Harpers

106 / CHAPTER 13CH 2 OHCH 2 OHCOCH 2 OHCH 2 OHCOHOCHCOCOHOCHHCOHCH 2 OHHOCHHCOHHCOHHCOHCOHCOHHCOHHCOHHCOHCH 2 OHCH 2 OHCH 2 OHCH 2 OHDihydroxyacetone D-XyluloseD-RibuloseD-FructoseFigure 13–7. Examples of ketoses of physiologic significance.CH 2 OHD-Sedoheptuloseother ⎯OH group. If the hemiacetal portion is glucose,the resulting compound is a glucoside; if galactose, agalactoside; and so on. If the second group is an amine,an N-glycosidic bond is formed, eg, between adenine andribose in nucleotides such as ATP (Figure 10–4).Glycosides are widely distributed in nature; the aglyconemay be methanol, glycerol, a sterol, a phenol, or abase such as adenine. The glycosides that are importantin medicine because of their action on the heart (cardiacglycosides) all contain steroids as the aglycone.These include derivatives of digitalis and strophanthussuch as ouabain, an inhibitor of the Na + -K + ATPase ofcell membranes. Other glycosides include antibioticssuch as streptomycin.Deoxy Sugars Lack an Oxygen AtomDeoxy sugars are those in which a hydroxyl group hasbeen replaced by hydrogen. An example is deoxyribose(Figure 13–9) in DNA. The deoxy sugar L-fucose (Figure13–15) occurs in glycoproteins; 2-deoxyglucose is usedexperimentally as an inhibitor of glucose metabolism.Amino Sugars (Hexosamines) AreComponents of Glycoproteins,Gangliosides, & GlycosaminoglycansThe amino sugars include D-glucosamine, a constituentof hyaluronic acid (Figure 13–10), D-galactosamine(chondrosamine), a constituent of chondroitin; andD-mannosamine. Several antibiotics (eg, erythromycin)contain amino sugars believed to be important for theirantibiotic activity.MALTOSE, SUCROSE, & LACTOSE AREIMPORTANT DISACCHARIDESThe physiologically important disaccharides are maltose,sucrose, and lactose (Table 13–4; Figure 13–11).Hydrolysis of sucrose yields a mixture of glucose andHHOCOO – HOOH HH COO –OHHHOHOHHOOHFigure 13–8. α-D-Glucuronate (left) andβ-L-iduronate (right).Figure 13–9.5HOCH 2HOCH 2HHO4H H 3OHHOHHOH1H HOHH+ NH3Figure 13–10. Glucosamine (2-amino-D-glucopyranose)(α form). Galactosamine is 2-amino-D-galactopyranose.Both glucosamine and galactosamine occur asN-acetyl derivatives in more complex carbohydrates,eg, glycoproteins.O2HOHHHOHHOH2-Deoxy-D-ribofuranose (β form).