Carbohydrates
Structure and Biological Function
Monosaccharides
Carbohydrates in Cyclic Structures
Reactions of Glucose and Other
Monosaccharides
Polysaccharides
Glycoproteins
1
Carbohydrates
Compounds containing C, H and O
General formula
:
Cx(H2O)y
All have C=O and -OH functional groups.
Classified based on
• Size of base carbon chain
• Number of sugar units
• Location of C=O
• Stereochemistry
2
Types of carbohydrates
Classifications based on number of sugar
units in total chain.
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides
- single sugar unit
- two sugar units
- 2 to 10 sugar units
- more than 10 units
Chaining relies on ‘bridging’ of oxygen
atoms
glycoside bonds
3
Monosaccharides
Based on location of C=O
H
|
C=O
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
CH2OH
Aldose
- aldehyde C=O
CH2OH
|
C=O
|
HO-C-H
|
H-C-OH
|
H-C-OH
|
CH2OH
Ketose
- ketone C=O
4
Monosaccharide classifications
Number of carbon atoms in the chain
H
|
C=O
|
H-C-OH
|
CH2OH
triose
H
|
C=O
|
H-C-OH
|
H-C-OH
|
CH2OH
tetrose
H
H
|
C=O
|
C=O
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
CH2OH
pentose
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
CH2OH
hexose
Can be either aldose or ketose sugar.
5
Examples
CH2OH
|
C=O
H
|
C=O
|
H-C-OH
|
CH2OH
D-glyceraldehyde
triose
aldose
aldotriose sugar
|
HO-C-H
|
H-C-OH
|
H-C-OH
|
CH2OH
D-fructose
hexose
ketose
ketohexose sugar
6
Examples
H
H
|
C=O
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
CH2OH
D-ribose
pentose, aldose
aldopentose sugar
|
C=O
|
H-C-OH
|
H-C-OH
|
HO-C-H
|
HO-C-H
|
CH2OH
L-mannose
hexose, aldose
aldohexose sugar
7
Stereoisomers
Stereochemistry
Study of the spatial arrangement of molecules.
Stereoisomers have
• the same order and types of bonds.
• different spatial arrangements.
• different properties.
Many biologically important chemicals, like
sugars, exist as stereoisomers. Your body can
tell the difference.
8
Enantiomers
Pairs of stereoisomers
Designated by D- or L- at the start of the name.
They are mirror images that can’t be
overlapped.
If you don’t believe it,
give it a try!
9
Enantiomers
10
L- and D- glyceraldehyde
CHO
CHO
HO
H
H
OH
C
C
CH2OH
CH2OH
CHO
CHO
HO
H
CH2OH
H
OH
CH2OH
11
Enantiomers
Chiral center.
Asymmetric carbon - 4 different things
are attached to it.
Cl
|
I- C - F
|
Chiral center
Br
You must have at least one asymmetric
carbon to have stereoisomers.
12
Examples
Is the ‘red’ carbon chiral?
H
C=O
H
H3C- C-OH
Cl
CH2CH3
I
Cl
C=C
Br
F
H
H
H3C- C-OH
H
|
C=O
|
H-C-OH
|
HH H
H2N-C-C-C-SH
Cl H Cl
CH2OH
13
Physical properties
Optical activity
ability to rotate plane polarized light.
dextrorotatory
- rotate to right
- use + symbol
- usually D isomers
levorotatory
- rotate to left
- use - symbol
- usually L isomers
14
Plane polarized light
Light is passed through a polarized filter.
A solution of an optical isomer will rotate the
light one direction.
15
Stereochemistry
Properly drawing enantiomers in 3-D is hard.
Use Fischer Projections
Specific type of formula that designates
the orientation of groups.
H
H
|
C=O
|
H-C-OH
|
CH2OH
|
C=O
|
H-C-OH
|
H-C-OH
|
CH2OH
16
Fischer projections
With this system, a tetrahedral carbon atom is
represented by two crossed lines.
H
H
|
C=O
O
|
H-C-OH
H
OH
H
OH
|
H-C-OH
|
CH2OH
CH2OH
A horizontal bond to an asymmetric carbon
designates bonds in the front plane of the page.
Vertical bonds are behind the plane of the page.
17
Some important monosaccharides
D-glyceraldehyde
D-glucose
D-fructose
D-galactose
D-ribose
Simplest sugar
Most important in diet
Sweetest of all sugars
Part of milk sugar
Used in RNA
note that each is a D- enantiomer
18
D-glyceraldehyde
Three carbon sugar
Aldose sugar
Triose sugar
H
|
aldotriose
C=O
|
H-C-OH
|
CH2OH
19
D-glucose
•
Glucose is an aldohexose sugar.
•
Common names include
dextrose, grape sugar, blood
sugar.
