2. Carbohydrates
• Most abundant class of biological
molecules on Earth
• Originally produced through CO2
fixation during photosynthesis
3. Roles of Carbohydrates
• Energy storage (glycogen, starch)
• Structural components (cellulose, chitin)
• Cellular recognition
• Components of monomeric unit of DNA
and RNA,
• Glycoconjugates: glycoproteins, glycolipids
4. Carbohydrates
• Carbohydrates are polyhydroxy aldehydes or
ketones, or substances that yield such compounds
on hydrolysis
• Carbohydrates (glycans) have the following basic
composition:
I
(CH2O)n or H - C - OH
I
5. Classification of Carbohydrates
Monosaccharides - simple sugars with multiple OH
groups.
Disaccharides - 2 monosaccharides covalently linked.
Oligosaccharides - a few monosaccharides (upto 10)
covalently linked.
Polysaccharides - polymers consisting of chains of
monosaccharide or disaccharide units.
6. Monosaccharides
Aldoses (e.g., glucose) have
an aldehyde group at one end.
Ketoses (e.g., fructose) have
a keto group, usually at C2.
C
C OH
H
C H
HO
C OH
H
C OH
H
CH2OH
D-glucose
O
H
C H
HO
C OH
H
C OH
H
CH2OH
CH2OH
C O
D-fructose
8. Stereoisomers and Stereochemistry
• Prefixes D- and L- in a monosaccharide name
identify one of two isomeric forms
– These isomers differ in the spatial arrangement of
atoms and are stereoisomers
• Stereochemistry is the study of different spatial
arrangements of atoms
• The stereoisomers D- and L- glyceraldehyde are
non-superimposable mirror image molecules and
are called enantiomers (a subset of stereoisomers)
9. D vs L Designation
D & L designations
are based on the
configuration about
the single asymmetric
C in glyceraldehyde.
The lower
representations are
Fischer Projections.
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehyde
D-glyceraldehyde
L-glyceraldehyde
D-glyceraldehyde
10. Sugar Nomenclature
For sugars with more
than one chiral center,
D or L refers to the
asymmetric C farthest
from the aldehyde or
keto group.
Most naturally occurring
sugars are D isomers.
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
11. D & L sugars are mirror images of one another.
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
12. Stereochemistry
•Enantiomers = mirror images
•Pairs of isomers that have opposite configurations at one or more
chiral centers but are NOT mirror images are diastereomers
•Epimers = Two sugars that differ in configuration at only one
chiral center
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
H OH
HO H
HO H
H
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
HO H
H OH
H
D-glucose
L-glucose
Enantiomers
D-glucose D-mannose
Epimers
D-mannose D-galactose
Diastereomers
14. Chirality
• A carbon atom that has four different groups
bonded to it is called a chiral carbon atom
• Any molecule containing a chiral carbon can
exist as a pair of enantiomers
• Chirality in glyceraldehyde (the simplest carbohydrate)
is conveyed by a single chiral carbon
15. Monosaccharides are chiral
• Aldoses with 3C or more and
ketoses with 4C or more are chiral
• The number of chiral carbons
present in a ketose is always one
less than the number found in the
same length aldose
• Number of possible stereoisomers =
2n (n = the number of chiral
carbons)
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
CH2OH
C
C*
C*
C*
CH2OH
O
HO H
H OH
H OH
D-glucose D-fructose
16. Optical Activity
• Enantiomers are also called optical isomers
• Enantiomers interact with plane polarized
light to rotate the plane of the light in
opposite directions
– This interaction with polarized light is called
optical activity
– Optical activity distinguishes the isomers
– It is measured in a device called a polarimeter
17. Polarized Light
• Normal light vibrates in an infinite number of
directions perpendicular to the direction of travel
– When the light passes through a polarizing filter
(Polaroid sunglasses) only light vibrating in one plane
reaches the other side of the filter
– A polarimeter allows the determination of the specific
rotation of a compound
• Measures its ability to rotate plane-polarized light
20. Molecular Structure and Optical Activity
• When an enantiomer in a solution is placed in the
polarimeter, the plane of rotation of the polarized light is
rotated
– One enantiomer always rotates light in a clockwise (+)
direction
• This is the dextrorotatory isomer
– The other isomer rotates the light in a counterclockwise (-)
direction
• It is the levorotatory isomer
• Under identical conditions, the enantiomers always rotate
light to exactly the same degree, but in opposite directions
21. Hemiacetal & hemiketal formation
An aldehyde can
react with an
alcohol to form
a hemiacetal.
