HL Chemistry - Option B: Human Biochemistry
“The Discovery of Honey” by Piero de Cosimo (1462)
Carbohydrates
Part 1
Overview of Carbohydrates
General Characteristics
•
•
•
•
•
The term carbohydrate is derived from the
French: “hydrate de carbone”
All carbohydrates are compounds composed of
(at least) C, H, and O
The general formula for a carbohydrate is:
(CH2O)n (e.g. when n = 5 then the formula
would be C5H10O5)
Not all carbohydrates have this empirical
formula (e.g. deoxysugars, aminosugars, etc.)
Carbohydrates are the most abundant
compounds found in nature (e.g. cellulose: 100
billion tons annually)
General Characteristics
•
In nature, most carbohydrates are found bound
to other compounds rather than as simple sugars
• Polysaccharides (starch, cellulose, inulin,
gums)
• Glycoproteins and proteoglycans
(hormones, blood group substances,
antibodies)
• Glycolipids (cerebrosides, gangliosides)
• Glycosides
• Mucopolysaccharides (hyaluronic acid)
• Nucleic acid polymers
Carbohydrate Functions
Carbohydrates can be:
• Sources of energy
• Intermediates in the biosynthesis of other
basic biochemical entities (fats and
proteins)
• Associated with other entities such as
glycosides, vitamins and antibiotics)
• Structural tissues in plants and in
microorganisms (cellulose, lignin, murein)
• Involved in biological transport, cell-cell
recognition, activation of growth factors,
modulation of the immune system
Classification of Carbohydrates
Carbohydrates can be classified by size:
• Monosaccharides (monoses or glycoses)
• Trioses, tetroses, pentoses, hexoses
• Oligosaccharides
• Di, tri, tetra, penta …up to 10
• (The disaccharides are the most important)
• Polysaccharides (or glycans)
• Homopolysaccharides (all the same type)
• Heteropolysaccharides (mixtures of
momomer types)
• Complex carbohydrates (joined to noncarbohydrate molecules)
Monosaccharides
•
•
•
•
•
Monosaccharides are also known as
“simple sugars”
They are classified by (1) the number of
carbons and (2) whether they are aldoses
or ketoses (more to come on this!)
Most (99%) simple sugars are straight
chain compounds
D-glyceraldehyde is the simplest of the
aldoses (aldotriose)
All other sugars have the ending ose
(glucose, galactose, ribose, lactose, etc…)
Monosaccharides
Aldoses (e.g. glucose)
have an aldo (aldehyde)
group at one end
H
Ketoses (e.g. fructose)
have a keto (ketone)
group (usually at C2)
O
CH2OH
C
C
O
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
CH2OH
D-glucose
D-fructose
Aldose sugars
H
(H
C
O
C
O H)n
C H2O H
Aldose
H
H
H
H
C
O
C
OH
C H2O H
Aldotriose
n =1
C
O
H
C
OH
H
C
OH
C H2O H
Aldote trose
n =2
H
C
O
H
C
OH
H
C
H
C
C
O
H
C
OH
OH
H
C
OH
OH
H
C
OH
H
C
OH
C H2O H
Aldope ntose
n =3
C H2O H
Aldohe xose
n =4
Ketose sugars
C H2O H
(H
C
O
C
O H)n
C H2O H
C
O
C H2O H
C H2O H
Ke tose
H
C
O
C
OH
C H2O H
Ke totriose
n =0
C H2O H
C H2O H
Ke tote trose
n =1
C
O
C H2O H
H
C
OH
C
O
H
C
OH
C
OH
C H2O H
Ke tope n tose
n =2
H
H
H
OH
C
OH
C H2O H
Ke toh e xose
n =3
D- vs L- Designation
D & L designations
are based on the
configuration about
the single
asymmetric C in
glyceraldehyde
The lower diagrams
are Fischer
Projections.