•
Most important sugar in our diet.
•
Most abundant organic
compound found in nature.
•
H
O
C
H C OH
HO C H
H C OH
H C OH
CH 2OH
Level in blood can be as high as
0.1%
20
D-fructose
Another common sugar.
CH2OH
|
C=O
|
HO-C-H
It is a ketohexose.
|
H-C-OH
|
H-C-OH
Sweetest of all sugars.
|
CH2OH
21
Carbohydrates in cyclic structures
If optical isomers weren’t enough, sugars also
form rings. For many sugars, its the most
common form.
hemiacetal - forms from alcohol and aldehyde
hemiketal - forms from alcohol and ketone
R
OR’’
\
|
C=O + ROH
/
R’
R - C - OH
|
R’
22
Intramolecular cyclization
Cyclization.
Remember - chains can bend and rotate.
CH2OH
C
CH2OH
OH
C
H
C
C
C
C
O
O
C
C
C
OH
C
23
Intramolecular cyclization
The -OH group that forms can be above or
below the ring resulting in two forms anomers
 and  are used to identify the two forms.
  - OH group is down compared to CH2OH
(trans).
  - OH group is up compared to CH2OH
(cis).
24
Intramolecular cyclization
The  and  forms are in equilibrium so one
form can convert to the other mutarotation.
Haworth projections can be used to help
see  and  orientations.
O
O
25
Cyclization of D-glucose
 -D - glucose
CH2 OH
H
O
H
H
OH
O
C
H C OH
H C OH
H
OH
OH
H C OH
HO C H
H
H
OH
 - D - glucose
CH 2 OH
O
H
OH
H
CH 2OH
OH
H
OH
H
H
OH
26
Fischer vs. Haworth projections
 -D-glucose
H C OH
H C OH
HO C H
H C OH
HO-CH2 C H
CH2 OH
H
O
H
O
OH
H
H
OH
OH
H
OH
27
Cyclization of D-fructose
This can also happen
CH2 OH O
to ketose sugars.
CH2 OH
C O
HO C H
H C OH
H C OH
CH 2OH
H
CH2OH
OH
H
OH
CH2 OH O
H
H
OH
OH
OH
H
OH

H

CH2OH
28
D-galactose
•
•
Not found in many biological systems
Common part of lactose - disaccharide
CH 2 OH
OH
H
H
OH
H
H
HO
C
C
OH
H
HO
H
C
C
H
OH
CH 2 OH
H
H
OH
O
C
O
H
OH
CH 2 OH
OH
O
H
OH
OH
H
H
H
H
OH
29
D-glucose vs. D-galactose
D-glucose
H
O
C
D-galactose
H
O
C
H C OH
HO C H
H C OH
HO C H
H C OH
H C OH
HO C H
H C OH
CH 2OH
CH 2 OH
Can you find a difference? Your body can!
You can’t digest galactose - it must be
converted to glucose first.
30
D-ribose
An important sugar used
in genetic material.
This sugar is not used as
an energy source but is a part
of the backbone of RNA.
When the C-2 OH is removed,
the sugar becomes
deoxyribose which is used
in the backbone of DNA.
H
|
C=O
|
H-C-OH
|
H-C-OH
|
H-C-OH
|
CH2OH
31
Reactions of glucose
and other monosaccharides
Oxidation-Reduction. Required for their
complete metabolic breakdown.
Esterification. Production of phosphate
esters.
Amino derivatives. Used to produce
structural components and glycoprotein.
Glycoside formation. Linkage of
monosaccharides to form polysaccharides.
32
Oxidation-Reduction.
Aldehyde sugars (reducing sugars) are readily
oxidized and will react with Benedict’s reagent.
O-
H
|
|
C=O
|
C=O
+ 2 Cu 2+ + 5 OH-
H-C-OH
|
CH2OH
|
H-C-OH
+ 2 Cu2O + 3H2O
|
CH2OH
This provides a good test for presence of glucose in
urine - forms a red precipitate.
Other tests - Tollen’s or Fehling’s solutions.
33
Benedict’s reagent
glucose
0.5%
2%
Benedict's
Reagent
34
Ketone sugars
Ketones are not easy to oxidize except ketoses.
Enediol reaction.
H
O
C
H C OH
H C OH
HO C H
C OH
HO C H
H C OH
H C OH
H C OH
H C OH
CH 2OH
CH 2OH
CH2OH
H C O
HO C H
H C OH
H C OH
CH 2OH
So all monosaccharides are reducing sugars.
35
Esterification
Esters are formed by reaction of hydroxyl
groups (alcohols) with acids.
O
R OH +
C R'
HO
O
R O C R' + H2O
The hydroxyl groups of carbohydrates react
similarly to alcohols.
36
Esterification
The most important biological esters of
carbohydrates are phosphate esters.
