Post on 19-Dec-2015
Prentice Hall c2002 Chapter 8 1
Chapter 8 - Carbohydrates
• Carbohydrates (“hydrate of carbon”) have empirical formulas of (CH2O)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 c2002 Chapter 8 8
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 c2002 Chapter 8 9
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 c2002 Chapter 8 10
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 c2002 Chapter 8 11
Fig 8.9 Cyclization of D-ribose to form -
and -D-ribopyranose and
- and -D-ribofuranose
Prentice Hall c2002 Chapter 8 12
8.4 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 c2002 Chapter 8 14
Deoxy Sugars
• In deoxy sugars an H replaces an OH
Fig 8.14 Deoxy sugars
Prentice Hall c2002 Chapter 8 15
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 c2002 Chapter 8 16
Sugar Alcohols (polyhydroxy alcohols)
• Sugar alcohols: carbonyl oxygen is reduced
Fig 8.16 Several sugar alcohols
Prentice Hall c2002 Chapter 8 17
Sugar Acids
• Sugar acids are carboxylic acids
Fig 8.17 Sugar acids derived from glucose
Prentice Hall c2002 Chapter 8 18
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 c2002 Chapter 8 19
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 c2002 Chapter 8 21
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 c2002 Chapter 8 23
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 c2002 Chapter 8 24
Amylose and Amylopectin form helical structures in starch granules of plants
Prentice Hall c2002 Chapter 8 27
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 c2002 Chapter 8 28
Cellulose, a structural polysaccharide in plants has -(1-4) glycosidic bonds
Fig 8.25 Structure of cellulose
Prentice Hall c2002 Chapter 8 29
Fig 8.26 Cellulose fibrils
• Intra- and interchain Hydrogen bonds give strength
Prentice Hall c2002 Chapter 8 30
Humans digest starch and glycogen ingested in theirdiet using amylases, enzymes that hydrolyze -1-4) glycosidic bonds.
Humans cannot hydrolyze -1-4) linkages ofcellulose. Therefore cellulose is not a fuel sourcefor humans. It is fiber.
Certain microorganisms have cellulases, enzymes thathydrolyze -1-4) linkages of cellulose.
Cattle have these organisms in their rumen.Termites have them in their intestinal tract.
Prentice Hall c2002 Chapter 8 31
Fig 8.27 Structure of chitinThe exoskeleton of arthropods
• Repeating units of -(1-4)GlcNAc residues
GlcNAc = N-acetylglucosamine
Prentice Hall c2002 Chapter 8 32
Glycoconjugates
• Heteroglycans appear in 3 types of glycoconjugates:
1. Proteoglycans
2. Peptidoglycans
3. Glycoproteins
Prentice Hall c2002 Chapter 8 33
Proteoglycans
• Proteoglycans - glycosaminoglycan-protein complexes
• Glycosaminoglycans - unbranched heteroglycans of repeating disaccharides of amino sugars
(D-galactosamine or D-glucosamine)
Prentice Hall c2002 Chapter 8 34
Fig 8.28 Repeating disaccharide of hyaluronic acid, a glycosaminoglycan
• GlcUA =D-glucuronate
• GlcNAc= N-acetylglucosamine
Prentice Hall c2002 Chapter 8 36
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 c2002 Chapter 8 38
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 c2002 Chapter 8 39
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 c2002 Chapter 8 40
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. 8.33 O-Glycosidic and N-glycosidic linkages