CHO
CHO
H
C
OH
HO
L-glyceraldehyde
CHO
H
C
OH
CH2OH
D-glyceraldehyde
H
CH2OH
CH2OH
D-glyceraldehyde
C
CHO
HO
C
H
CH2OH
L-glyceraldehyde
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 (in yellow)
Most naturally
occurring sugars are
D isomers
O
H
C
H – C – OH
HO – C – H
H – C – OH
H – C – OH
CH2OH
D-glucose
O
H
C
HO – C – H
H – C – OH
HO – C – H
HO – C – H
CH2OH
L-glucose
O
H
O
H
D & L sugars are mirror
images of one another
C
C
They have the same root
H – C – OH
HO – C – H
name (but a different
HO – C – H
H – C – OH
D/L designation),
H – C – OH
HO – C – H
[e.g. D-glucose
& L-glucose]
H – C – OH
HO – C – H
Other stereoisomers
CH2OH
CH2OH
have unique names,
D-glucose
L-glucose
(e.g. glucose, mannose,
galactose, etc)
The number of stereoisomers is 2n, where n is the
number of asymmetric (chiral) centers
The 6-C aldoses have 4 asymmetric centers. Thus
there are 16 stereoisomers (8 D-sugars and 8 Lsugars).
Structure of a Simple Aldose and a Simple Ketose
Enantiomers and Epimers
H
H
H
H
C
O
H
C
OH
H
C
OH
C
O
C
O
C
O
HO
C
H
HO
C
H
OH
C
H
HO
C
H
HO
C
H
OH
C
H
H
C
OH
HO
C
H
H
th e se two aldote trose s are e n an tiom e rs.
Th e y are ste re oisom e rs th at are m irror
im age s of e ach oth e r
C
OH
H
C
OH
C H2O H
C H2O H
C H2O H
C H2O H
th e se two aldoh e xose s are C -4 e pim e rs.
th e y diffe r on ly in th e position of th e
h ydroxyl grou p on on e asym m e tric carbo
(carbon 4)
Relationship Between D- & L-Fructose
Properties of Optical Isomers
• The differences in structures (configurations) of
sugar optical isomers are responsible for variations
in properties
• Physical Differences Between D- & L- forms
• Crystalline structure; solubility; rotatory power
• Chemical Differences Between D- & L- forms
• Reactions (oxidations, reductions,
condensations)
• Physiological Differences Between D- & L- forms
• Nutritive value (human, bacterial); sweetness;
absorption
Structural Representation of Sugars
Biomolecules (in this case sugars)
can be represented in three main
ways (visualized in the following slides):
• Fischer Projection: straight chain
representation
• Haworth Projection: simple ring in
perspective
• Conformational Representation:
chair and boat configurations
Pentoses and hexoses
can cyclize as the
ketone or aldehyde
reacts with a distal
OH. The top diagram is
a Fischer Projection
of D-Glucose
1
H
HO
H
H
2
3
4
5
6
Glucose forms an intramolecular hemiacetal,
as the C1 aldehyde &
C5 OH react, to form a
6-member pyranose
ring, named after pyran
CHO
C
OH
C
H
C
OH (linear form)
C
OH
D-glucose
CH2OH
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
OH
-D-glucose
The representations of the cyclic sugars (bottom) are
called Haworth Projections
H
More Pyran Cyclization
CH2OH
1
HO
H
H
2C
O
C
H
C
OH
C
OH
3
4
5
6
HOH2C 6
CH2OH
D-fructose (linear)
H
5
H
1 CH2OH
O
4
OH
HO
2
3
OH
H
-D-fructofuranose
Fructose forms either a:
 6-member pyranose ring: reaction of the C2 keto
group with the OH on C6, or
 5-member furanose ring: reaction of the C2 keto
group with the OH on C5
6 CH2OH
6 CH2OH
5
H
4
OH
H
OH
3
H
O
H
H
1
2
OH
-D-glucose
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
H
OH
-D-glucose
Cyclization of glucose produces a new asymmetric center
at C1. The 2 stereoisomers are called anomers,  & 
Haworth projections represent the cyclic sugars as having
