Biology of carbohydrates

1. Carbohydrates and Glycobiology
Chapter 7

2. CHAPTER 7
Carbohydrates and Glycobiology
▫ Structures and names of monosaccharides
▫ Open-chain and ring forms of monosaccharides
▫ Structures and properties of disaccharides
▫ Biological function of polysaccharides
▫ Biological function of glycoconjugates
Key topics about carbohydrates:

3. Carbohydrates
• Named so because many have formula Cn(H2O)n
• Produced from CO2 and H2O via photosynthesis in plants
• Range from as small as glyceraldehyde (Mw = 90 g/mol)
to as large as amylopectin (Mw = 200,000,000 g/mol)
• Fulfill a variety of functions including:
▫ energy source and energy storage
▫ structural component of cell walls and exoskeletons
▫ informational molecules in cell-cell signaling
• Can be covalently linked with proteins to form
glycoproteins and proteoglycans

4. Carbohydrate Basics
• Carbohydrates are poly-hydroxy aldehydes or
polyhydroxy ketones
• The basic unit is a sugar molecule
▫ Also known as a saccharide
• Monosaccharide = single sugar unit
• Disaccharide = two sugar units, covalently bound
• Oligosaccharide = short chains of sugars
• Polysaccharide = >20 sugar units

5. Monosaccharides
• Single sugar unit
• The most common biological
monosaccharide: D-glucose
(dextrose)
• Most biologically relevant
monosaccharides have 3-6
carbons
• >4 C can readily form cyclic
structures
• Each monosaccharide—in its
linear form—contains one
carbonyl (C=O)
▫ rest of the Cs are bound to OH
groups
Drawing convention:
in the Fischer projection,
always put C=O towards
the top

6. Naming monomeric sugars
• All mono- and disaccharide names end in “-ose”
• Typical naming will include the number of
carbon atoms in the sugar molecule
▫ 3=triose
▫ 4=tetrose
▫ 5=pentose
▫ 6=hexose
▫ Rare examples of heptoses, octoses & nonoses in
biology
• Carbohydrates, especially simple ones, will often
have common names

7. Aldose v. Ketose
• Monosaccharides can be further subdivided into
aldoses and ketoses
• If the last carbon in the chain is the carbonyl,
this is an aldose
• Ketoses have the carbonyl anywhere else
OTHER than the final position

8. Aldoses and Ketoses
• An aldose contains an aldehyde functionality
• A ketose contains a ketone functionality
Carbonyl: C=O

10. Sugars are CHIRAL
• Chirality adds more
structural diversity
and another level of
nomenclature
• Most carbons in a
sugar are chiral
• Aldoses naturally
have—on average—
more chiral centers
than ketoses

11. Glyceraldehyde
• Glyceraldehyde is a simple 3-carbon sugar
• The central C is chiral
• This is the standard by which the
configurational stereochemistry of D & L are set
for other sugars (and amino acids)…

12. Sugar Chirality
• With the vast number of multi-carbon sugars, we
need a simple way of diagramming them
• Fischer projections use the D- & L- representations
of glyceraldehyde to name sugars

13. Drawing Fisher projections
• Simpler than using perspective formulae
▫ chiral carbohydrates are commonly represented by
Fischer projections
• Horizontal bonds are pointing toward you
• Vertical bonds are projecting away from you

14. Sugar Chirality
• Compare the stereochemistry of the chiral center
most distant from the carbonyl to D and L
glyceraldehyde
• D-sugars will be drawn with the hydroxyl on the
RIGHT of the reference carbon, whereas L-sugars
will have the reference hydroxyl on the LEFT

15. Which one is D? Which one is L?
D-Ribose L-Ribose

16. Stereoisomers and Epimers
• Longer sugars allow for chemically identical yet
stereochemically different molecules
• Sugars that differ only in the stereochemistry of
ONE chiral center are called epimers
• Enantiomers
▫ Stereoisomers that are nonsuperimposable mirror
images
• Diastereomers
▫ stereoisomers not related through a reflection
operation

18. Diastereomers
• Diastereomers:
stereoisomers that are
not mirror images
• Diastereomers have
different
physiochemical
properties 
Boiling points of threose
(290.6 °C) & erythrose
(311.11 °C) are different

19. Epimers
• Epimers are two sugars that differ only in the
configuration around one carbon atom

20. Use of common names
• Carbohydrates, due to multiple stereocenters, can
have complex IUPAC names
▫ D-(−)-Erythrose = (2R,3R)-2,3,4-Trihydroxybutanal
▫ D-(−)-Threose = (2S,3R)-2,3,4-Trihydroxybutanal
Common names always refer to same
stereochemistry, so serve as useful
shorthand.
How many common names would you
need to cover all aldopentoses?

