Hh

Pyran

Furan

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.

whereas two configurations can be interconverted only by breaking a covalent bond—for example, in the case of a and 3 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.

Organisms Contain a Variety of Hexose Derivatives

In addition to simple hexoses such as glucose, galactose, and mannose, there are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group (Fig. 7-9). In glucosamine, galactosamine, and mannosamine, the hydroxyl at C-2 of the parent compound is replaced with an amino group. The amino group is nearly always condensed with acetic acid, as in ^-acetylglucosamine. This glucosamine derivative is part of many structural polymers, including those of the bacterial cell wall. Bacterial cell walls also contain a derivative of glucosamine, ^-acetylmu-ramic acid, in which lactic acid (a three-carbon car-boxylic acid) is ether-linked to the oxygen at C-3 of ^-acetylglucosamine. The substitution of a hydrogen for the hydroxyl group at C-6 of l-galactose or l-mannose produces l-fucose or l-rhamnose, respectively; these deoxy sugars are found in plant polysaccharides and in the complex oligosaccharide components of glycopro-teins and glycolipids.

Oxidation of the carbonyl (aldehyde) carbon of glucose to the carboxyl level produces gluconic acid; other aldoses yield other aldonic acids. Oxidation of the carbon at the other end of the carbon chain—C-6 of glucose, galactose, or mannose—forms the corresponding uronic acid: glucuronic, galacturonic, or mannuronic acid. Both aldonic and uronic acids form stable intramolecular esters called lactones (Fig. 7-9, lower left). In addition to these acidic hexose derivatives, one nine-carbon acidic sugar deserves mention: ^-acetylneuraminic acid (a sialic acid, but often referred to simply as "sialic acid"), a derivative of ^-acetylmannosamine, is a component of many glycoproteins and glycolipids in animals. The carboxylic acid groups of the acidic sugar derivatives are ionized at pH 7, and the compounds are therefore correctly named as the carboxylates—glucuronate, galacturonate, and so forth.

Axis

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Two possible chair forms (a)

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