Monosaccharides and Disaccharides:- The Common Monosaccharides Have Cyclic Structures

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7–3, 7–4). In fact, in aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl

FIGURE 7–3 Aldoses and ketoses. The series of (a)D-aldoses and (b)D-ketoses having from three to six carbon atoms, shown as projection formulas. The carbon atoms in red are chiral centers. In all these Disomers, the chiral carbon most distant from the carbonyl carbonhas the same configuration as the chiral carbon in D-glyceraldehyde. The sugars named in boxes are the most common in nature; you will encounter these again in this and later chapters.

FIGURE 7–4 Epimers.D-Glucose and two of its epimers are shown as projection formulas. Each epimer differs from D-glucose in the con figuration at one chiral center (shaded red).

FIGURE 7–5 Formation of hemiacetals and hemiketals. An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond (p. 245).

group along the chain. The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals (Fig. 7–5), which contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms. For example, D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbona symmetric and producing two stereoisomers, designated and (Fig. 7–6). These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran (Fig. 7–7). The systematic names for the two ring forms of D-glucose are-D-glucopyranose and -D-glucopyranose. Aldohexoses also exist in cyclic forms having five membered rings, which, because they resemble the five membered ring compound furan, are called furanoses. However, the six-membered Aldo pyranose ring is much more stable than the Aldo furanose ring and predomi nates in aldohexose solutions. Only aldoses having five or more carbon atoms can form pyranose rings. Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. The and anomers of D-glucose interconvert in aqueous solution by a process called mutarotation. Thus, a solution of -D-glucose and a solution of -D-glucose eventually form identical equilibrium mixtures having identical optical properties. This mixture consists of about one-third -D-glucose, two-thirds -D-glucose, and very small amounts of the linear and five-membered ring (glucofuranose) forms. Ketohexoses also occur in and anomeric forms. In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or pyranose) ring containing a hemiketal linkage (Fig. 7–5). D-Fructose readily forms the furanose ring (Fig. 7–7); the more common anomer of this sugar in combined forms or in derivatives is -D-fructofuranose. Haworth perspective formulas like those in Fig ure 7–7 are commonly used to show the stereochemistry of ring forms of monosaccharides. However, the six-membered pyranose ring is not planar, as Haworth perspectives suggest, but tends to assume either of two “chair” conformations (Fig. 7–8). Recall from Chapter 1 (p. 19) that two conformations of a molecule are in terconvertible without the breakage of covalent bonds, whereas two configurations can be interconverted only by breaking a covalent bond—for example, in the case α of β and configurations, the bond involving the ring oxygen atom. The specific three-dimensional conformations of the monosaccharide units are important in determining the biological properties and functions of some polysaccharides, as we shall see.

FIGURE 7–6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the and anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of and anomers is called mutarotation.

FIGURE 7–7 Pyranoses and furanoses. The pyranose forms of D glucose and the furanose forms of D-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Fig. 7–6). Pyran and furan are shown for comparison.

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Polysaccharides:- Glycosaminoglycans Are Heteropolysaccharides of the Extracellular Matrix

Polysaccharides:- Bacterial and Algal Cell Walls Contain Structural Hetero polysaccharides

Polysaccharides: -Steric Factors and Hydrogen Bonding Influence Homopolysaccharide Folding

Polysaccharides:- Some Homopolysaccharides Serve Structural Roles

Polysaccharides:- Some Homopolysaccharides Are Stored Forms of Fuel

Monosaccharides and Disaccharides:- Disaccharides Contain a Glycosidic Bond

Monosaccharides and Disaccharides:- Monosaccharides Are Reducing Agents

Monosaccharides and Disaccharides:- Organisms Contain a Variety of Hexose Derivatives

Monosaccharides and Disaccharides:- Monosaccharides Have Asymmetric Centers

Monosaccharides and Disaccharides: - The Two Families of Monosaccharides Are Aldoses and Ketoses

Regulatory Enzymes:- Some Enzymes and Other Proteins Are Regulated by Proteolytic Cleavage of an Enzyme Precursor

Regulatory Enzymes:- Multiple Phosphorylations Allow Exquisite Regulatory Control