21. 5-C and 6-C aldoses

22. 5-C and 6-C ketoses

23. Sugars are also cyclic
• Most common 5-C & 6-C sugars
adopt a cyclic structure in
solution
• This occurs most commonly via
the reaction of the alcohol on
C5 with the aldehyde (aldoses)
or ketone (ketoses) on the
carbonyl carbon
• The cyclization results in
hemiacetals or hemiketals

24. Hemiacetals and Hemiketals
• Aldehyde and ketone carbons are electrophilic
• Alcohol oxygen atom is nucleophilic
• When aldehydes are attacked by alcohols, hemiacetals form
• When ketones are attacked by alcohols, hemiketals form

26. Cyclization of monosaccharides
• Pentoses and hexoses readily undergo intramolecular cyclization
• The former carbonyl carbon becomes a new chiral center, called
the anomeric carbon
• The former carbonyl oxygen becomes a hydroxyl group; the
position of this group determines if the anomer is  or 
• If the anomeric OH group is on the opposite side (trans) of the
ring as the CH2OH moiety the configuration is 
 This is functionally equivalent to the anomeric OH cis to the reacting OH
group—best visualized in the Fischer projection
• If the hydroxyl group is on the same side (cis) of the ring as the
CH2OH moiety, the configuration is 
 This is functionally equivalent to the anomeric OH trans to the reacting OH
group—best visualized in the Fischer projection

27. Cyclic Sugars
• Cyclization of sugar monomers
introduces another source of
variability
• Monosaccharides that differ
only in the stereochemistry
around anomeric carbon,
as hemi-acetals or hemi-
ketals, are called anomers

28. The anomeric carbon is
drawn on the right side

29. Pyranoses and furanoses
• Aldohexoses can form 5- or
6-membered rings
• Five membered rings are
furanoses, named after the
similarity to furan
• Six membered rings are
pyranoses, named after the
similarity to pyran

30. Haworth Perspective Formulae
• Haworth perspectives depict
cyclic sugars
• Lines drawn up from the
carbon indicate the H or OH
is above the plane of the ring

31. Rings, chairs and boats
• Ring structures of monosaccharides in solution are not
actually planar
• Most monosaccharides form chair structures
▫ Conformational isomers or conformers
▫ Chair conformation review
• Which of the 2 chair structures is predominant depends
on the steric hindrance of the groups on the sugar—axial
vs. equatorial groups

32. Conformational
change
Configurational
change
No bond-
breaking required
Bond-breaking
required
Mutarotation →

33. β is preferred over α→
Quick mini-quiz:
• What makes these
α or β?
• Why is the β form
here “preferred”?
~46 kJ required for
transition

34. Carbohydrates in solution reach
equilibrium
• The stability of each structural variation of a
given sugar molecule determines its equilibrium
population
• A solution of glucose will reach an equilibrium:
▫ ~36% α-D-glucopyranose
▫ ~64% β-D-glucopyranose
▫ Trace linear
▫ Trace glucofuranose 
α-D-glucofuranose

35. Chain-Ring Equilibrium & Reducing Sugars
• Ring forms exist in equilibrium with the open-chain
form. All monosaccharides are thus reducing sugars
• Aldehydes can reduce Cu2+ to Cu+ (Fehling’s test)↓
• Aldehydes can reduce Ag+ to Ag0 (Tollens’ test)
• This allows detection of reducing sugars, such as
glucose

36. Substituted Rings
• The most common class of biological
monosaccharides are the hexoses
• Hexoses are often substituted at one or more of
the carbon atoms
• Deoxy sugars are the most common substituted
groups
▫ one hydroxyl is converted to a H

37. Ribose vs. Deoxyribose

38. Important Hexose Derivatives

39. Disaccharides
• Disaccharides are 2
monosaccharides joined by a
covalent linkage known as the
O-glycosidic bond
• Two sugar molecules are joined
between an anomeric carbon
and a hydroxyl carbon

40. The Glycosidic Bond
• The glycosidic bond (an acetal) between monomers
is now relatively unreactive compared to the
hemiacetal at the second monomer
▫ 2nd monomer, often contains hemiacetal, is reducing
▫ 1st monomer: anomeric carbon is involved in the glycosidic
linkage, thus it’s is nonreducing
• The disaccharide formed upon condensation of two
glucose molecules via 1  4 bond is called maltose

41. Helpful link:
http://butane.chem.uiuc.edu
/pshapley/GenChem2/B10/
1.html
Hydroxyls are
nucleophilic—why?
Anomeric carbon is
electrophilic—why?

