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CARBOHYDRATES

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Title: CARBOHYDRATES


1
MS1005N General Biochemistry
CARBOHYDRATES
Dr Sundus Tewfik s.tewfik_at_londonmet.ac.uk
2
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.
3
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.
4
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
5
Monosaccharides
3. simplest CHOs are aldehydes or ketones that
have 2 or more hydroxyl groups. empirical
formula
the smallest ones (where n3) are glyceraldehyde
and dihydroxyacetone
6
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
7
5. Based on number of carbons, a monosaccharide
is a
 tetrose
 pentose
 hexose
 heptose
8
6. 2 common hexoses are D-glucose (an aldose) and
D-fructose (a ketose)
Aldoses have an aldehyde group at one end.
Ketoses have a keto group usually at C2.
each have the same formulae
9
Fig. Mathews, van Holde, and Ahern Biochemistry
3rd edition.
10
Aldoses
11
Fig. Mathews, van Holde, and Ahern Biochemistry
3rd edition.
12
Ketoses
13
7. D and L refer to the configuration at the
asymmetric carbon atom furthest from the aldehyde
or keto group
D because of configuration at asymmetric carbon
atom furthest from
Most naturally occurring sugars are D isomers.
14
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.
15
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
16
10. Fewer D-ketoses since D-erythrulose only has
1 asymmetric centre (dihydroxyacetone is not
optically active). D-fructose is the most
abundant ketohexose.
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
17
13. In glucose (open-chain) the C-1 aldehyde
reacts with the C-5 hydroxyl group ?
intramolecular hemiacetal
18
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 a and ß-isomers are
called anomers.
19
The six membered ring is called pyranose -
similar to pyran.
14. Similarly a ketone can react with an alcohol
? hemiketal
20
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
21
16. Ring forms are called Haworth projections
(straight chains are Fischer projections).
22
17. There is an additional asymmetric centre
created on cyclisation at C-1 with aldose sugars
eg with glucose either a or ß possible (OH below
ring -a, OH above ring -ß ). The forms are
anomers of each other.
23
18. Fructose can also have a or ß forms, here the
OH group is attached to C-2 (which is anomeric).
24
19. Fructose also forms pyranose rings. This form
predominates in fructose but furanose is the main
form in derivatives.
25
20. 5 carbon sugars such as D-ribose and
D-deoxyribose form furanose rings.
26
21. In water, a- D- glucopyranose and ß- D-
glucopyranose interconvert through the open chain
form of the sugar.
27
22. The specific rotations of the a and ß 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
28
This change is called mutarotation and results
from an equilibrium mixture of 1/3 a and 2/3 ß
anomers.
Note very little (lt1) open chain is present
23. Fructose also shows mutarotation.
29
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)
30
They are classified as reducing sugars since they
reduce the Cu2 to Cu  which forms as a red
precipitate, copper (I) oxide.
31
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.
32
Sugar derivatives
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.
33
Sugar derivatives
  • amino sugar - an amino group substitutes for a
    hydroxyl. An example is glucosamine. The amino
    group may be acetylated, as in
    N-acetylglucosamine.

34
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
35
26. If glucose is warmed in anhydrous methanol
containing HCl, the anomeric carbon atom reacts
with the methanol to form a methyl glucoside and
ß methyl glucoside
36
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 ß
configuration
37
28. Three highly abundant disaccharides are
maltose, lactose and sucrose
38
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 non-reducing Sucrase (invertase) Fruits seeds roots and honey
39
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)aD20 66.5? to D-glucose
aD20 52.5? and D-fructose aD20 -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.
40
Optical activity of sugars
D- and L- define the positions of the atoms in
space relative to reference compound and not the
optical rotation (designated d and l or and -)
Ability to rotate plane polarized light
dextrorotatory rotate to right use ()
symbol usually D isomers
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
41
30. Lactose (the main sugar in milk) consists of
galactose joined to glucose by a ß 1,4 glycosidic
linkage. It is hydrolysed to these
monosaccharides by ß-galactosidase 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.
42
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.
43
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.
44
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.
45
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
46
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
47
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 Non-hygroscopicity Heat and pH stability
Stability with basic drug High compactibility Taste and odor-masking effect
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