Prentice Hall c2002Chapter 81 Chapter 8 - Carbohydrates Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH 2 O) n, where n ≥ 3 Monosaccharides one monomeric unit Oligosaccharides ~2-20 monosaccharides Polysaccharides > 20 monosaccharides Glycoconjugates linked to proteins or lipids Trioses - 3 carbon sugars Tetroses - 4 carbon sugars Pentoses - 5 carbon sugars Hexoses - 6 carbon sugars

Prentice Hall c2002Chapter 82 Trioses - 3 carbon sugars Fig. 8.3

Prentice Hall c2002Chapter 83 Tetroses - 4 carbon sugars Fig. 8.3

Prentice Hall c2002Chapter 84 Pentoses - 5 carbon sugars Fig. 8.3

Prentice Hall c2002Chapter 85 Pentoses - 5 carbon sugars Fig. 8.3

Prentice Hall c2002Chapter 86 Hexoses - 6 carbon sugars Fig 8.3

Prentice Hall c2002Chapter 87 Hexoses - 6 carbon sugars Fig 8.3

Prentice Hall c2002Chapter 88 Enantiomers and epimers D-Sugars predominate in nature Enantiomers - pairs of D-sugars and L-sugars Epimers - sugars that differ at only one of several chiral centers Example: D-galactose is an epimer of D-glucose

Prentice Hall c2002Chapter 89 Fig 8.7 (a) Pyran and (b) furan ring systems (a) Six-membered sugar ring is a “pyranose” (b) Five-membered sugar ring is a “furanose”

Prentice Hall c2002Chapter 810 Fig 8.8 Cyclization of D-glucose to form glycopyranose In aqueous solution hexoses and pentoses will cyclize, forming alpha (  ) and beta (  ) forms

Prentice Hall c2002Chapter 811 Fig 8.9 Cyclization of D-ribose to form  - and  -D-ribopyranose and  - and  -D- ribofuranose

Prentice Hall c2002Chapter Derivatives of Monosaccharides Many sugar derivatives are found in biological systems Some are part of monosaccharides, oligosaccharides or polysaccharides These include sugar phosphates, deoxy and amino sugars, sugar alcohols and acids

Prentice Hall c2002Chapter 813 Sugar Phosphates Fig 8.13 Some important sugar phosphates

Prentice Hall c2002Chapter 814 Deoxy Sugars In deoxy sugars an H replaces an OH Fig 8.14 Deoxy sugars

Prentice Hall c2002Chapter 815 Amino Sugars An amino group replaces a monosaccharide OH Amino group is sometimes acetylated Amino sugars of glucose and galactose occur commonly in glycoconjugates

Prentice Hall c2002Chapter 816 Sugar Alcohols (polyhydroxy alcohols) Sugar alcohols: carbonyl oxygen is reduced Fig 8.16 Several sugar alcohols

Prentice Hall c2002Chapter 817 Sugar Acids Sugar acids are carboxylic acids Fig 8.17 Sugar acids derived from glucose

Prentice Hall c2002Chapter 818 Sugar Acids L-Ascorbic acid (Vitamin C) is derived from D-glucuronate Fig 8.18 L-Ascorbic acid L-Ascorbic acid (Vitamin C)

Prentice Hall c2002Chapter 819 Disaccharides and Other Glycosides Glycosidic bond - primary structural linkage in all polymers of monosaccharides Glucosides - glucose provides the anomeric carbon Fig 8.20 Structures of disaccharides (a) maltose, (b) cellobiose

Prentice Hall c2002Chapter 820 Fig 8.20 Structures of disaccharides (c) lactose, (d) sucrose

Prentice Hall c2002Chapter 821 Polysaccharides Homoglycans - homopolysaccharides containing only one type of monosaccharide Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide Lengths and compositions of a polysaccharide may vary within a population of these molecules

Prentice Hall c2002Chapter 822

Prentice Hall c2002Chapter 823 Starch D-Glucose is stored intracellularly in polymeric forms Plants and fungi store glucose as starch Starch is a mixture of amylose (unbranched) and amylopectin (branched every 25 sugars) (a)Amylose is a linear polymer Figure 8.22 (a)Amylopectin is a branched polymer Figure 8.23

Prentice Hall c2002Chapter 824 Amylose and Amylopectin form helical structures in starch granules of plants

Prentice Hall c2002Chapter 825 Starch is stored by plants and used as fuel.

