohydrates are establish products. Human milk contains about 7.5% lactose, and cow's milk contains virtually 4.5%. This sugar is one of the lowest ranking in terms of sweetness, being about i-sixth as sugariness as sucrose (see Tabular array 6.1 "The Relative Sweetness of Some Compounds (Sucrose = 100)" in Section half-dozen.iii "Of import Hexoses"). Lactose is produced commercially from whey, a by-product in the industry of cheese. It is important as an infant food and in the production of penicillin.

Lactose is a reducing carbohydrate equanimous of one molecule of D-galactose and 1 molecule of D-glucose joined by a β-1,4-glycosidic bond (the bail from the anomeric carbon of the first monosaccharide unit beingness directed upward). The ii monosaccharides are obtained from lactose by acid hydrolysis or the catalytic action of the enzyme lactase:

Many adults and some children suffer from a deficiency of lactase. These individuals are said to be lactose intolerant considering they cannot digest the lactose plant in milk. A more serious problem is the genetic disease galactosemia, which results from the absenteeism of an enzyme needed to convert galactose to glucose. Certain bacteria tin metabolize lactose, forming lactic acid equally one of the products. This reaction is responsible for the "souring" of milk.

Case three

For this trisaccharide, signal whether each glycosidic linkage is α or β.

Solution

The glycosidic linkage betwixt sugars ane and 2 is β considering the bond is directed up from the anomeric carbon. The glycosidic linkage between sugars 2 and 3 is α considering the bond is directed down from the anomeric carbon.

Skill-Building Exercise

1. For this trisaccharide, point whether each glycosidic linkage is α or β.

   

To Your Health: Lactose Intolerance and Galactosemia

Lactose makes up about 40% of an infant's diet during the first year of life. Infants and pocket-size children have 1 form of the enzyme lactase in their small-scale intestines and tin digest the carbohydrate easily; however, adults ordinarily have a less agile form of the enzyme, and virtually 70% of the globe'due south adult population has some deficiency in its production. Every bit a result, many adults experience a reduction in the ability to hydrolyze lactose to galactose and glucose in their small intestine. For some people the inability to synthesize sufficient enzyme increases with age. Upward to xx% of the Usa population suffers some degree of lactose intolerance.

In people with lactose intolerance, some of the unhydrolyzed lactose passes into the colon, where it tends to describe water from the interstitial fluid into the intestinal lumen by osmosis. At the aforementioned time, intestinal bacteria may act on the lactose to produce organic acids and gases. The buildup of water and bacterial decay products leads to abdominal distention, cramps, and diarrhea, which are symptoms of the condition.

The symptoms disappear if milk or other sources of lactose are excluded from the diet or consumed simply sparingly. Alternatively, many nutrient stores now carry special brands of milk that have been pretreated with lactase to hydrolyze the lactose. Cooking or fermenting milk causes at least partial hydrolysis of the lactose, so some people with lactose intolerance are however able to savour cheese, yogurt, or cooked foods containing milk. The nigh common treatment for lactose intolerance, nonetheless, is the use of lactase preparations (east.g., Lactaid), which are available in liquid and tablet class at drugstores and grocery stores. These are taken orally with dairy foods—or may exist added to them directly—to assist in their digestion.

Galactosemia is a condition in which one of the enzymes needed to convert galactose to glucose is missing. Consequently, the blood galactose level is markedly elevated, and galactose is found in the urine. An infant with galactosemia experiences a lack of ambition, weight loss, diarrhea, and jaundice. The illness may consequence in impaired liver part, cataracts, mental retardation, and even death. If galactosemia is recognized in early infancy, its effects can be prevented past the exclusion of milk and all other sources of galactose from the diet. As a child with galactosemia grows older, he or she commonly develops an alternate pathway for metabolizing galactose, and then the demand to restrict milk is not permanent. The incidence of galactosemia in the United States is 1 in every 65,000 newborn babies.

Sucrose

Sucrose, probably the largest-selling pure organic compound in the earth, is known as beet saccharide, cane saccharide, table saccharide, or simply sugar. Most of the sucrose sold commercially is obtained from sugar cane and saccharide beets (whose juices are 14%–20% sucrose) by evaporation of the water and recrystallization. The dark brownish liquid that remains later on the recrystallization of sugar is sold as molasses.

