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Carbohydrates and Carbohydrate metabolism (Chemistry of Carbohydrate )
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 )
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
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
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)
(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
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
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
–C=O
–C–OH
–C–H
–C–OH
–C–OH
CH2OH
H
H
HO
H
H
D-Glucose D-Galactose
–C=O
–C–OH
–C–H
–C–H
–C–OH
CH2OH
H
H
HO
HO
H
–C=O
–C–H
–C–H
–C–OH
–C–OH
CH2OH
H
HO
HO
H
H
D-Mannose
If only one of several stereocentres in a molecule is diferent, such isomers are epimers.
–C=O
–C–OH
–C–H
–C–OH
–C–OH
CH2OH
H
H
HO
H
H
In Fischer projection:Chiral C farthestaway from highest oxidized C has OH to right D-sugar.
In Haworth projection:if C1-OH andC5-CH2OH on same side of ring = , if on different sides = (anders, alternative)
–C–OH
–C=O
–C–H
–C–OH
–C–OH
CH2OH
H
H
HO
H
H
D-Fructose
Cyclization of monosaccharide 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
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–C=O
–C–OH
–C–H
–C–OH
–C–OH
CH2OH
H
H
HO
H
H
D-glucoseopen chain
D-glucosering form
-D-glucoseHaworth projection
-D-glucoseChair conformation
anomeric C
Fischerprojection
Chemistry of Carbohydrate
Reducing sugar: If the O2 of anomeric C- atom 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
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
Maltose
Sucrose
Gal--1-4-Glu
Glu--1-4-Glu
Glu--1--1-Fru
polysaccharidepolysaccharide 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
-1-6 branch pointin glycogen
Polysaccharides - Cellulose
Stability of cellulose is increased through formation of crystalline regions with extensive hydrogen bonding
Complex carbohydrates CHO can also attached by glycosidic bonds to
non-CHO 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 – C6CHO
E T
RAXL
All Altruists Gladly Make Gum In Gallon Tanks
The Ketoses C3 – C6
Related Aldose+ ending “ulose“
Related Aldose+ ending “ulose“
P F S T
Dihydroxyacetone
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 % -D-Glucopyranose, ca. 35 % -D-Glucopyranose, and <1 % of the open-chain form.
O
Furan
O
Pyran ~ 65 %
~ 35 %
< 1 %
Fructose structure in solution
Fructose adopts a furanose structure, preferring the -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