C–OH

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Carbohydrates and Carbohydrate
metabolism (Chemistry of Carbohydrate )
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Objective:
Understand classification and structure
of carbohydrates
Understand multistep sequences
(pathways) for carbohydrates
metabolism.
Study the metabolic disorders in
carbohydrates metabolism.
Carbohydrates and Carbohydrate
metabolism (Chemistry of Carbohydrate )
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Carbohydrate: They are polyhydroxy
aldhydes or ketones or any substances
derived from them.
OR
Compounds that contains at least 3
carbon atoms, a number of OH group,
in addition to aldhyde or ketone
Formula for simple is (CH2O)n .
Carbohydrates
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Importance and distribution of CHO in
animal and plant tissue:
Plants:
(a) Cellulose: which form the frame
work of the plant and has supporting
action.
(b) Starch: Which is the stored form of
CHO.
Carbohydrates
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Classification of CHO:
(1) Monosaccharide: They are the
simplest units of CHO, cannot
hydrolyzed to the simpler form.
They can be classified according to
either number of carbon atoms, or
whether they contain aldhydes or
ketons.
Carbohydrates (Monosaccharide)
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(1) according to number of carbons:
Trioses:Ex.Glyceraldehydes (aldotriose) Ex.
Dihydroxyacetone ( ketotrioses)
Tetrosis: Ex. Erythrose (aldotetrosis) Ex.
Erythulose (ketotetrosis)
Pentoses:Ex. Ribose (aldopentosis) Ex.
Ribulose (ketoopentosis)
Hexoses: glucose , fructose , galactose,
mannose
Carbohydrates (Monosaccharide),
Stereoisomerism
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Asymetric carbon atom: A carbon atom that
attached to four different atoms or groups of
atoms.
Any substance containing asymetric carbon
atom, it has two different optically active
isomers.
Isomers: Compounds that have the same
chemical formula but have different structure.
Ex. Glucose, fructose, mannose, galactose.
Monosaccharide, Stereoisomerism
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Epimers: Compounds that have the same
chemical formula but differ in configuration
around one carbon atom. Ex. ( D. glucose, D
galactose C-4 not galactose and mannose)
Enantiomers: A special type of isomers is
found in the pairs of structure that are mirror
images of each other.( L. glucose, D. glucose)
D.glyceraldhyde: in which OH group attached
to asymmetric C atom is towards the right.
L.glyceraldhyde: in which OH group attached
to asymmetric C atom is towards the left.
Chemistry of Carbohydrate
Stereochemical relations in
carbohydrates were explored by
Emil Fischer, who also devised a
way to represent these molecules.
 Fischer projection
Important Monosaccharides
H–C=O
H–C=O
H–C–OH
H–C–OH
HO–C–H
H–C–OH
H–C–OH
CH2OH
D-Glucose
HO–C–H
HO–C–H
H–C–OH
CH2OH
D-Galactose
H–C=O
H–C–OH
HO–C–H
H–C=O
HO–C–H
HO–C–H
H–C–OH
H–C–OH
H–C–OH
H–C–OH
CH2OH
CH2OH
D-Mannose
D-Fructose
If only one of several stereocentres in a molecule is diferent, such isomers are epimers.
H–C=O
H–C–OH
HO–C–H
H–C–OH
H–C–OH
CH2OH
In Fischer projection:
Chiral C farthest
away from highest
oxidized C has OH to
right
D-sugar.
b
a
In Haworth projection:
if C1-OH and
C5-CH2OH on same
side of ring = b,
if on different sides = a
(anders, alternative)
Cyclization of monosaccharide
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The simple chain formula fails to explain the many
reaction so,
Less than 1% of each of monosaccharides are found
in a ring form, in which the aldhyde or keton group
has reacted with an alcohol group in the same sugar.
Formation of the ring results in the creation of
anomeric carbon atomat C-1 of an aldose and on C-2
of a ketose
These structure are designated the α & β
configuration of the sugar.
