Harpers

Table 13–2. Pentoses of physiologic importance.CARBOHYDRATES OF PHYSIOLOGIC SIGNIFICANCE / 105Sugar Where Found Biochemical Importance Clinical SignificanceD-Ribose Nucleic acids. Structural elements of nucleic acids andcoenzymes, eg, ATP, NAD, NADP, flavoproteins.Ribose phosphates are intermediatesin pentose phosphate pathway.D-Ribulose Formed in metabolic processes. Ribulose phosphate is an intermediate inpentose phosphate pathway.D-Arabinose Gum arabic. Plum and cherry gums. Constituent of glycoproteins.D-Xylose Wood gums, proteoglycans, Constituent of glycoproteins.glycosaminoglycans.D-Lyxose Heart muscle. A constituent of a lyxoflavin isolated fromhuman heart muscle.L-Xylulose Intermediate in uronic acid pathway. Found in urine in essentialpentosuria.nucleic acids, and several coenzymes (Table 13–2).Glucose, galactose, fructose, and mannose are physiologicallythe most important hexoses (Table 13–3). Thebiochemically important aldoses are shown in Figure13–6, and important ketoses in Figure 13–7.In addition, carboxylic acid derivatives of glucose areimportant, including D-glucuronate (for glucuronideformation and in glycosaminoglycans) and its metabolicderivative, L-iduronate (in glycosaminoglycans)(Figure 13–8) and L-gulonate (an intermediate in theuronic acid pathway; see Figure 20–4).Sugars Form Glycosides With OtherCompounds & With Each OtherGlycosides are formed by condensation between the hydroxylgroup of the anomeric carbon of a monosaccharide,or monosaccharide residue, and a second compoundthat may—or may not (in the case of an aglycone)—beanother monosaccharide. If the second group is a hydroxyl,the O-glycosidic bond is an acetal link because itresults from a reaction between a hemiacetal group(formed from an aldehyde and an ⎯OH group) and an-Table 13–3. Hexoses of physiologic importance.Sugar Source Importance Clinical SignificanceD-Glucose Fruit juices. Hydrolysis of starch, cane The “sugar” of the body. The sugar carried Present in the urine (glycosuria)sugar, maltose, and lactose. by the blood, and the principal one used in diabetes mellitus owing toby the tissues.raised blood glucose (hyperglycemia).D-Fructose Fruit juices. Honey. Hydrolysis of Can be changed to glucose in the liver Hereditary fructose intolerancecane sugar and of inulin (from the and so used in the body. leads to fructose accumulationJerusalem artichoke).and hypoglycemia.D-Galactose Hydrolysis of lactose. Can be changed to glucose in the liver Failure to metabolize leadsand metabolized. Synthesized in the to galactosemia and cataract.mammary gland to make the lactose ofmilk. A constituent of glycolipids andglycoproteins.D-Mannose Hydrolysis of plant mannans and A constituent of many glycoproteins.gums.