Chapter 8 Carbohydrates

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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 gt 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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Fig. 8.3

• Trioses - 3 carbon sugars

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Fig. 8.3

• Tetroses - 4 carbon sugars

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Fig. 8.3

• Pentoses - 5 carbon sugars

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Fig. 8.3

• Pentoses - 5 carbon sugars

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Fig 8.3

• Hexoses - 6 carbon sugars

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Fig 8.3

• Hexoses - 6 carbon sugars

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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

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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

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Fig 8.8 Cyclization of D-glucose to form
glycopyranose
In aqueous solution hexoses and pentoses will
cyclize, forming alpha (a) and beta (b) forms
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Fig 8.9 Cyclization of D-ribose to form a- and
b-D-ribopyranose and a- and b-D-ribofuranose
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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

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Sugar Phosphates
Fig 8.13 Some important sugar phosphates
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Deoxy Sugars

• In deoxy sugars an H replaces an OH
• Fig 8.14 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

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Sugar Alcohols (polyhydroxy alcohols)

• Sugar alcohols carbonyl oxygen is reduced
• Fig 8.16 Several sugar alcohols

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Sugar Acids

• Sugar acids are carboxylic acids

Fig 8.17 Sugar acids derived from glucose
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Sugar Acids

• L-Ascorbic acid (Vitamin C) is derived from
  D-glucuronate

L-Ascorbic acid (Vitamin C)
Fig 8.18 L-Ascorbic acid
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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
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Fig 8.20 Structures of disaccharides (c) lactose,
(d) sucrose
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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)

• Amylose is a linear polymer
• Figure 8.22

• Amylopectin is a branched polymer
• 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
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Fig 8.26 Cellulose fibrils

• Intra- and interchain Hydrogen bonds give strength

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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 chitinThe exoskeleton of
arthropods

• Repeating units of b-(1-4)GlcNAc residues

GlcNAc N-acetylglucosamine
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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

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Fig 8.29 Proteoglycan aggregate of cartilage
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Peptidoglycans
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Fig 8.30 Glycan moiety of peptidoglycan
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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
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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

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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
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Fig 8.34 and 8.35. Types of glycosidic linkages