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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 11.1Monosaccharides Are Aldehydes or Ketones with Multiple Hydroxyl Groups

Monosaccharides, the simplest carbohydrates, are aldehydes or ketones that have two or more hydroxyl groups; the empirical formula of many is (C-H2O)n, literally a “carbon hydrate.” Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n = 3, are dihydroxyacetone and d- and l-glyceraldehyde.

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They are referred to as trioses (tri- for 3). Dihydroxyacetone is called a ketose because it contains a keto group, whereas glyceraldehyde is called an aldose because it contains an aldehyde group. Glyceraldehyde has a single asymmetric carbon and, thus, there are two stereoisomers of this sugar. d-Glyceraldehyde and l-glyceraldehyde are enantiomers, or mirror images of each other. As mentioned in Chapter 3, the prefixes d and l designate the absolute configuration. Monosaccharides and other sugars will often be represented in this book by Fischer projections (Figure 11.1). Recall that, in a Fischer projection of a molecule, atoms joined to an asymmetric carbon atom by horizontal bonds are in front of the plane of the page, and those joined by vertical bonds are behind (see the Appendix in Chapter 1). Fischer projections are useful for depicting carbohydrate structures because they provide clear and simple views of the stereochemistry at each carbon center.

Figure 11.1. Fischer Projections of Trioses.

Figure 11.1

Fischer Projections of Trioses. The top structure reveals the stereochemical relations assumed for Fischer projections.

Simple monosaccharides with four, five, six, and seven carbon atoms are called tetroses, pentoses, hexoses, and heptoses, respectively. Because these molecules have multiple asymmetric carbons, they exist as diastereoisomers, isomers that are not mirror images of each other, as well as enantiomers. In regard to these monosaccharides, the symbols d and l designate the absolute configuration of the asymmetric carbon farthest from the aldehyde or keto group. Figure 11.2 shows the common d-aldose sugars. d-Ribose, the carbohydrate component of RNA, is a five-carbon aldose. d-Glucose, d-mannose, and d-galactose are abundant six-carbon aldoses. Note that d-glucose and d-mannose differ in configuration only at C-2. Sugars differing in configuration at a single asymmetric center are called epimers. Thus, d-glucose and d-mannose are epimeric at C-2; d-glucose and d-galactose are epimeric at C-4.

Figure 11.2. d-Aldoses containing three, four, five, and six carbon atoms.

Figure 11.2

d-Aldoses containing three, four, five, and six carbon atoms. d-Aldoses contain an aldehyde group (shown in blue) and have the absolute configuration of d-glyceraldehyde at the asymmetric center (shown in red) farthest from the aldehyde group. The numbers (more...)

Dihydroxyacetone is the simplest ketose. The stereochemical relation between d-ketoses containing as many as six carbon atoms are shown in Figure 11.3. Note that ketoses have one fewer asymmetric center than do aldoses with the same number of carbons. d-Fructose is the most abundant ketohexose.

Figure 11.3. d -Ketoses containing three- four, five, and six carbon atoms.

Figure 11.3

d -Ketoses containing three- four, five, and six carbon atoms. The keto group is shown in blue. The asymmetric center farthest from the keto group, which determines the d designation, is shown in red.

11.1.1. Pentoses and Hexoses Cyclize to Form Furanose and Pyranose Rings

The predominant forms of ribose, glucose, fructose, and many other sugars in solution are not open chains. Rather, the open-chain forms of these sugars cyclize into rings. In general, an aldehyde can react with an alcohol to form a hemiacetal.

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For an aldohexose such as glucose, the C-1 aldehyde in the open-chain form of glucose reacts with the C-5 hydroxyl group to form an intramolecular hemiacetal. The resulting cyclic hemiacetal, a six-membered ring, is called pyranose because of its similarity to pyran (Figure 11.4). Similarly, a ketone can react with an alcohol to form a hemiketal.

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Figure 11.4. Pyranose Formation.

Figure 11.4

Pyranose Formation. The open-chain form of glucose cyclizes when the C-5 hydroxyl group attacks the oxygen atom of the C-1 aldehyde group to form an intramolecular hemiacetal. Two anomeric forms, designated α and β, can result.

The C-2 keto group in the open-chain form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal (Figure 11.5). The five-membered ring is called a furanose because of its similarity to furan.

Figure 11.5. Furanose Formation.

Figure 11.5

Furanose Formation. The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown.

The depictions of glucopyranose and fructofuranose shown in Figures 11.4 and 11.5 are Haworth projections. In such projections, the carbon atoms in the ring are not explicitly shown. The approximate plane of the ring is perpendicular to the plane of the paper, with the heavy line on the ring projecting toward the reader. Like Fischer projections, Haworth projections allow easy depiction of the stereochemistry of sugars. We will return to a more structurally realistic view of the conformations of cyclic monosaccharides shortly.

