Transcript Document

MS1005N General Biochemistry
CARBOHYDRATES
Dr Sundus Tewfik
[email protected]
Learning outcomes:
To know some of the roles of carbohydrates in biology.
To relate this to their structures and basic chemistry.
To know the structures and names of some common
carbohydrates.
CARBOHYDRATES
1. Carbohydrates (CHOs) are ketone or aldehyde compounds
with multiple hydroxyl groups
2. They serve as
a) energy stores, fuels and metabolic intermediates.
b) part of the structural framework of DNA and RNA.
c) structural elements in the cell walls of bacteria and plants
and exoskeleton of invertebrates. and they are linked to
many proteins and lipids.
Some Carbohydrate Functions
• Developmental
processes
• Protein localization
• Regulation of activity
• Cell-cell interactions
• Structural integrity, e.g. cell
walls
• Phytohormones
• Plant and animal defence
• Prevention of desiccation
(bacteria)
• Adherence of bacteria to
surfaces
• Bacterial resistance to host
defences
• Cell-cell recognition and
signalling
Monosaccharides
3. simplest CHOs are aldehydes or ketones that have 2 or more
hydroxyl groups.
I
CHO
CHO
empirical
formula :
H
C
(CH
2O)n or H - C - OH
HO C H
I
OH
CH2OH
CH2OH
CHO
CHO
the smallest ones (where n=3) are glyceraldehyde and
D-glyceraldehyde
L-glyceraldehyde
dihydroxyacetone
H
C
OH
HO
CH2OH
C
H
CH2OH
D-glyceraldehyde
L-glyceraldehyde dihydroxyacetone
aldoses
a ketose
4. Glyceraldehyde has an asymmetric carbon atom,
therefore it has 2 stereoisomers the D and L forms
(these are enantiomers ie mirror images, not
superimposable).
Asymmetric carbon; four different things are attached to it
Stereoisomers have
The same chemical formula
The same order and types of bonds
Different spatial arrangements
Different properties
5. Based on number of carbons, a monosaccharide is a
Number of carbon atoms
4
5
6
7
Name of sugar
tetrose
pentose
hexose
heptose
6. 2 common hexoses are D-glucose (an aldose) and
D-fructose (a ketose)
Aldoses have an aldehyde
group at one end.
H
Ketoses have a keto group
usually at C2.
O
CH2OH
C
H
C
OH
HO
C
H
H
C
OH
H
C
OH
CH2OH
D-glucose
Tautomers
C6H12O6
C
O
HO
C
H
H
C
OH
H
C
OH
each have the same formulae
CH2OH
D-fructose
Fig. Mathews, van Holde, and
Ahern Biochemistry 3rd edition.
Aldoses
Fig. Mathews, van Holde, and Ahern
Biochemistry 3rd edition.
Ketoses
7. D and L refer to the configuration at the asymmetric
carbon atom furthest from the aldehyde or keto group
H
O
D because of configuration at asymmetric
carbon atom furthest from
C
H
C
OH
HO
C
H
H
C
OH
H
C
OH
CH2OH
D-glucose
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
Most naturally occurring sugars are D isomers.
8. A molecule with n chiral carbon atoms (asymmetric centers) has
2n stereoisomeric forms.
9. Possible to draw up 2 series of D-sugars - the D-aldoses and
the D-ketoses.
Note that the chiral C furthest from -CHO always has -OH to
right therefore D series of sugars
Important sugars are ribose, glucose, mannose, galactose, xylose.
Glucose, mannose, galactose are the most abundant.
D-sugars differing at a single asymmetric centre are
called epimers.
glucose/mannose are epimers at
C2
glucose/galactose are epimers at
C4
10. Fewer D-ketoses since D-erythrulose only has 1 asymmetric
centre (dihydroxyacetone is not optically active). D-fructose is
the most abundant ketohexose.
D-erythrulose
11. In solution, glucose and fructose are not mainly
open chains. They cyclise into rings (as follows)
12. An aldehyde can react with an alcohol to form a hemiacetal
H
C
H
O
+
R'
OH
R
aldehyde
R'
O
C
R
alcohol
hemiacetal
OH
13. In glucose (open-chain) the C-1 aldehyde reacts with the C-5
hydroxyl group  intramolecular hemiacetal
The linear aldehyde is tipped on its side, and rotation about
the C4-C5 bond brings the C5-hydroxyl function close to the
aldehyde carbon. For ease of viewing, the six-membered
hemiacetal structure is drawn as a flat hexagon, but it actually
assumes a chair conformation. The hemiacetal carbon atom
(C-1) becomes a new stereogenic center, commonly referred
to as the anomeric carbon, and the α and β-isomers are
called anomers.
The six membered ring is called pyranose - similar to pyran.