Prentice Hall c2002Chapter 826 Glycogen is is stored by animals and used as fuel.

Prentice Hall c2002Chapter 827 Glycogen Glycogen is the main storage polysaccharide of humans. Glycogen is a polysaccharide of glucose residues connected by  -  1-4) linkages with  -(1-6) branches (one branch per 10 sugars). Glycogen is present in large amounts in liver and skeletal muscle.

Prentice Hall c2002Chapter 828 Cellulose, a structural polysaccharide in plants has  -(1-4) glycosidic bonds Fig 8.25 Structure of cellulose

Prentice Hall c2002Chapter 829 Fig 8.26 Cellulose fibrils Intra- and interchain Hydrogen bonds give strength

Prentice Hall c2002Chapter 830 Humans digest starch and glycogen ingested in their diet using amylases, enzymes that hydrolyze  -  1-4) glycosidic bonds. Humans cannot hydrolyze  -  1-4) linkages of cellulose. Therefore cellulose is not a fuel source for humans. It is fiber. Certain microorganisms have cellulases, enzymes that hydrolyze  -  1-4) linkages of cellulose. Cattle have these organisms in their rumen. Termites have them in their intestinal tract.

Prentice Hall c2002Chapter 831 Fig 8.27 Structure of chitin The exoskeleton of arthropods Repeating units of  -(1-4)GlcNAc residues GlcNAc = N-acetylglucosamine

Prentice Hall c2002Chapter 832 Glycoconjugates Heteroglycans appear in 3 types of glycoconjugates: 1. Proteoglycans 2. Peptidoglycans 3. Glycoproteins

Prentice Hall c2002Chapter 833 Proteoglycans Proteoglycans - glycosaminoglycan-protein complexes Glycosaminoglycans - unbranched heteroglycans of repeating disaccharides of amino sugars (D-galactosamine or D-glucosamine)

Prentice Hall c2002Chapter 834 Fig 8.28 Repeating disaccharide of hyaluronic acid, a glycosaminoglycan GlcUA = D-glucuronate GlcNAc= N-acetylglucosamine

Prentice Hall c2002Chapter 835 Fig 8.29 Proteoglycan aggregate of cartilage

Prentice Hall c2002Chapter 836 Peptidoglycans Peptidoglycans - heteroglycan chains linked to peptides Major component of bacterial cell walls Heteroglycan composed of alternating N- acetylglucosamine (GlcNAc) and N- acetylmuramic acid (MurNAc)  -(1-4) linkages connect the units

Prentice Hall c2002Chapter 837 Fig 8.30 Glycan moiety of peptidoglycan

Prentice Hall c2002Chapter 838 Fig 8.31 Structure of the peptidoglycan of the cell wall of Staphylococcus aureus (a) Repeating disaccharide unit, (b) Cross-linking of the peptidoglycan macromolecule

Prentice Hall c2002Chapter 839 Penicillin inhibits a transpeptidase involved in bacterial cell wall formation Fig 8.32 Structures of penicillin and -D-Ala-D-Ala Penicillin structure resembling -D-Ala- D-Ala is shown in red

Prentice Hall c2002Chapter 840 Glycoproteins Proteins that contain covalently-bound oligosaccharides, either to serine (O-Glycosidic linkage) or asparagine (N-glycosidic linkage) Oligosaccharide chains exhibit great variability in sugar sequence and composition Fig O-Glycosidic and N-glycosidic linkages

Prentice Hall c2002Chapter 841 Fig 8.34 and Types of glycosidic linkages