If the remaining OH is on the right  α- sugar
If the remaining OH is on the left  β - sugar
Cyclization of monosaccharide
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Mutarotation: The cyclic α & β anomers
of sugar are in the equilibrium with
each other, and can be spontaneously
inter-converted in a process called
mutarotation.
Representation of sugar conformation:
(1)Fisher projection.
(2)Haworth projection
Cyclization of monosaccharide
H–C=O
b
H–C–OH
HO–C–H
H–C–OH
a
H–C–OH
CH2OH
D-glucose
open chain
Fischer
projection
D-glucose
ring form
a-D-glucose
Haworth projection
a-D-glucose
Chair conformation
anomeric C
Chemistry of Carbohydrate
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Reducing sugar: If the O2 of anomeric Catom is not attached to any other compound,
that sugar is a reducing sugar
A reducing sugar can react with the chemical
reagent (Ex. Bendict solution& fehling
solution) and reduce the reactive compound,
with the anomeric C- atom is oxidized.
Important Monosaccharides
Disaccharide & polysaccharide
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Disaccharides: These are formed by
condensation of 2 molecules of
monosaccharide by a glycosidic linkage.
oligosaccharides: contain from3 to
about 12 of monosaccharide units.
polysaccharides: contain more than 12
of monosaccharide units.
Disaccharides
Lactose
Gal-b-1-4-Glu
Maltose
Glu-a-1-4-Glu
Sucrose
Glu-a-1-b-1-Fru
polysaccharide
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Starch: It is the most important polysacharide. It is a
polyglucose, a-1-4 linked. Ther are two main
components:
amylose – linear, ca. 500 – 20 000 linked glucose
units
amylopectin – branched through a-1-6 bonds every
~25 AGU
glycogen : body polysachharide: similar to
amylopectin, higher branched
Cellulose: is composed of b-1-4 linked glucose units.
This bond cannot be cleaved by our digestive
enzymes. Important part of cell walls and dietary
fibre.
Polysaccharides – Starch, glycogen
a-1-6 branch point
in glycogen
Polysaccharides - Cellulose
Stability of cellulose is
increased through formation
of crystalline regions with
extensive hydrogen bonding
Complex carbohydrates
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CHO can also attached by glycosidic bonds to nonCHO structure (a glycone) (Ex. Purine and
pyrimidine as in nucleic acids, aromatic ring as those
found in steroid & bilirubin, proteins as
glycoproteins& glycosaminoglycans, and lipids as in
glycolipids) to form glycosides.
O- and N- glycosides: If the group on the non-CHO is
an OH group, the structure is an O- glycosides,
whereas If the group on the non-CHO is an NH2
group, the structure is an N- glycosides.
The Aldoses C3 – C6
CHO
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All
Altruists
Gladly
Make
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In
Gallon
Tanks
The Ketoses C3 – C6
Dihydroxyacetone
Related Aldose
+ ending “ulose“
Related Aldose
+ ending “ulose“
PFST
Glucose structure in solution
Pentoses and hexoses can adopt two ring structures: 5-membered (Furanoses, after furan),
and 6-membered (Pyranoses, after pyran).
Glucose is in equilibrium between two pyranose forms. At equilibrium, there is ca.
65 % b-D-Glucopyranose, ca. 35 % a-D-Glucopyranose, and <1 % of the open-chain form.
O
~ 35 %
Furan
O
~ 65 %
Pyran
<1%
Fructose structure in solution
Fructose adopts a furanose structure, preferring the a-anomer.
Other roles and modifications of carbohydrates
In addition to their role as fuel molecules, carbohydrates are important molecules as:
- building blocks of nucleic acids
- antigens (blood groups, cellular interaction through glycosylated surface proteins)
- glycosylation of proteins  quality control system for protein folding
- glycosylation also determines functional properties of proteins
- metabolic intermediates and specialised molecules
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