An additional asymmetric center is created when a cyclic hemiacetal is formed. In glucose, C-1, the carbonyl carbon atom in the open-chain form, becomes an asymmetric center. Thus, two ring structures can be formed: α-d-glucopyranose and β-d-glucopyranose (see Figure 11.4). For d sugars drawn as Haworth projections, the designation α means that the hydroxyl group attached to C-1 is below the plane of the ring; β means that it is above the plane of the ring. The C-1 carbon atom is called the anomeric carbon atom, and the α and β forms are called anomers. An equilibrium mixture of glucose contains approximately one-third α anomer, two-thirds β anomer, and <1% of the open-chain form.

The same nomenclature applies to the furanose ring form of fructose, except that α and β refer to the hydroxyl groups attached to C-2, the anomeric carbon atom (see Figure 11.5). Fructose forms both pyranose and furanose rings. The pyranose form predominates in fructose free in solution, and the furanose form predominates in many fructose derivatives (Figure 11.6). Pentoses such as d-ribose and 2-deoxy-d-ribose form furanose rings, as we have seen in the structure of these units in RNA and DNA.

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Figure 11.6. Ring Structures of Fructose.

Figure 11.6

Ring Structures of Fructose. Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both α and β anomers are possible.

11.1.2. Conformation of Pyranose and Furanose Rings

The six-membered pyranose ring is not planar, because of the tetrahedral geometry of its saturated carbon atoms. Instead, pyranose rings adopt two classes of conformations, termed chair and boat because of the resemblance to these objects (Figure 11.7). In the chair form, the substituents on the ring carbon atoms have two orientations: axial and equatorial. Axial bonds are nearly perpendicular to the average plane of the ring, whereas equatorial bonds are nearly parallel to this plane. Axial substituents sterically hinder each other if they emerge on the same side of the ring (e.g., 1,3-diaxial groups). In contrast, equatorial substituents are less crowded. The chair form of β-d-glucopyranose predominates because all axial positions are occupied by hydrogen atoms. The bulkier -OH and -CH2OH groups emerge at the less-hindered periphery. The boat form of glucose is disfavored because it is quite sterically hindered.

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Figure 11.7. Chair and Boat Forms of β - d -glucopyranose.

Figure 11.7

Chair and Boat Forms of β - d -glucopyranose. The chair form is more stable because of less steric hindrance as the axial positions are occupied by hydrogen atoms.

Furanose rings, like pyranose rings, are not planar. They can be puckered so that four atoms are nearly coplanar and the fifth is about 0.5 Å away from this plane (Figure 11.8). This conformation is called an envelope form because the structure resembles an opened envelope with the back flap raised. In the ribose moiety of most biomolecules, either C-2 or C-3 is out of the plane on the same side as C-5. These conformations are called C2-endo and C3-endo, respectively.

Figure 11.8. Envelope Conformations of β - d -ribose.

Figure 11.8

Envelope Conformations of β - d -ribose. The C2-endo and C3-endo forms of β-d-ribose are shown. The color indicates the four atoms that lie approximately in a plane.

11.1.3. Monosaccharides Are Joined to Alcohols and Amines Through Glycosidic Bonds

Monosaccharides can be modified by reaction with alcohols and amines to form adducts. For example, d-glucose will react with methanol in an acid-catalyzed process: the anomeric carbon atom reacts with the hydroxyl group of methanol to form two products, methyl α-d-glucopyranoside and methyl β-d-glucopyranoside. These two glucopyranosides differ in the configuration at the anomeric carbon atom. The new bond formed between the anomeric carbon atom of glucose and the hydroxyl oxygen atom of methanol is called a glycosidic bond—specifically, an O-glycosidic bond. The anomeric carbon atom of a sugar can be linked to the nitrogen atom of an amine to form an N-glycosidic bond.

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Indeed, we have previously encountered such reaction products; nu-cleosides are adducts between sugars such as ribose and amines such as adenine (Section 5.1.1). Some other important modified sugars are shown in Figure 11.9. Compounds such as methyl glucopyranoside show differences in reactivity from that of the parent monosaccharide. For example, unmodified glucose reacts with oxidizing agents such as cupric ion (Cu2+) because the open-chain form has a free aldehyde group that is readily oxidized.

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Figure 11.9. Modified Monosaccharides.

Figure 11.9

Modified Monosaccharides. Carbohydrates can be modified by the addition of substituents (shown in red) other than hydroxyl groups. Such modified carbohydrates are often expressed on cell surfaces.

Glycosides such as methyl glucopyranoside do not react, because they are not readily interconverted with a form that includes a free aldehyde group. Solutions of cupric ion (known as Fehling's solution) provide a simple test for sugars such as glucose. Sugars that react are called reducing sugars; those that do not are called nonreducing sugars.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, W. H. Freeman and Company.
Bookshelf ID: NBK22547


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