H
C
H
O
+
R'
OH
R'
O
R
C
OH
R
aldehyde
alcohol
hemiacetal
14. Similarly a ketone can react with an alcohol  hemiketal
R
C
R
O
+
"R
OH
R'
ketone
"R
O
C
R'
alcohol
hemiketal
OH
15. The C-2 keto group in open chain fructose can react
with the C-5 hydroxyl group intramolecular hemiketal.
The five membered ring is called furanose
-similar to
16. Ring forms are called Haworth projections (straight
chains are Fischer projections).
17. There is an additional asymmetric centre created on
cyclisation at C-1 with aldose sugars eg with glucose
either α or β possible (OH below ring -α, OH above ring β ). The forms are anomers of each other.
6 CH2OH
6 CH2OH
5
H
4
OH
O
H
OH
3
H
H
2
OH
-D-glucose
H
1
OH
5
H
4
OH
H
OH
3
H
O
OH
H
1
2
OH
-D-glucose
H
18. Fructose can also have α or β forms, here the OH
group is attached to C-2 (which is anomeric).
D- fructose
α-D- Fructofuranose
β-D- Fructofuranose
19. Fructose also forms pyranose rings. This form
predominates in fructose but furanose is the main
form in derivatives.
α-D- Fructopyranose
β-D- Fructopyranose
20. 5 carbon sugars such as D-ribose and
D-deoxyribose form furanose rings.
21. In water, α- D- glucopyranose and β- D- glucopyranose
interconvert through the open chain form of the sugar.
22. The specific rotations of the α and β anomers of glucose are
+112 and +18.7. When a crystalline sample of either anomer is
dissolved in water the specific rotation changes over time until
+52.7is reached.
Initial state 1
Aqueous solution
of D-glucose with
X g/ml and a
specific rotation of
+112.2°
Final state for
both
Equilibrium solution
with a specific
rotation of +52.7°
Initial state 2
Aqueous solution
of D-glucose with X
g/ml and a specific
rotation of +18.7°
This change is called mutarotation and results from an
equilibrium mixture of 1/3 α and 2/3 β anomers.
Note: very little (<1%) open chain is present
23. Fructose also shows mutarotation.
24. The open chain form of aldose or ketose is the
reducing agent in tests for reducing sugars.
This is because the aldehyde group that is present can
be oxidized to form a carboxylic acid group, or in the
presence of a base, a carboxylate ion group.
Common test reagents are :
Benedicts reagent (CuSO4 / citrate)
Fehlings reagent (CuSO4 / tartrate)
They are classified as reducing sugars since they reduce the
Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide.
red precipitate
D-glucose
25. Conformation of pyranose or furanose rings:
The 6-membered pyranose ring cannot be planar
because of the geometry of its saturated carbon atoms.
They adopt chair or boat conformations
Substituents to the ring are of 2 types, axial (a) or equatorial (e).
Axial bonds are nearly perpendicular to the plane of the ring () to
the plane of the ring, equatorial bonds are nearly parallel () to
the plane of the ring to plane.
Sugar derivatives
H
C
OH
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
H
C
OH
H
C
OH
HO
C
H
H
C
OH
H
C
CH2OH
H
C
D-ribitol
CHO
COOH
CH2OH
CH2OH
D-gluconic acid
COOH
D-glucuronic acid
sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol.
sugar acid - the aldehyde at C1, or OH at C6, is oxidized
to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
Sugar derivatives
CH
CH22OH
OH
CH
CH22OH
OH
OO
HH
HH
OH
OH
HH
HH
OH
OH
HH
OH
OH
OH
OH
HH
OO
HH
NH
NH22
HH
HH
OH
OO OH
OH
OH
HH
NN CC CH
CH33
HH
-DD-glucosamine
-glucosamine
-DD-N-acetylglucosamine
-N-acetylglucosamine
amino sugar - an amino group substitutes for a hydroxyl.
An example is glucosamine.
The amino group may be acetylated, as in
N-acetylglucosamine.
Glycosidic bond
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
26. If glucose is warmed in anhydrous methanol
containing HCl, the anomeric carbon atom reacts with
the methanol to form α methyl glucoside and β methyl
glucoside
27. Each single Haworth structure is a monosaccharide.
These can be linked together by O-glycosidic bonds 
disaccharides  polysaccharides.
For example, in cellulose, D-glucose residues are joined by
glycosidic linkages between C-1 of one glucose and the hydroxyl
oxygen atom of C-4 of another glucose.
