Download Principles of BIOCHEMISTRY

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
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•Trioses - 3 carbon sugars
Fig. 8.3
•Tetroses - 4 carbon sugars
Fig. 8.3
•Pentoses - 5 carbon sugars
Fig. 8.3
•Pentoses - 5 carbon sugars
Fig. 8.3
•Hexoses - 6 carbon sugars
Fig 8.3
•Hexoses - 6 carbon sugars
Fig 8.3
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
Fig 8.6 (a) Pyran and (b) furan ring systems
• (a) Six-membered sugar
ring is a “pyranose”
• (b) Five-membered sugar
ring is a “furanose”
Fig 8.8 Cyclization of D-glucose
to form glycopyranose
In aqueous solution
hexoses and
pentoses will
cyclize, forming
alpha (a) and beta
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(b) forms
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Fig 8.10 Cyclization
of D-ribose to form aand
b-D-ribopyranose and
a- and b-Dribofuranose
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
Sugar Phosphates
Fig 8.14 Some important sugar phosphates
Deoxy Sugars
• In deoxy sugars an H replaces an OH
Fig 8.15 Deoxy sugars
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Amino Sugars
• An amino group replaces a monosaccharide OH
• Amino group is sometimes acetylated
• Amino sugars of glucose and galactose occur
commonly in glycoconjugates
Sugar Alcohols (polyhydroxy alcohols)
• Sugar alcohols: carbonyl oxygen is reduced
Fig 8.17 Several sugar alcohols
Sugar Acids
• Sugar acids are carboxylic acids
Fig 8.18 Sugar acids derived from glucose
Sugar Acids
• L-Ascorbic acid (Vitamin C) is derived from D-glucuronate
L-Ascorbic acid
(Vitamin C)
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Fig 8.18 L-Ascorbic
acid
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
Fig 8.20 Structures of disaccharides (c) lactose, (d) sucrose
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
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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
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Figure
8.23
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Amylose and Amylopectin form helical structures
in starch granules of plants
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Starch is stored by plants and used as fuel.
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Glycogen is is stored by animals and used as fuel.
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Glycogen
Glycogen is the main storage polysaccharide of humans.
Glycogen is a polysaccharide of glucose residues connected by
a -(1-4) linkages with a -(1-6) branches
(one branch per 10 sugars).
Glycogen is present in large amounts in liver and skeletal muscle.
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Cellulose, a structural polysaccharide
in plants has b-(1-4) glycosidic bonds
Fig 8.25 Structure of cellulose
Fig 8.26 Cellulose fibrils
• Intra- and interchain Hydrogen bonds give strength
Humans digest starch and glycogen ingested in their
diet using amylases, enzymes that hydrolyze
a-(1-4) glycosidic bonds.
Humans cannot hydrolyze b-(1-4) linkages of
cellulose. Therefore cellulose is not a fuel source
for humans. It is fiber.
Certain microorganisms have cellulases, enzymes that
hydrolyze b-(1-4) linkages of cellulose.
Cattle have these organisms in their rumen.
Termites have them in their intestinal tract.
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Fig 8.27 Structure of chitin
The exoskeleton of arthropods
• Repeating units of b-(1-4)GlcNAc residues
GlcNAc = N-acetylglucosamine
Glycoconjugates
• Heteroglycans appear in 3 types of glycoconjugates:
1. Proteoglycans
2. Peptidoglycans
3. Glycoproteins
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Proteoglycans
• Proteoglycans - glycosaminoglycan-protein
complexes
• Glycosaminoglycans - unbranched
heteroglycans of repeating disaccharides of
amino sugars
(D-galactosamine or D-glucosamine)
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Fig 8.28 Repeating disaccharide
of hyaluronic acid, a glycosaminoglycan
• GlcUA =
D-glucuronate
• GlcNAc=
N-acetylglucosamine
Fig 8.29 Proteoglycan aggregate of cartilage
Peptidoglycans
• Peptidoglycans - heteroglycan chains linked to
peptides
• Major component of bacterial cell walls
• Heteroglycan composed of alternating Nacetylglucosamine (GlcNAc) and Nacetylmuramic acid (MurNAc)
• b-(1-4) linkages connect the units
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Fig 8.30 Glycan moiety of peptidoglycan
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
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-AlaD-Ala is shown in red
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
Fig 8.34 and 8.35. Types of
glycosidic linkages