42. Naming Disaccharides
1. Name the configuration (/) of the
anomeric carbon of the first sugar
2. Add -pyranosyl or -furanosyl to
distinguish 6- from 5-membered rings,
respectively
3. Indicate the 2 carbon atoms joined via
the glycosidic bond in parentheses
 If the two anomeric carbons are linked,
specify the α/β confirmations of each
4. Name the second residue using the
same -pyranose or -furanose
designation system
5. Repeat the process if there is a 3rd sugar,
etc. Why isn’t there a β here?

43. Chain-Ring Equilibrium & Reducing disaccharides
• The anomeric carbon in the 1st saccharide is an
acetal or ketal, thus non-reducing
• If the 2nd anomeric carbon is free, it can still
linearize into an aldehyde or ketone
Benedict’s Reagent test

44. Nonreducing Disaccharides
• Two sugar molecules can be joined via a glycosidic
bond between two anomeric carbons
• The product has two acetal/ketal groups and no
hemiacetals/hemiketals
• Thus, there are no reducing ends: this is a
nonreducing sugar

45. Polysaccharides
• Polysaccharides are long
sugar polymers
• Homopolysaccharides
▫ all one type of
monosaccharide
• Heteropolysaccharides
▫ different monosaccharides
• Polysaccharides can be
linear or branched
• Different structural &
storage roles…

46. Glycogen
• Glycogen is a branched homopolysaccharide of glucose
– Glucose monomers form (1  4) linked chains
– Branch-points with (1  6) linkers every 8–12
residues
– Molecular weight reaches several millions
– Functions as the main storage polysaccharide in
animals

47. Starch
• Starch is a mixture of two homopolysaccharides of
glucose
• Amylose is an unbranched polymer of (1  4) linked
residues
• Amylopectin is branched like glycogen but the branch-
points with (1  6) linkers occur every 24–30
residues
• Molecular weight of amylopectin is up to 200 million
• Starch is the main storage polysaccharide in plants

48. Glycosidic Linkages in Glycogen and Starch
In starch granules:

49. Polymeric glucose
• Single glycogen
molecule has 1
reducing end & many
non-reducing ends
• Advantages?
Why not store glucose in monomeric form?
• 0.01 μM glycogen can break up into 0.4M glucose
• Consider the consequences of 0.4M glucose inside a cell
•Osmolarity
•Transport

50. Glycogenin

51. Metabolism of Glycogen and Starch
• Glycogen and starch often form
granules in cells
• Granules contain enzymes that
synthesize and degrade these
polymers
• Glycogen and amylopectin have
one reducing end but many
nonreducing ends
• Enzymatic processing occurs
simultaneously @ many
nonreducing ends to release
glucose

52. Structural Polysaccharides
• Cellulose is a structural
component of plants
▫ Also composed of amylose and
amylopectin, but the linkages
are β, not α
• Chitin is a homopolymer of N-
acetylglucosamine (GlcNac) in
β linkages
▫ Found in the exoskeletons of
insects

53. • Cellulose is a branched homopolysaccharide of glucose
▫ Glucose monomers form (1  4) linked chains
▫ Hydrogen bonds form between adjacent monomers
▫ Additional H-bonds between chains
▫ Structure is now tough and water-insoluble
▫ Most abundant polysaccharide in nature
▫ Cotton is nearly pure fibrous cellulose

54. Cellulose Metabolism
• The fibrous structure and water-insolubility make cellulose a
difficult substrate on which to act
• Fungi, bacteria, and protozoa secrete cellulase, which allows
them to use wood as source of glucose
▫ Most animals cannot use cellulose as a fuel source because they
lack enzymes to hydrolyze (1 4) linkages
• Ruminants and termites live symbiotically with
microorganisms that produces cellulase
• Cellulases hold promise in the fermentation of biomass into
biofuels