A ketone can
react with an
alcohol to form
a hemiketal.
O C
H
R
OH
O C
R
R'
OH
C
R
R'
O
aldehyde alcohol hemiacetal
ketone alcohol hemiketal
C
H
R
O R'
R' OH
"R OH "R
+
+
These reaction facilitates formation of ring structures
of monosaccharides
22. In aqueous solution, aldotetroses and all
monosaccharides with five or more carbon
atoms in the backbone occur predominantly as
cyclic (ring) structures.
In cyclic structures the carbonyl group has
formed a covalent bond with the oxygen of a
hydroxyl group along the chain.
The formation of these ring structures is the result
of a general reaction hemiacetals or hemiketals
reactions.
23. Cyclic Structures of common
monosaccharide
Haworth projections.
6-member pyranose ring, named after pyran
24. Fructose forms either
a 6-member pyranose ring, by reaction of the C2 keto
group with the OH on C6, or
a 5-member furanose ring, by reaction of the C2 keto
group with the OH on C5.
CH2OH
C O
C H
HO
C OH
H
C OH
H
CH2OH
HOH2C
OH
CH2OH
H
OH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
25. Cyclization of glucose produces a new asymmetric center at
C1. The hemiacetal (or carbonyl) carbon atom is called the
anomeric carbon. The 2 stereoisomers are called anomers,
& b.
Haworth projections represent the cyclic sugar anomers as:
(OH below the ring)
b (OH above the ring).
H O
OH
H
OH
H
OH
CH2OH
H
-D-glucose
OH
H H O
OH
H
OH
H
OH
CH2OH
H
H
OH
b-D-glucose
2
3
4
5
6
1 1
6
5
4
3 2
26. The α and β anomers of D-glucose interconvert in
aqueous solution by a process called
mutarotation.
Thus, a solution of α-D-glucose and a solution of
β-D-glucose eventually form identical equilibrium
mixtures having identical optical properties.
This mixture consists of about one-third α-D-
glucose, two-thirds β-D-glucose, and very small
amounts of the linear and five-membered ring
(glucofuranose) forms.
27. Reducing Sugars
• The aldehyde groups of aldoses are oxidized by
Benedict’s reagent, an alkaline copper(II) solution
• The blue color of the reagent fades as reaction occurs
reducing Cu2+ to Cu+ with a red-orange precipitate
forming as Cu2O results
• Test can measure glucose in urine
+ Cu2O (red-orange)
28. Reducing Sugars
• All monosaccharides and the disaccharides
except sucrose are reducing sugars
• Ketoses can isomerize to aldoses and react
D-glucose
CH
C
C
OH
O
H
C OH
H
C
H
H OH
CH2OH
H
O
D-fructose
CH2OH
C
C
O
O
H
C OH
H
C
H
H OH
CH2OH
CH
C
C
OH
O
H
C OH
H
C
H
H OH
CH2OH
O
H
enediol
30. Sugar Phosphates
Involved in catabolism and
anabolism, cellular catabolism it must
first be converted to glucose 6-
phosphate
Fructose 1,6-bisphosphate lies
within the glycolysis metabolic
pathway
32. Amino Sugars
a prominent precursor in the
biochemical synthesis of
glycosylated proteins and
lipid. It is part of the structure of
the polysaccharides, chitosan,
and chitin.
It is a constituent of some
glycoprotein hormones
such as follicle-
stimulating hormone and
luteinizing hormone
34. Sugar derivatives
Acidic sugar, found in complex glycans on
mucins and glycoproteins found at the cell
membrane
Sulfoquinovose, sulfosugar.
Component of sulfolipid (SQDG)
sulfoquinovosyl diacylglycerides
35. Disaccharides
Disaccharides (such as maltose, lactose, and
sucrose) consist of two monosaccharides
joined covalently by an O-glycosidic bond,
which is formed when a hydroxyl group of
one sugar reacts with the anomeric carbon of
the other.