The sucrose molecule is unique among the common disaccharides in having an α-ane,β-two-glycosidic (head-to-head) linkage. Because this glycosidic linkage is formed by the OH group on the anomeric carbon of α-D-glucose and the OH grouping on the anomeric carbon of β-D-fructose, information technology ties up the anomeric carbons of both glucose and fructose.

This linkage gives sucrose certain properties that are quite different from those of maltose and lactose. As long as the sucrose molecule remains intact, neither monosaccharide "uncyclizes" to grade an open-concatenation structure. Thus, sucrose is incapable of mutarotation and exists in merely one form both in the solid state and in solution. In addition, sucrose does non undergo reactions that are typical of aldehydes and ketones. Therefore, sucrose is a nonreducing sugar.

The hydrolysis of sucrose in dilute acid or through the activity of the enzyme sucrase (besides known as invertase) gives an equimolar mixture of glucose and fructose. This 1:1 mixture is referred to as capsize sugar because it rotates plane-polarized lite in the opposite direction than sucrose. The hydrolysis reaction has several practical applications. Sucrose readily recrystallizes from a solution, simply invert sugar has a much greater tendency to remain in solution. In the industry of jelly and candy and in the canning of fruit, the recrystallization of sugar is undesirable. Therefore, weather leading to the hydrolysis of sucrose are employed in these processes. Moreover, because fructose is sweeter than sucrose, the hydrolysis adds to the sweetening issue. Bees carry out this reaction when they make honey.

The average American consumes more 100 lb of sucrose every year. Most two-thirds of this amount is ingested in soft drinks, presweetened cereals, and other highly candy foods. The widespread utilize of sucrose is a contributing factor to obesity and molar decay. Carbohydrates such as sucrose, are converted to fat when the caloric intake exceeds the body's requirements, and sucrose causes tooth decay past promoting the germination of plaque that sticks to teeth.

Concept Review Exercise

one. What monosaccharides are obtained by the hydrolysis of each disaccharide?

a. sucrose

b. maltose

c. lactose

Reply

1.

a. D-glucose and D-fructose

b. ii molecules of D-glucose

c. D-glucose and D-galactose

Key Takeaways

• Maltose is equanimous of 2 molecules of glucose joined by an α-i,4-glycosidic linkage. It is a reducing sugar that is plant in sprouting grain.
• Lactose is equanimous of a molecule of galactose joined to a molecule of glucose by a β-one,4-glycosidic linkage. It is a reducing sugar that is constitute in milk.
• Sucrose is composed of a molecule of glucose joined to a molecule of fructose by an α-1,β-2-glycosidic linkage. Information technology is a nonreducing sugar that is found in sugar pikestaff and sugar beets.

Exercises

1. Identify each saccharide past its common chemic proper name.

a. milk sugar

b. table sugar

2. Identify each saccharide by its common chemical name.

a. cane sugar

b. malt saccharide

3. For each disaccharide, bespeak whether the glycosidic linkage is α or β.

a.

b.

4. For each disaccharide, indicate whether the glycosidic linkage is α or β.

a.

b.

5. Place each disaccharide in Practice 3 every bit a reducing or nonreducing sugar. If it is a reducing saccharide, describe its construction and circumvolve the anomeric carbon. Land if the OH group at the anomeric carbon is in the α or the β position.

6. Identify each disaccharide in Exercise four as a reducing or nonreducing sugar. If it is a reducing sugar, draw its structure and circumvolve the anomeric carbon. State if the OH grouping at the anomeric carbon is in the α or β position.

seven. Melibiose is a disaccharide that occurs in some found juices. Its structure is as follows:

a. What monosaccharide units are incorporated into melibiose?

b. What blazon of linkage (α or β) joins the two monosaccharide units of melibiose?

c. Melibiose has a free anomeric carbon and is thus a reducing sugar. Circumvolve the anomeric carbon and indicate whether the OH group is α or β.

8. Cellobiose is a disaccharide composed of ii glucose units joined by a β-ane,four-glycosidic linkage.

a. Draw the structure of cellobiose.

b. Is cellobiose a reducing or nonreducing sugar? Justify your answer.

Answers

1.

a. lactose

b. sucrose

three.

a.

b.

five.

3a: nonreducing; 3b: reducing

7.

a. galactose and glucose

b. α-glycosidic linkage

c.

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Source: https://guides.hostos.cuny.edu/che120/chapter6

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