O
R OH + HO P OH
OH
O
HO P OH + H2O
O-R
Example. Phosphoryl group from ATP forms an
ester with D-glucose, catalyzed by kinases.
kinase
D-glucose + ATP
D-glucose-6-phosphate + ADP
37
Amino derivatives
The replacement of a hydroxyl group on a
carbohydrate results in an amino sugar.
CH2OH
CH2OH
O
H
H
OH
OH
H
OH
H
H
OH
H
OH
-D-glucose
O
H
OH
H
H
OH
H
NH2
-D-2-aminoglucose
(glucosamine)
38
Amino derivatives
Uses for amino sugars.
Structural components of bacterial cell
walls.
As a component of chitin, a polymer found
in the exoskeleton of insects and
crustaceans.
A major structural unit of chondroitin
sulfate - a component of cartilage.
Component of glycoprotein and glycolipids.
39
Glycoside formation
 or  -OH group of cyclic monosaccharide
can form link with another one (or more).
CH2OH
glycosidic bond
CH2OH
O
H
H
OH
OH
H
H
H
O
H
OH
sugar -O- sugar
H
H
H
OH
OH
OH
OH
OH
H
CH2OH
CH2OH
oxygen bridge
H
O
H
H
OH
o
H
H
OH
H
H
H
OH
OH
H
OH
O
OH
+ H2 O
H
40
Glycosidic bonds
Type is based on the position of the C-1 OH
 glycosidic bond
- linkage between a C-1  OH and a C-4 OH
 glycosidic bond
- linkage between a C-1  OH and a C-4 OH
 bonds
 bonds
O
O
O
O
C-4 end can be either up or down depending
on the orientation of the monosaccharide.
41
Glycosidic bonds
 bonds
 bonds
O
O
O
42
O
Glycosidic bonds
General format used to describe bond.
OH type
( or )
(
carbon# of
first sugar
carbon# of
second sugar
)
As we work through the next few examples
this will become clear.
For disaccharides - the sugar is either  or 
based on form of the
remaining C-1 OH.
43
-Maltose
Malt sugar. Not common in nature except in
germinating grains.
CH2 OH
CH2 OH
H
H
OH
O
H
H
OH
H
OH
-D-glucose
H
O
H
OH
O
OH
H
H
H
OH
-D-glucose
-D-glucose and -D-glucose,  (1
4) linkage.
44
-Maltose
It is referred to as -maltose because the
unreacted C-1 on -D-glucose is in the 
position.
CH2 OH
CH2 OH
H
H
OH
O
H
OH
H
OH
H
H
O
H
OH
O
OH
H
H
H
OH
45
-Maltose
Uses for -maltose
Ingredient in infant formulas.
Production of beer.
Flavoring - fresh baked aroma.
It is hydrolyzed the in body by:
maltose + H2O
maltase
2 glucose
46
Cellobiose
Like maltose, it is composed of two molecules of
D-glucose - but with a  (1
4) linkage.
CH2OH
H
CH2OH
H
H
OH
H
H
H
OH
H
O
O
H
OH
O
OH
H
OH
H
OH
47
Cellobiose
CH 2 OH
CH 2 OH
H
The difference in
the linkage results
in cellobiose
being unusable
H
OH
O
H
H
H
H
H
OH
O
OH
O
H
H
OH
OH
H
maltose,  (1
OH
4)
CH2OH
We lack an enzyme
that can hydrolyze
cellobiose.
H
CH2OH
H
H
H
OH
OH
H
H
H
H
OH
H
OH
O
O
H
OH
O
OH
cellobiose
 (1
4)
48
Lactose
Milk sugar - dimer of -D-galactose and
either the  or  - D-glucose.
CH2 OH
CH2 OH
-Lactose
OH
H
OH
O
H
O
H
OH
H
OH
-D-galactose
OH
H
H
H
H
 (1
H
O
H
OH
-D-glucose
4) linkage,  disaccharide.
49
Lactose
We can’t directly use galactose. It must be
converted to a form of glucose.
Galactosemia - absence of needed
enzymes needed for conversion.
Build up of galactose or a metabolite like
dulcitol (galactitol) causes toxic effects.
Can lead to retardation, cataracts, death.
50
Lactose
Lactase
Enzyme required to hydrolyze lactose.
Lactose intolerance
Lack or insufficient amount of the
enzyme.
If lactase enters lower
GI, it can cause gas
and cramps.
51
Sucrose
CH2OH
Table sugar - most
common sugar in all
plants.
H
Sugar cane and beet,
are up to 20% by
mass sucrose.
OH
Disaccharide of
-glucose and
-fructose.
(1
2) linkage
O
H
H
OH
H
H
OH
O
CH2OH O
H
OH
H
OH
H
CH2OH
52
Sucrose
glucose
fructose
53
How sweet it is!