36. Maltose
• Disaccharide of two glucose molecules.
• Has a α -1,4-glycosidic bond (between two glucoses).
• Is obtained from the breakdown of starches.
• Is used in cereals and candies.
• Is a reducing sugar (carbon 1 can open to give a free
aldehyde to oxidize).
b
O
CH2OH
OH
OH
OH
O
O
CH2OH
OH
OH
OH
1 4
-1,4-glycosidic
bond
b- maltose
α-glucose
OH
OH
OH
OH
CH2OH
O
OH
OH
OH
OH
CH2OH
O
+
1 4
α-glucose
37. Maltose
Anomeric carbon (C1) of the second glucose
molecule, which is not involved in a
glycosidic bond, could be either an α- or β-
anomer depending on the bond direction of
the attached hydroxyl group, resulting in
either α-maltose or β-maltose
38. • Disaccharide of galactose and glucose.
• Has a β-1,4-glycosidic bond (between β-galactose and glucose).
• with the glucose moiety being potentially free (i.e., a reducing
sugar) and existing as either an α- or β-anomer.
• Is found in milk and milk products (almost no sweet).
• Is a reducing sugar (carbon 1 can open to give a free aldehyde to
oxidize).
b
Lactose
b-lactose
39. • Is found in table sugar (obtained from sugar cane and
sugar beets).
• Consists of glucose and fructose.
• Has an α,β-1,2-glycosidic bond (between α-glucose and
b-fructose).
• Is not a reducing sugar (carbon 1 cannot open to give a
free aldehyde to oxidize).
Sucrose
40.
41. Invert Sugar
Invert or sugar syrup is a mixture of glucose and fructose.
It is obtained by splitting sucrose into these two components.
Compared with its precursor sucrose, inverted sugar is sweeter
and its products tend to retain moisture and are less prone to
crystallization.
Invert sugar is therefore valued by bakers.
C12H22O11 + H2O → C6H12O6 + C6H12O6
Sucrose Glucose Fructose
+66.5° +52.7° −92°
43. 1) Homoglycans: single type of monomer (Starch,
Cellulose, Glycogen, Dextrins or Inulin)
2) Heteroglycans : contain two or more different
kinds (Mucopolysaccharides)
Characteristics:
-polymers (MW from 200,000)
-White and amorphous products (glassy)
-not sweet
-not reducing; do not give the typical aldose or ketose
reactions
-form colloidal solutions or suspensions
Polysaccharides or glycans
43
44. Homopoysaccharides are polymers composed of
a single type of sugar monomers
Homopoysaccharides
44
Homo polysaccharides
Fructosan
e.g. Inulin
Glucosans
e.g. Starch
Glycogen
Cellulose
Galactosan
e.g. Agar
45. 1) Glycogen (Storage Polysaccharide)
-Also known as animal starch
-Stored in muscle and liver
-Present in cells as granules (high MW)
-Contains both α(1,4) links and α (1,6) branches at every
8 to 12 glucose unit
-Complete hydrolysis yields glucose
- With iodine gives a red-violet color
-Hydrolyzed by both α and β-amylases and by glycogen
phosphorylase
Glucosans /Glucans
45
47. -In the liver, glycogen synthesis and degradation are regulated
to maintain blood-glucose levels as required to meet the needs
of the organism as a whole. Glycogen serves as a buffer to
maintain blood glucose level.
-In contrast, in muscle, these processes are regulated to meet the
energy needs of the muscle itself.
- The concentration of glycogen is higher in the liver than in
muscle (10% versus 2% by weight), but more glycogen is stored
in skeletal muscle overall because of its much greater mass.
Glycogen
47
Structure of
Glycogen
48. -Most common storage polysaccharide in plants
-Composed of 10 – 30% Amylose and 70-90% amylopectin
depending on the source
(a) Amylose is a linear polymer of α-D-glucose, linked
together by α 1→4 glycosidic linkages. It is soluble in water,
reacts with iodine to give a blue color and the molecular
weight of Amylose ranges between 50, 000 – 200, 000.