essentially planar rings, with the OH at the anomeric C1:
  (OH below the ring)
  (OH above the ring)
H OH
4
H OH
6
H O
HO
HO
H O
HO
H
HO
5
3
H
H
2
H
OH 1
OH
-D-glucopyranose
H
OH
OH
H
-D-glucopyranose
Because of the tetrahedral nature of carbon bonds,
pyranose sugars actually assume a "chair" or
"boat" configuration, depending on the sugar
The representation above reflects the chair
configuration of the glucopyranose ring more
accurately than the Haworth projection
Chair (top) and Boat (bottom)
forms of the Pyranose Ring
Optical Isomerism and Polarimetry
•
•
Recall that optical isomerism is a property exhibited by
any compound whose mirror images are nonsuperimposable
Also, compounds with asymmetric carbons rotate plane
polarized light –
– Measurement of optical activity in chiral or
asymmetric molecules uses plane polarized light
– Molecules may be chiral because of certain
atoms or because of chiral axes or chiral planes
– Measurement uses an instrument called a
polarimeter (Lippich type)
– Rotation is either (+) dextrorotatory or (-)
levorotatory
Polarimeter
Polarimetry
• Magnitude of rotation depends
upon:
1. The nature of the compound
2. The length of the tube (cell or
sample container) usually
expressed in decimeters (dm)
3. The wavelength of the light
source employed; usually either
sodium D line at 589.3 nm or
mercury vapor lamp at 546.1 nm
4. Temperature of sample
5. Concentration of carbohydrate in
grams per 100 ml
Selected Rotations
D-glucose
+52.7
D-fructose
-92.4
D-galactose +80.2
L-arabinose +104.5
D-mannose +14.2
D-arabinose -105.0
D-xylose
+18.8
Lactose
+55.4
Sucrose
+66.5
Maltose+
+130.4
Invert sugar -19.8
Dextrin
+195
Part 2
Oligosaccharides
and selected derivatives
Oligosaccharides
•
The most common oligosaccarides are
the disaccharides
• Sucrose, lactose, and maltose
• Maltose hydrolyzes to 2 molecules of Dglucose
• Lactose hydrolyzes to a molecule of glucose
and a molecule of galactose
• Sucrose hydrolyzes to a molecule of glucose
and a molecule of fructose
Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another
sugar or some other compound can join together,
splitting out water to form a glycosidic bond:
R-OH + HO-R'  R-O-R' + H2O
e.g. methanol reacts with the anomeric OH on
glucose to form methyl glucoside (methylglucopyranose).
H OH
H OH
H2O
H O
HO
HO
H
H
H
+
CH3-OH
H O
HO
HO
H
OH
H
OH
-D-glucopyranose
methanol
H
OH
OCH3
methyl--D-glucopyranose
Disaccharides:
Maltose, a cleavage
product of starch (i.e.
amylose), is a
disaccharide with an
(1 4) glycosidic
link between the C1 C4 OH’s of 2 glucoses.
It is the  anomer (C1
O points down)
6 CH2OH
6 CH2OH
H
5
O
H
OH
4
OH
3
H
H
H
1
H
4
4
maltose
OH
H
H
1
OH
2
H
OH
2OH
H
H
1
O
4
5
O
H
OH
H
H
3
H
6 CH
O
H
OH
H
OH
3
OH
5
O
O
2
6 CH2OH
H
5
2
OH
3
cellobiose
H
2
OH
1
H
OH
Cellobiose, a product of cellulose breakdown, is the
otherwise equivalent  anomer (O on C1 points up).
The (1 4) glycosidic linkage is represented as a zig-zag,
but one glucose is actually flipped over relative to the other
Other disaccharides include:

Sucrose, common table sugar, has a glycosidic
bond linking the anomeric hydroxyls of glucose
& fructose.
Because the configuration at the anomeric C of
glucose is  (O points down from ring), the
linkage is (12)
The full name of sucrose is -D-glucopyranosyl(12)--D-fructopyranose.)

