Disaccharides and Polysaccharides

Describe the monosaccharide units and iinkages in disaccharides and polysaccharides. 

A disaccharide is composed of two monosaccharides linked together. The most common disaccharides are maltose, lactose, and sucrose. 

FIGURE 18.2 Lactose is a disaccharide found in miik and miik products. 

Cl What type of giycosidic bond iinks gaiactose and glucose in lactose  

FIGURE 18.3 Sucrose is a disaccharide obtained from sugar beets and sugar cane. Cl What monosaccharides form sucrose  

The hydroxyl group on the anomeric carbon of the monosaccharide molecule can be replaced by another monosaccharide molecule. The resulting dimer is called a disaccharide. Binding of a-D-glucopyranose to p-D-fractofuranose gives the 

In the same way, glucose molecules can be connected by a glycosidic bond into long polymeric structures of polysaccharides. The structure of the polysaccharide formed from glucose depends mostly on the starting anomer. Polymerization of a-D-glucopyranose by the a(l,4)-glycosidic bond yields amylose, the polysaccharide which is a component of starch. 

Polymerization of another anomer, p-D-glucopyranose by the P(l,4)-glycosidic bond forms cellulose. 

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Carbohydrates may be divided into monosaccharides, disaccharides and polysaccharides. The monosaccharides under certain conditions react as polyhydroxy-aldehydes or polyhydroxy-ketones two important representatives are glucose CjHjjO (an aldose) and fructose (laevulose) CgHuO, (a ketose). Upon hydrolysis di- and polysaccharides 3deld ultimately monosaccharides. Common disaccharides are sucrose, lactose and maltose (all of molecular formula C,2H2. 0,), whilst starch, dextrin and cellulose, (CjHjoOj), in which n > 4, are typical polysaccharides. 

Sections Disaccharides are carbohydrates in which two monosaccharides are 25.14-25.15 joined by a glycoside bond. Polysaccharides have many monosaccharide units connected through glycosidic linkages. Complete hydrolysis of disaccharides and polysaccharides cleaves the glycoside bonds, yielding the free monosaccharide components. 

It seems quite possible that not only true phosphorylase but also other transglycosidases may play an important role in the biological syntheses, decompositions and interconversions of disaccharides and polysaccharides. 

Carbohydrates are literally hydrates of carbon, containing only the elements carbon, oxygen, and hydrogen. In the human diet, they are considered macronutrients, along with proteins and fats (triacylglycerols). The three types of carbohydrates are monosaccharides, disaccharides, and polysaccharides. 

Disaccharides and polysaccharides form a high proportion of the diet, and are present in natural and processed foods starch, the polysaccharide... 

Digestible carbohydrates include simple sugars, disaccharides, and polysaccharides (such as starches). 

Carbohydrates are divided into three groups called monosaccharides, disaccharides, and polysaccharides. The monosaccharides are the simple sugars, like glucose. The disaccharides, which are composed of two monosaccharides bonded together, include sucrose, or table sugar. The polysaccharides are polymers of monosaccharides and include starch and cellulose, which are both made from glucose. 

Glucose is by far the most abundant monosaccharide it occurs free in fruits, plants, honey, in the blood of animals, and combined in many glycosides, disaccharides, and polysaccharides. The structure and properties of glucose will be considered in greater detail than those of the other monosaccharides, not only because of its importance, but because much of what can be said about glucose also can be said about the other monosaccharides. 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a. and p structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 ratio of p-D-glucose to a-D-glucose. Although their chemical constituents are identical, the biochemical properties between the a and the P forms can be quite different. Monosaccharides linked together to form disaccharides and polysaccharides cannot continue to interconvert and are therefore frozen in the a or p forms. Changing one monosaccharide in a complex carbohydrate to its opposite... 

There are several disaccharides and polysaccharides for which you should know the common name. 

Wine is one of the most complex and interesting matrices for a number of reasons. It is composed of volatile compounds, some of them responsible for the odor, and nonvolatile compounds which cause taste sensations, such as sweetness (sugars), sourness (organic acids), bitterness (polyphenols), and saltiness (mineral substances Rapp and Mandary, 1986). With a few exceptions, those compounds need to be present in levels of 1%, or even more, to influence taste. Generally, the volatile components can be perceived in much lower concentrations, since our organs are extremely sensitive to certain aroma substances (Rapp et ah, 1986). Carbohydrates (monosaccharides, disaccharides, and polysaccharides), peptides, proteins, vitamins, and mineral substances are among the other wine constituents. 

Disaccharides and polysaccharides are also oxidized by the same degradative oxidation at first they react rapidly then, when the point of branching is reached, they react much more slowly. When subjected to similar oxidations, ascorbic acid affords threonic acid and oxalic acid, and 2-deoxy sugars yield 2-deoxyaldonic acids and lower alditols. 

Hydrolysis of acetal groups. Disaccharides and polysaccharides can be converted into monosaccharides by hydrolysis. The following is an example ... 

Understanding the General Structural Features of Disaccharides and Polysaccharides... 

The major reason that the formation of glycosides is so important is that disaccharides and polysaccharides are formed from monosaccharide units held together by gly-cosidic bonds. The oxygen of a hydroxy group from one sugar is used to form a bond to the acetal carbon of another monosaccharide. This process is discussed in Sections 25.6 and 25.7. 

Understand the general structural features of disaccharides and polysaccharides. (Problem 25.47)... 

To understand the chemistry of these more complex carbohydrates, we must first learn the principles of carbohydrate structure and reactions, using the simplest monosaccharides as examples. Then we will apply these principles to more complex disaccharides and polysaccharides. The chemistry of carbohydrates applies the chemistry of alcohols, aldehydes, and ketones to these polyfunctional compounds. In general, the chemistry of biomolecules can be predicted by applying the chemistry of simple organic molecules with similar functional groups. 

Disaccharides and polysaccharides are glycosides in which the alcohol forming the glycoside bond is an —OH group of another monosaccharide. We will consider disaccharides and polysaccharides in Sections 23-17 and 22-18. 

Q Draw the common types of glycosidic linkages, and identify these linkages in disaccharides and polysaccharides. Problems 23-59, 60, 61, and 64... 

Hydrolysis is the reverse process of condensation as a water molecule and specific enzymes break all the glycosidic linkages in disaccharides and polysaccharides into their constituting monosaccharides. 

Reaction with acids in acid medium, disaccharides and polysaccharides may be hydrolyzed to monosaccharides, which in turn may be dehydrated and degraded... 

Glycosides are common in nature. All disaccharides and polysaccharides are formed by joining monosaccharides together with glycosidic linkages. These compounds are discussed in detail beginning in Section 27.12. 

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