These glycosidic bonds have a β configuration
28. Three highly abundant disaccharides are maltose, lactose and sucrose
Disaccharides
Maltose
Glc(14)Glc
reducing
maltase
Plants (starch)
Animals
(glycogen)
Cellobiose
Glc(14)Glc
reducing
cellulase
Plants (cellulose
dimer)
Lactose
Gal (14)Glc
reducing
Lactase
Milk (major
energy source)
Sucrose
Glc(12)Fru
nonreducing
Sucrase
(invertase)
Fruits seeds roots
and honey
29. Sucrose obtained commercially from sugar cane or sugar beet
It is .unusual in that the anomeric carbon atoms of glucose and
fructose are involved in the glycosidic linkage. This means no
aldehydes are free therefore no reducing groups therefore
sucrose is non-reducing sugar.
The hydrolysis of sucrose to glucose and fructose is
catalysed by invertase (also called sucrase). This
hydrolysis from sucrose (value for the optical
rotation)[α]D20 = +66.5 to D-glucose [α]D20 = +52.5
and D-fructose [α]D20 = -92 is called inversion
(because the net change is from dextro to laevo). The
final mixture of glucose and fructose is called invert
sugar.
Trehalose is another example of a non-reducing
disaccharide.
Optical activity of sugars
and L- define the positions of the
atoms in space relative to reference
compound and not the optical rotation
Ability to rotate plane polarized light (designated d and l or + and -)
dextrorotatory
rotate to right
use (+) symbol
usually D isomers
D-
Laevorotatory
rotate to left
use (-) symbol
usually L isomers
Light is passed through a polarized filter. A solution of an
optical isomer will rotate the light one direction
30. Lactose (the main sugar in milk) consists of
galactose joined to glucose by a β 1,4 glycosidic linkage.
It is hydrolysed to these monosaccharides by βgalactosidase in bacteria and lactase in humans.
31. Lactase deficiency:
In certain populations adults are deficient in lactase. For
these adults when milk is ingested, lactose is not broken
down and accumulates in the small intestine. This has
an osmotic effect drawing fluid from the cells into the
small intestine therefore lactose causes distention,
nausea, cramping pain and watery diarrhoea.
Note:
3% incidence among Danes but 97% incidence among Thais.
Trehalose
Trehalose is a disaccharide composed of two glucose
molecules bound by an alpha, alpha-1,1 linkage.
Since the reducing end of a glucosyl residue is
connected with the other, trehalose has no reducing
power.
α, 1
1 glucosyl glucopyranose
Trehalose is widely distributed in nature. It is known to
be one of the sources of energy in most living
organisms and can be found in many organisms,
including bacteria, fungi, insects, plants, and
invertebrates. Mushrooms contain up to 10-25 %
trehalose by dry weight. Furthermore, trehalose
protects organisms against various stresses, such as
dryness, freezing, and osmopressure. In the case of
resurrection plants, which can Jive in a dry state, when
the water dries up, the plants dry up too. However,
they can successfully revive when placed in water.
Empirical evidence shows that high concentrations of trehalose
in the tissues of certain insects and desert plants allows them to
survive in a state of suspended animation under conditions of
water deficiency. Trehalose helps frogs to survive in a frozen
state and also helps to revive the DNA of salmon sperms from
dehydration.
Its relative sweetness is 45% of sucrose. Trehalose has high
thermostability and a wide pH-stability range. Therefore, it is one
of the most stable saccharides.
In trehalose, one glucose molecule is upside-down relative to the
other. In maltose, the two glucose molecules are in the same
orientation. This small difference reflects in the properties of
trehalose. It does not brown when heated; it does not promote
bacterial growth or tooth decay as much as maltose or sugar, and
it is less attractive to moisture.
Trehalose creates a more crystalline formation with
neighbouring water molecules than that created between water
molecules and the two similar disaccharides. Trehalose modifies
the structural and dynamic properties of water, forming a unique
entity with water molecules which makes it better able to protect
biological structures
The term "trehalose" was coined when the same substance was
identified as a component of the secretions of a beetle in the Iraqi
desert. These secretions, known by native peoples to be edible
and sweet, were called the "trehala manna.
" Some people believe that this is a similar
substance to the manna that was gathered
and eaten by the Israelites of the
Old Testament
There is a variety of manna obtained from the nests and cocoons
of a Syrian coleopterous insect (Larinus maculatus, L. nidificans,
etc.) which feeds on the foliage of a variety of thistle. It is used as
an article of food, and is called also nest sugar
Larinus maculatus
The Bedouin gather this sweet product from leaves of trees and
bushes, and use it as a sugar substitute in coffee. The sugar
fraction was found to consist mainly of trehalose. Two samples
contained 30% and 45% of trehalose, respectively, calculated on
the basis of total dry matter, and 70% and 80%, respectively,
calculated on the total carbohydrate content.
Features attracts pharmaceutical industry
Low reactivity
Stability with basic
drug
Non-hygroscopicity
High compactibility
Heat and pH stability
Taste and odor-masking
effect