Lactose, milk sugar, is composed of galactose
& glucose, with (14) linkage from the
anomeric OH of galactose. Its full name is -Dgalactopyranosyl-(1 4)--D-glucopyranose
Sucrose
•
•
•
•
•
Probably the most famous sugar, and everyone’s
favorite, is sucrose:
 -D-glucopyranosido--D-fructofuranoside
 -D-fructofuranosido--D-glucopyranoside
Also known as table sugar
Commercially obtained from sugar cane or sugar
beet
Hydrolysis yield glucose and fructose (invert sugar) (
sucrose: +66.5o ; glucose +52.5o; fructose –92o)
Used pharmaceutically to make syrups
Lactose
•
•

•
Lactose is another famous disaccharide, resulting
from -D-galactose joining to -D-glucose via a
-(1,4) linkage
Milk contains the a and b-anomers in a 2:3 ratio
-lactose is sweeter and more soluble than ordinary
- lactose
Used in infant formulations, medium for penicillin
production and as a diluent in pharmaceuticals
Starch
•
•
•
Starch is the most common storage
polysaccharide in plants
It is composed of 10 – 30% amylose and 70-90% amylopectin
(depending on the source)
The chains are of varying length,
having molecular weights from
several thousands to half a million
Polysaccharides
CH 2OH
H
O
H
OH
H
H
H
1
O
OH
6CH OH
2
5
O
H
4 OH
3
H
OH
H
H
H
H 1
O
H
OH
CH 2OH
CH 2OH
CH 2OH
H
H
H
O
H
OH
H
O
O
H
H
O
H
OH
H
H
O
OH
2
OH
H
OH
H
OH
H
OH
amylose
Plants store glucose as amylose or amylopectin.
Glucose polymers collectively are called starch.
Glucose storage in polymeric form minimizes osmotic
effects.
• Amylose is a glucose polymer with (14)
linkages. It adopts a helical conformation (see above)
• The end of the polysaccharide with an anomeric C1
not involved in a glycosidic bond is called the
reducing end
•
CH 2OH
CH 2OH
O
H
H
OH
H
H
OH
H
O
OH
CH 2OH
H
H
OH
H
H
OH
H
H
OH
CH 2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
amylopectin
H
1
O
6 CH 2
5
H
OH
3
H
CH 2OH
O
H
2
OH
H
H
1
O
CH 2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Amylopectin is a glucose polymer with mainly
(14) linkages, but it also has branches formed by
(16) linkages (see above). Branches are generally
longer than shown above.
• The branches produce a compact structure & provide
multiple chain ends at which enzymatic cleavage can
occur.
•
Another view of amylose and amylopectin, the two forms of starch. Amylopectin
is a highly branched structure, with branches occurring every 12
to 30 residues
Glycogen
•
•
•
•
•
•
•
Glycogen is also known as “animal starch”
(not really an accurate description!)
It is stored in muscle and liver tissue
Also present in cells as granules (high MW)
It contains both -(1,4) links and -(1,6)
branches at every 8 to 12 glucose unit
Complete hydrolysis yields glucose
Glycogen and iodine gives a red-violet color
Hydrolyzed by both  and -amylases and by
glycogen phosphorylase [these are enymes]
CH 2OH
CH 2OH
O
H
H
OH
H
H
OH
H
O
OH
CH 2OH
H
H
OH
H
H
OH
H
H
OH
CH 2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH 2
5
H
OH
3
H
CH 2OH
O
H
2
OH
H
H
1
O
CH 2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is
similar in structure to amylopectin, but glycogen has
more (16) branches
• The highly branched structure permits rapid release
of glucose from glycogen stores, i.e. in muscle during
exercise. The ability to rapidly mobilize glucose is
more essential to animals than to plants
•
Cellulose
• Cellulose is a polymer of -D-glucose
attached by -(1,4) linkages
• It yields glucose upon complete hydrolysis
• Partial hydrolysis yields cellulobiose
• Cellulose is the most abundant of all
carbohydrates
• Cotton flax: 97-99% cellulose
• Wood: ~ 50% cellulose
• Cellulose gives no color with iodine
• Held together with “lignin” in woody plant
tissues
CH 2OH
H
O
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH 2OH
CH 2OH
CH 2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
H
OH
cellulose
Cellulose, a major constituent of plant cell walls,
consists of long linear chains of glucose with (14)
linkages.