Sugar
lactose
galactose
maltose
sucrose
fructose
aspartame
saccharin
Sweetness relative
to sucrose
0.16
0.32
0.33
1.00
1.73
180
450
54
Polysaccharides
Carbohydrate polymers
Storage Polysaccharides
Energy storage - starch and glycogen
Structural Polysaccharides
Used to provide protective walls or
lubricative coating to cells - cellulose
and mucopolysaccharides.
Structural Peptidoglycans
Bacterial cell walls
55
Starch
Energy storage used by plants
Long repeating chain of -D-glucose
Chains up to 4000 units
Amylose
straight chain
major form of starch
Amylopectin
branched structure
56
Amylose starch
Straight chain that forms coils  (1 4)
linkage. Most common type of starch.
O
H
H
OH
H
O
H
H
OH
H
H
H
H
O
O
O
H
OH
H
H
O
O
O
O
H
H
OH
O
O
O
O
O
O
O
O
H
O
OH
O
O
O
H
O
OH
O
O
H
O
OH
O
O
H
H
OH
O
H
CH2OH
CH2OH
CH2OH
CH2OH
O
O
O
O
57
Amylose starch
Example showing coiled structure
- 12 glucose units
- hydrogens and side chains are omitted.
58
Amylopectin starch
Branched structure due to crosslinks.
O
H
H
OH
H
O
H
H
OH
H
H
H
O
H
H
OH
H
OH
H
CH2OH H
OH
H
O
H
H
OH
H
OH
O
H
H
O
CH2OH H
OH
H
O
H
H
OH
H
OH
CH2
OH
H
O
H
H
OH
H
H
OH
H
H
O
O
O
H
H
O
O
H
H
O
CH2OH H
OH
O
H
H
OH
O
H
CH2OH
CH2OH
CH2OH
CH2OH
H
OH
 (1
6) linkage
at crosslink
59
Glycogen
•
•
•
Energy storage of animals.
Stored in liver and muscles as granules.
Similar to amylopectin.
 (1 6) linkage
at crosslink
O
O
O
O
O
c
O
O
O
O
O
c
O
O
O
O
60
Cellulose
•
•
•
Most abundant polysaccharide.
 (1
4) glycosidic linkages.
Result in long fibers - for plant structure.
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
O
H
H
OH
O
H
H
OH
O
H
H
H
OH
O
H
H
H
OH
O
H
H
H
OH
O
H
H
H
H
H
H
H
H
O
H
H
O
O
H
O
H
OH
OH
OH
OH
OH
61
Mucopolysaccharides
These materials provide a thin, viscous, jelly-like
CH OH
coating to cells.
O
H
2
The most abundant form is
hyaluronic acid.
H
(1
3)
(1
4)
H
OH
O
H
H
O
HO
H
H
H
OH
H
H
H
H
NH
C O
CH3
HO
O
H
COO-
O
OH
O
H
COOCH2OH
O
OH
O
H
COO
H
CH2OH
O
H
H
O
HO
O
H
-
O
H
H
H
H
H
NH
C O
CH3
OH
NH
C O
CH3
OH
Alternating units of
N-acetylglucosamine and
D-glucuronic acid.
62
Structural peptidoglycans
Bacterial cell walls are composed primarily of an
unbranched polymer of alternating units of Nacetylglucosamine and N-acetylmuramic acid.
CH 2OH
O
H
H
OH
CH 3
CH 2OH
H
O
O
H
O
H
OR
NH
R=
O
H
H
CH
H
H
H
O
NH
C O
C O
CH 3
CH 3
L-Ala
D-Isoglu
(Gly) 5
L-Lys
D-Ala
(Gly) 5
Peptide crosslinks between the polymer
strands provide extra strength
crosslink for
- varies based on bacterium.
Staphylococcus
aureus
63
Glycoproteins
Proteins that carry covalently bound
carbohydrate units.
They have many biological functions.
• immunological protection
• cell-cell recognition
• blood clotting
• host-pathogen interaction
64
Glycoprotein structure
Carbohydrates only account for 1-30% of
the total weight of a glycoprotein.
The most common monosaccharides are
glucose
mannose
galactose
fucose
sialic acid
N-acetylgalactosamine
N-acetylglucosamine
65
Glycoprotein structure
Linking sugars to proteins.
CH 3
CH 2OH
O
O
H
OH
O
C
H
H
H
H
N-glycosidic bonds using
side chain amide
nitrogen of asparagine
residue
C
H
polypeptide chain
O-glycosidic bonds using
hydroxyl groups of
serine
and threonine
threonine
NHCOCH
3
asparagine
O
CH 2OH
H
N
O
H
H
OH
O
C
C
H2
C
H
H
H
NHCOCH
3
66