55. Chitin
• Chitin is a linear homopolysaccharide of N-acetylglucosamine
– N-acetylglucosamine monomers form (1  4)-linked chains
– Forms extended fibers that are similar to those of cellulose
– Hard, insoluble, cannot be digested by vertebrates
– Structure is tough but flexible, and water-insoluble
– Found in cell walls in mushrooms, and in exoskeletons of insects, spiders,
crabs, and other arthropods

56. Polysaccharides have higher order
structures
• Disaccharides can rotate around the glycosidic
bonds
• The monosaccharide rings act as “planes” that
rotate around the point of the glycosidic oxygen
• Therefore they have f & y angles
• Hydrogen bonding between residues is also
important for the structural integrity of a
polysaccharide.

57. α vs. β linkages
• Structural polysaccharides often utilize β linkages rather
than α.
• In amylose—with its (α1→4) glucose linkages—the most
stable configuration is a 60°bend between the planes
• This limits H-
bond availability
between sugar
residues &
promotes loose
helical structure

58. • In cellulose—with its (β1→4) glucose linkages—the most
stable configuration is a 180°bend between the planes
• This readily allows H-bonds to form between monomers
& across strands
α vs. β linkages

59. Ramachandran plots revisited
Most favored
Least favored
Relative energy
Gal(β1→3)Gal

60. Agar and Agarose
• Agar is a complex mixture of hetereopolysaccharides
containing modified galactose unitsthat serves as a cell wall
component of some algaes & seaweeds
▫ ~70% Agarose
▫ ~30% Agaropectin

61. Agarose gels are useful in
separating nucleic acids

…and in growing microbial
cultures ↓

62. Polysaccharides in cell walls
• The cell walls of bacteria are made up of crosslinked
hetero-polysaccharides
• Peptidoglycan is made of strands of alternating
N-acetylglucosamine (GlcNac) 1→4 N-acetylmruanic
acid (MurNac)
▫ Crosslinked to protein scaffold via MurNac residues
• Gives the cells structure and resistance to osmotic
rupture

63. N-acetylglucosamine
(GlcNac)
N-acetylmruanic acid
(MurNac)

64. N-acetylglucosamine
(GlcNac)
N-acetylmruanic acid
(MurNac)

65. Peptidoglycan
• Peptidoglycan is vulnerable to the enzyme lysozyme
▫ Found in tears
• Antibiotics often work by preventing the proper formation
of these cell walls
▫ Penicillin prevents synthesis of the crosslinks

66. Glycoconjugates
• Many proteins and lipids have
mono- or polysaccharides
attached to them
• Glycoproteins – found in the
cell membrane, secreted from
cells, and inside organelles in the
secretory pathway
• Proteoglycans – found in the
cell membrane & extracellular
matrix
• Glycolipids – found in the outer
leaflet of the cell membrane.
Serve as receptors for certain
extracellular proteins

67. Extracellular Matrix
• The extracellular matrix
is the mesh between
cells
▫ Allows for passage of
materials but also
provides structural
support and cushioning
• Glycosaminoglycans:
heteropolysaccharides
found in the ECM along
with fibrous proteins

68. Glycosaminoglycans
• Linear polymers of repeating disaccharide units
• One monomer is an amino sugar
▫ N-acetyl-glucosamine (GlcNac)
▫ N-acetyl-galactosamine (GalNac)
• Other monomer is typically:
▫ Uronic acid (C6 oxidation) 
▫ Galactose
• Extended highly hydrated molecule
▫ Minimizes charge repulsion
• Forms meshwork with fibrous proteins to form extracellular
matrix
▫ Connective tissue
▫ Lubrication of joints

69. Glycosaminoglycans
• Most are proteoglycans
(linked to proteins)
Examples:
• Hyaluronic acid: long polysaccharide
involved in lubricating joints
• Chondroitin sulfate: structural
component of cartilage
• Keratan sulfate: found in cornea,
cartilage, & bone
• Heparin: naturally occurring
anticlotting agent, used as medication

70. Proteoglycan Aggregates
• Hyaluronan and aggrecan
form huge (Mr > 2  108)
noncovalent aggregates
• Hold lots of water (1000
its weight) & provides
lubrication
• Very low friction material
• Covers joint surfaces:
articular cartilage
▫ Reduced friction
▫ Load balancing

71. Glycoconjugates: Glycoprotein
• A protein with relatively small oligosaccharides
attached
▫ Carbohydrate attached via its anomeric carbon
▫ About half of mammalian proteins are glycoproteins
▫ Carbohydrates play role in protein-protein recognition
▫ Only some bacteria glycosylate few of their proteins
▫ Viral proteins are heavily glycosylated; helps evade the
immune system

72. Glycoproteins vs. proteoglycans
• Glycoproteins are proteins with carbohydrate
groups
• Proteoglycans are glycoproteins that are so
heavily glycosylated that carbohydrates
dominate the molecular weight and the
physiological functions, reflected in the
nomenclature

73. Glycosylation
Glycosylated proteins are
membrane-bound
extracellular, secreted, or in
the secretory pathway
▫ Glycosylation occurs in
the golgi apparatus
▫ Glycosylation is most
frequent on
 Ser & Thr (O-linked)
 Asn (N-linked)

74. Why glycosylate proteins?
• Not completely sure in many cases…
• Many cellular processes are seemingly
unaffected by a complete block of glycosylation
• May play a role in protein solubility or folding
• Cell-cell adhesion controlled by sugar-binding
proteins called lectins

75. Why glycosylate proteins?
• ~50% of all human proteins are glycosylated
▫ So it must be important, right?
• Improperly glycosylated proteins are degraded
rather rapidly
▫ Perhaps a sign of protein “health”?

76. Glycosylation = Outside
• Only proteins that are outside the cell are
glycosylated
• Glycophorin A (GpA) is a membrane protein in
erythrocytes with >60% of its mass coming from
glycosylations
▫ gives cells very hydrophilic/charged coat, reducing
adhesion
• GpA is essentially one TM domain with the
extracellular domain highly glycosylated

77. Glycophorin A
• Extremely well-studied
• Forms tight TM-dimers
• Dimerization driven by
VDW-packing in the
membrane

78. Glycosylation = Outside
• Secreted proteins also have glycosylations
• Antibodies, hormones, and many signaling
proteins have glycosylations
• Some signaling molecules have different
glycosylation patterns depending on which
tissue they are found in

79. Glycolipids
• A lipid with covalently bound
oligosaccharide
▫ Parts of plant and animal cell
membranes
▫ In vertebrates, ganglioside
carbohydrate composition
determines blood groups
▫ In gram-negative bacteria,
lipopolysaccharides cover the
peptidoglycan layer
• Only found in the outer-leaflet
of the cell membrane
• Linked to the headgroup of
lipid molecules
Bacterial lipopolysaccharide

80. Lipopolysaccharide (LPS)
• Major component of gram negative bacterial outer membranes
• LPS has a core region and an O-specific region that differ from
bacterial strain to strain
• Very immunoreactive!

81. Sugars = Complex Information
• Diversity in sugar structure allows for increased
combinatorial possibilities
• The ability to form O-glycosidic bonds at
multiple positions on the sugar ring increases
combinations exponentially
• Allows for very specific structural information
storage—proteins can “read” surface
carbohydrates and act

82. Lectins
• Lectins = proteins that bind carbohydrates with
high affinity
• Many hormones and signaling molecules are
glycosylated. Having high affinity binding
allows for quick responses to small
concentrations

83. Selectins
• Membrane proteins
lining vasculature are
expressed in differing
patterns in response to
inflammation
• Selectins bind to
glycosylated proteins on
T-cells circulating in the
vessel
• This allows for a second
interaction between
epithelial integrins
and the T-cell

84. Bacterial Toxins and Viruses
• Many bacterial toxins are also lectins
• Cholera and Ricin toxins bind gangliosides (see
textbook) as the cellular receptor
• A variety of viruses also use binding to
glycosylations on cell surface proteins as the
target

85. Study Questions
• Due next regular lecture
Draw & name the cyclic structures of the following:
1. D-Sorbose (can make pyranose & furanose
forms)
2. L-Gulose (pyranose)
3. L-Ribose (furanose)
4. D-Altrose (pyranose)

86. Study Questions
Draw & name the structures of the following:
5. Draw & name the result of a β2→4 bond
between β-D-Sorbose & β-L-Gulose
6. Draw & name the result of an α1→ α1 bond
between α-L-Ribose & α-D-Altrose
7. Can these two disaccharides react to form a
tetrasaccharide? If not, why? If so, what are the
possible reaction routes?