• Every other glucose is flipped over, due to the 
linkages. This promotes intra-chain and inter-chain Hbonds and van der Waals interactions. This cause
cellulose chains to be straight & rigid, and pack with a
crystalline arrangement in thick bundles called
microfibrils
•
Schematic of arrangement of
cellulose chains in a microfibril.
The Linear Structures of Cellulose and Chitin
(chitin is found in the exoskeleton of insects, crayfish, etc]
(these are the two most abundant polysaccharides in nature)
The Molecular Structure of Cellulose
(Notice the presence of “sheets” that can be pealed away. Think about a piece
of celery and how you can strip off the fibers)
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
(a) The structure of starch – shows  linkages
(b) The structure of cellulose – shows  linkages
Oligosaccharides
that are covalently
attached to proteins
or to membrane
lipids may be linear
or branched chains
C
CH2OH
O
H
H
OH
O
CH2
CH
NH
H
O
serine
residue
O H
OH
H
HN
C
CH3
-D-N-acetylglucosamine
O-linked oligosaccharide chains of glycoproteins vary
in complexity.
• They link to a protein via a glycosidic bond between a
sugar residue and a serine or threonine OH
• O-linked oligosaccharides have roles in recognition,
interaction, and enzyme regulation
The Structures of Serine or Threonine O-linked Saccharides
O-linked glycoproteins are found in the blood of Arctic
and Antarctic fish, enabling them to live at sub-zero
water temperatures
C
CH2OH
O
H
H
OH
O
CH2
CH
NH
H
O
serine
residue
O H
OH
H
HN
C
CH3
-D-N-acetylglucosamine
• N-acetylglucosamine (GlcNAc) is a common O-linked
glycosylation product of serine or threonine residues
• Many cellular proteins, including enzymes & transcription
factors, are regulated by reversible GlcNAc attachment
• Often attachment of GlcNAc to a protein OH alternates with
phosphorylation, with these 2 modifications having opposite
regulatory effects (stimulation or inhibition)
CH2OH
O
O
H
H
OH
HN
C
HN
CH2
C
H
H
OH
H
HN
C
CH3
O
N-acetylglucosamine
Initial sugar in N-linked
glycoprotein oligosaccharide
Asn
CH
O
HN
HC
R
C
O
X
HN
HC
R
C
O
Ser or Thr
N-linked oligosaccharides of glycoproteins tend to be
complex and branched. First N-acetylglucosamine is
linked to a protein via the side-chain N of an
asparagine residue in a particular 3-amino acid
sequence.
The Structure of Aspargine N-linked Glycoproteins
More Examples of N-Linked Glycoproteins
Selected Facts About Oligosaccharide Derivatives
• Many proteins secreted by cells have attached N-linked
oligosaccharide chains
• Genetic diseases have been attributed to deficiencies of
particular enzymes involved in synthesizing or modifying
these glycoprotein oligosaccharide chains
• Such genetic diseases, and gene knockout studies in
mice, have been used to define pathways of modification
of oligosaccharide chains in glycoproteins and glycolipids.
• Carbohydrate chains of plasma membrane glycoproteins
and glycolipids usually face the outside of the cell
• Plasma membrane glycoproteins and glycolipids have
roles in cell-cell interaction and signaling, as well as
forming a protective layer on the surface of some cells
Special Monosaccharides: Deoxy Sugars
•
•
•
Some monosaccharides lack one or
more hydroxyl groups on the molecule.
These are “deoxy sugars”
One ubiquitous deoxy sugar is 2’-deoxy
ribose which is the sugar found in DNA
6-deoxy-L-mannose (L-rhamnose) is
used as a fermentative reagent in
bacteriology
A Few Examples of Deoxysugar Structures