(b) Amylopectin is a highly branched polymer, insoluble in
water, reacts with iodine to give a reddish violet color. The
molecular weight ranges between 70, 000 - 1 000, 000.
Branches are composed of 25-30 glucose units linked by α
1→4 glycosidic linkage in the chain and by α 1→6
glycosidic linkage at the branch point.
Starch (Storage Polysaccharide)
48
50. Starch
50
Suspensions of Amylose
in water adopt a helical
conformation
Iodine (I2) can insert in
the middle of the Amylose
helix to give a blue color
that is characteristic and
diagnostic for starch
51. -Polymer of b-D-glucose linked by b(1,4) linkages
-Yields glucose upon complete hydrolysis
-Partial hydrolysis yields cellobiose
-Most abundant of all carbohydrates
-Gives no color with iodine
-Cellulose is tasteless, odorless and insoluble in water and
most organic solvents.
Cellulose (Structural Polysaccharide)
51
53. Cellulose
Biochemistry for medics 53
Cellulose consists of β -D-glucopyranose units linked by
β 1 →4 bonds to form long, straight chains strengthened
by cross-linking hydrogen bonds.
54. Mammals lack any enzyme that hydrolyzes the β 1→ 4
bonds, and hence cannot digest cellulose.
It is an important source of "bulk" in the diet, and the
major component of dietary fiber.
Microorganisms in the gut of ruminants and other
herbivores can hydrolyze the linkage and ferment the
products to short-chain fatty acids as a major energy
source.
Cellulose- Digestion
55. -Microcrystalline cellulose : used as binder-
disintegrant in tablets
-Methylcellulose: suspending agent and bulk
laxative
-Oxidized cellulose: hemostat
-Sodium carboxymethyl cellulose: laxative
-Cellulose acetate: rayon; photographic film;
plastics
-Cellulose acetate phthalate: enteric coating
-Nitrocellulose: explosives; collodion
(pyroxylin)
Significance of Cellulose
56. -Chitin is the second most abundant carbohydrate
polymer of N- Acetyl Glucosamine
-present in the cell wall of fungi and in the
exoskeletons of crustaceans, insects and spiders
-chitin is used commercially in coatings (extends the
shelf life of fruits and meats
Chitin- Structural Polysaccharide
56
57.
58. Mucopolysaccharides or Glycosaminoglycans.
It contain two or more different kinds
Usually contains an acid sugar and an amino sugar.
Acid sugar is generally D- Glucuronic acid or its C-5 epimer
Iduronic acid,
while amino sugar is either D- Glucosamine or D-Galactosamine,
amino group is generally acetylated eliminating its positive
charge.
The amino sugar may be sulfated on non acetylated nitrogen.
Heteropolysaccharides
58
59. Hyaluronic acid-(D-glucuronate + GlcNAc)n
Occurrence: synovial fluid, ECM of loose connective tissue.
Serves as a lubricant and shock absorber.
-It forms non-covalently linked complexes with Proteoglycans
in the Extracellular matrix
-Hyaluronic acid polymers are very large (100 - 10,000 kDa)
and can displace a large volume of water.
Dermatan sulfate (L-Iduronate + GalNAc sulfate) n
Occurrence: skin, blood vessels, heart valves
Chondroitin sulfate (D-glucuronate + GalNAc sulfate)n
Occurrence: cartilage, tendons, ligaments, heart valves and
aorta.
It is the most abundant GAG.
59
60. Heparin
(D-glucuronate sulfate + N-sulfo-D-glucosamine) n
Heparans have less sulfate groups than heparins
Occurrence:
Heparin : component of intracellular granules of mast cells lining the
arteries of the lungs, liver and skin ( Contrary to other GAGs that are
extra cellular compounds, it is intracellular). Acts as an anticoagulant.
Heparan sulfate : basement membranes, component of cell surfaces
Glycosaminoglycans of physiological
Significance
61. -Homopoysaccharides are polymers of similar monomer
monosaccharides linked together by Glycosidic linkages.
Functionally they may be storage or structural
polysaccharides
-Heteropolysaccharides are polymers of a repeating
disaccharide units with enormous diversity. They are
components of extra cellular matrix (ECM), joints, cartilages,
tendons, cornea and sclera. Heparin is an anticoagulant.
Summary
