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88607238-2721/21/037:38 AM Page244ac113ac11:aEDL 244 Part I Structure and Catalysis Glucose family Amino sugars CHoO CH。OH CHOOH CHOH CHoOHI OH H H NH2 H NH HH B-D-Galactosamine B-D-Mannosamine B-D-Glucosamine N-Acetyl-B-D-glucosamine Deoxy sugars CH2-0--POa CHOH CHOO OH H Hg XOH R=-0-C-H H HH HO 00 H NHo OH OH C=0 B-pD-Glucose 6-phosphate Muramic acid N-Acetylmuramie acid B-L-Fi g-L-Rhamnose CH3 CHoO H-C-OH CHoO B-D-Glucuronate D-Gluconate D-Glucono-s-lactone N-Acetylneuraminic acid (a sialic acid) FIGURE 7-9 Some hexose derivatives important in biology. In amino mers. The acidic sugars contain a carboxylate group, which confers a sugars,an-NH2 group replaces one of the -OH groups in the par- negative charge at neutral pH. D-Glucono-8-lactone results from for- ent hexose. Substitution of -H for -OH produces a deoxy sugar; mation of an ester linkage between the C-1 carboxylate group and the note that the deoxy sugars shown here occur in nature as the L iso- C-5 (also known as the 8 carbon) hydroxyl group of D-gluconate In the synthesis and metabolism of carbohydrates, Monosaccharides Are Reducing Agents e intermediates are very often not the sugars them selves but their phosphorylated derivatives. Condensation Monosaccharides can be oxidized by relative of phosphoric acid with one of the hydroxyl groups of a mild oxidizing agents such as ferric (Fe +)or sugar forms a phosphate ester, as in glucose 6-phosphate cupric(Cu-v ion(Fig. 7-10a). The carbonyl carbon is (Fig. 7-9). Sugar phosphates are relatively stable at neu- oxidized to a carboxyl group. Glucose and other sugars tral pH and bear a negative charge. One effect of sugar capable of reducing ferric or cupric ion are called re phosphorylation within cells is to trap the sugar inside the ducing sugars. This property is the basis of fehlings cell; most cells do not have plasma membrane trans- reaction, a qualitative test for the presence of reducing porters for phosphorylated sugars. Phosphorylation also sugar. By measuring the amount of oxidizing agent re- activates sugars for subsequent chemical transformation. duced by a solution of a sugar, it is also possible to es- Several important phosphorylated derivatives of sugars timate the concentration of that sugar. For many years are components of nucleotides(discussed in the next this test was used to detect and measure elevated glu- chapter) cose levels in blood and urine in the diagnosis of diaIn the synthesis and metabolism of carbohydrates, the intermediates are very often not the sugars themselves but their phosphorylated derivatives. Condensation of phosphoric acid with one of the hydroxyl groups of a sugar forms a phosphate ester, as in glucose 6-phosphate (Fig. 7–9). Sugar phosphates are relatively stable at neutral pH and bear a negative charge. One effect of sugar phosphorylation within cells is to trap the sugar inside the cell; most cells do not have plasma membrane transporters for phosphorylated sugars. Phosphorylation also activates sugars for subsequent chemical transformation. Several important phosphorylated derivatives of sugars are components of nucleotides (discussed in the next chapter). Monosaccharides Are Reducing Agents Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3) or cupric (Cu2) ion (Fig. 7–10a). The carbonyl carbon is oxidized to a carboxyl group. Glucose and other sugars capable of reducing ferric or cupric ion are called reducing sugars. This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar. By measuring the amount of oxidizing agent reduced by a solution of a sugar, it is also possible to estimate the concentration of that sugar. For many years this test was used to detect and measure elevated glucose levels in blood and urine in the diagnosis of dia- 244 Part I Structure and Catalysis CH2OH H O HO NH C PO3 N-Acetylmuramic acid R H OH H H H CH2 OH H HO D-Glucono--lactone OH H OH H H CH2OH H O HO NH2 -D-Mannosamine H OH H H H CH2OH H O OH HO OH H OH H H H H2N H O OH H OH H H H O CH3 -D-Glucose Muramic acid CH2OH H O OH HO NH C glucosamine H OH H H H O CH3 R Glucose family H OH HO -L-Rhamnose OH H OH H H O C O O C OH H HO -D-Glucuronate OH H OH H H H O O CH2OH CH2OH H N-Acetylneuraminic acid (a sialic acid) OH H O O -D-Glucosamine CH2OH H O OH HO NH2 H OH H H H -D-Galactosamine CH2OH H O OH HO CH3 H OH H H H CH2OH HO R O O O H H HO -D-Glucose 6-phosphate OH H OH H OH H O NH2 OH H HO -L-Fucose OH H OH H H H O CH3 H Amino sugars Acidic sugars Deoxy sugars O OH CH2OH H HO D-Gluconate OH H H H C OH H HN CH3 C O R H H 2 N-Acetyl--D- C OH H H OH C O COO O C H CH3 R FIGURE 7–9 Some hexose derivatives important in biology. In amino sugars, an ONH2 group replaces one of the OOH groups in the parent hexose. Substitution of OH for OOH produces a deoxy sugar; note that the deoxy sugars shown here occur in nature as the L isomers. The acidic sugars contain a carboxylate group, which confers a negative charge at neutral pH. D-Glucono--lactone results from formation of an ester linkage between the C-1 carboxylate group and the C-5 (also known as the carbon) hydroxyl group of D-gluconate. 8885d_c07_238-272 11/21/03 7:38 AM Page 244 Mac113 mac113:122_EDL: