1. Berg • Tymoczko • Stryer
Biochemistry
Sixth Edition
Chapter 11:
Carbohydrates
Copyright © 2007 by W. H. Freeman and Company

2. Carbohydrates
 Carbohydrates are aldehyde or ketone compounds with multiple hydroxyl
groups.
 They make up most of the organic matter on Earth and play extensive roles in
all forms of life
 They serve as energy stores, fuels, & metabolic intermediates
 Ribose & deoxyribose sugars form part of the structural framework of RNA &
DNA
 Polysaccharides are structural elements in the cell walls of bacteria and plants.
Cellulose is the most abundant organic compound in the biosphere.
 Carbohydrates are linked to many proteins and lipids
 key role in mediating interactions among cells and with other elements

3. Monosaccharides
 Simplest carbohydrates
 Aldehydes or ketones with two or more hydroxyl groups
 Empirical formula, (C-H2O)n, literally a “carbon hydrate”
 Important fuel molecules as well as building blocks for nucleic acid
 Smallest monosaccharides are trioses (n = 3)
Glyceraldehyde has
1 asymmetric C
2 stereoisomers of Glyceraldehyde
•(D- & L-)
•are enantiomers (mirror images of each other)

4. Monosaccharides
 Monosaccharides (simple sugar molecules) can be
classified according to the number of carbon atoms in the
chain. Those most commonly found in humans include the
following:
– 3 carbons: trioses (e.g., glyceraldehyde)
– 4 carbons: tetroses (e.g., erythrose)
– 5 carbons: pentoses (e.g., ribose, ribulose, xylose, xylulose)
– 6 carbons: hexoses (e.g., glucose, galactose, mannose, fructose)

5. Stereoisomers
 Stereoisomers are isomeric molecules that have
Same molecular formula and sequence of bonded atoms (constitution)
 But which differ only in the three-dimensional orientations of their atoms
in space
Two types
Enantiomers
Diastereomers

6. Enantiomers
 Enantiomers are two stereoisomers that are related to each other by a
reflection:
– they are mirror images of each other
– which are non-superimposable
– Human hands are a macroscopic example of stereoisomerism
S enantiomer R enantiomer

7. Diastereomers
 Diastereomers are stereoisomers
– that are not enantiomers
– they are distinct molecules with the same structural arrangement of atoms that
are
• non-superimposible
• Not mirror images of each other
– epimers are diastereomers that differ in configuration of only one stereogenic
center
1 2 3
(2S,3S)-tartaric acid (2R,3R)-tartaric acid (2R,3S)-tartaric acid
1 and 2 are enantiomers since they are mirror images
1and 3 are diastereomers
2 and 3 are diastereomers (epimers)

8. Monosaccharides
 Simple momosaccharides with more than 3 carbon atoms
– have multiple asymmetric carbon atoms
– exist as diastereomers
– Symbol D and L designate the absolute configuration of the
asymmetric carbon farthest from the aldehyde or keto
group.

9. D-Aldoses
Aldehyde group
Numbering
Distal asymmetric center
Epimers at C2

14. Pentoses and Hexoses cyclize to form Furanose and
Pyranose Rings
 Sugars in solution exist predominantly as rings and not open chains
 Open chain forms cyclize into rings
– Aldehyde reacts with an alcohol to form hemiacetal
– Ketone reacts with an alcohol to form a hemiketal

15. Cyclization of aldoses: pyranose
 For aldoses, eg glucose
– C-1 aldehyde in open chain reacts with the C-5 hydroxyl group to form an
intramolecular hemiacetal
– Resulting six membered ring: pyranose because it resembles pyran

16. Cyclization of ketoses: furanose
 For ketoses, eg Fructose
– C-2 of keto group in open chain reacts with the C-5 hydroxyl group to form an
intramolecular hemiketal
– Resulting five membered ring: furanose because it resembles furan
Haworth Projection
•Carbon atoms not shown
•Plane of ring is perpendicular to the plane of screen with heavy line towards you

17. Fructose ring structures
C5 to C2 bond
C6 to C2 bond

18. Nomenclature of cyclic hemiacetals and hemiketals
 Additional asymmetric center in cyclic forms
– In glucose, C-1 is the anomeric center
– In fructose (furanose form), C-2 is the anomeric center
• α means OH gp attached to anomeric carbon is below the plane of the ring
• β means OH gp attached to anomeric carbon is above the plane of the ring
C5, D
C2,

19. Confirmation of rings
 Pyranose and furanose rings are not planar
 Pyranose: chair and boat
 Furanose: puckered
 Pyranose : Chair form favored: Steric Reasons
Substituents on the ring carbon atom have 2
orientations
Axial
Equatorial
 Furanose: Puckered or envelope form
C-2 endo
C-2 out of the plane on the same side as C-5
C-3 endo
C-3 out of the plane on the same side as C-5

21. Reactions of Monosaccharides
 React with alcohol and amines to form adducts
 D-glucose reacts with methanol to form
– methyl-α-D-glucopyranoside
– methyl-β-D-glucopyranoside
– Differ in configuration at anomeric C
– New bond is called glycosidic bond (anomeric C and O of hydroxyl)

22. Reactions of Monosaccharides
 The anomeric carbon can be linked to the nitrogen atom of
an amine to form an N-glycosidic bond

23. Reducing vs. Non reducing Sugars
 Reducing Sugars eg., glucose
– React with Fehling’s solution
– Free aldehyde group is oxidized
 Non Reducing Sugars eg., methyl glucopyranoside
– Do not react with Fehling’s solution
– Not readily interconverted to a form with free aldehyde gp

24. Modified Monosaccharides

25. Oligosaccharides
 Oligosaccharides are built by the linkage of 2 or more monosaccharides by
O-glycosidic bonds.
– Maltose
• Two D-glucose residues are joined by a glycosidic linkage between
– the α-anomeric form of C-1 on one sugar and
– the hydroxyl oxygen atom on the C-4 of the adjacent sugar.

26. Disaccharides
Two sugars joined by an O-glycosidic bond

27. Polysaccharides
 Larger polymeric oligosaccharides
 Play vital roles in energy storage and in maintaining the structural
integrity of an organism.
 Homopolysaccharides : same monosaccharide
– Starch
– Glycogen
– Cellulose
– Chitin
 Heteropolysaccharides
– Provide extracellular support
– Bacterial cell wall
– Cartilage and tendons

28. Polysaccharides: Glycogen
 Glycogen is highly branched polymer of glucose residues
 Most of the glucose units are linked by α-1,4- glycosidic bonds
 Branches are formed by α-1,6- glycosidic bonds
 Branches present about once in 10 units

29. Polysaccharides: Starch
 Two forms
– Amylose
• Unbranched :α-1,4- glycosidic bonds
– Amylopectin
• Branched; α-1,6- glycosidic bonds about every 30 units

30. Polysaccharides: Cellulose
 One of the most abundant organic compounds in the biosphere
 Plays structural role in plants
 Unbranched glucose polymer with β-1,4- glycosidic bonds
 Forms long straight chains
Mammals lack cellulases and therefore cannot digest wood or
vegetable fibers

31. Polysaccharide structure
 Structure is determined by the type of linkages
– β-1,4-linkages
• Straight chains
• Optimal for structural purposes
- α-1,4- linkages
• Favor bent structure
• Suitable for storage

32. Glycosaminoglycans:Anionic Polysaccharides
 Made of repeating disaccharide units
 Glucosamine or
 Galactosamine
 At least 1 of the sugars has a negatively charged carboxyl or sulfate group
 Forms proteoglycans with proteins
 Lubricants
 Structural component
 Mediate adhesion and bind factors stimulating cell proliferation

33. Enzymes: Oligosaccharide assembly
 Glycosyltrasferases: catalyze formation of
glycosidic bonds
 Specific enzymes required
– Specific to the sugars being linked
 Sugar to be added is in activated form
– sugar nucleotide
 Proceeds with retention or inversion of
configuration at glycosidic carbon atom

34. Human ABO Blood groups
 One type of blood group possess either of these structures
 A antigen
 B antigen
 O antigen
 These structures have a common oligosaccharide foundation termed O present in O
antigen
 A antigen has N-acetylgalactosamine addition (α-1,3- linkage to galactose moiety of O
antigen)
 B antigen has galactose addition (α-1,3- linkage to galactose moiety of O antigen)
 Specific glycosyltransferases add the extra monosaccharide to the O antigen.

35. Glycoproteins
 Carbohydrates covalently attached to
proteins
 Small percentage of carbohydrates
 Components of cell membranes
 cell adhesion
 binding of sperm to egg
 Carbs linked to proteins through
Aspargine (N-linked)
Takes place in the lumen of ER and
in Golgi complex
Serine or threonine (O-linked)
Takes place exclusively in the golgi
complex

36. Golgi complex as sorting center
Golgi complex has 2 roles
• Carbohydrate units of glycoproteins are modified
• Major sorting center of the cell
• Targets proteins to Lysosomes, Secretory vesicles, and Plasma
membrane

37. Lectins: Specific carbohydrate-binding proteins
• Ubiquitous
• Animals, plants and microorganisms
• Promote interactions between cells
• In plants: can serve as potent insecticides
• Binding specificities in plants is well characterized

38. Animal lectin
Animal cell lectins facilitate cell-cell contact
In animal cell C-type lectins, Ca2+ ion links a mannose residue to the lectin

39. Selectins
C-type lectins
Bind immune-system cells to the sites of injury
Lymphocytes adhering to the lining of a lymph node

40. Influenza hemagglutinin
•Viruses infect by binding to particular
•Structures
•Receptors: carbohydrates
Example: Influenza virus
•Binds to sialic acid residues
on target cell surface through hemagglutinin
• Once inside the cell, viral protein,
neuraminidase, cleaves glycosidic bonds to
sialic acid and frees the virus for infection.
•Neuraminidase is a promising target for
anti-influenza agents.