Carbohydrate Chemistry

Carbohydrates are polyhydroxy aldehydes or ketones.

They are represented with general formulae Cn(H2O)n.

They are the chief source of energy in human body. 50-60% of daily energy requirment is derived from carbohydrate oxidation.

Carbohydrates are classified according to the hydrolysis products into four main groups as follows:

I- Monosaccharides: contain one basic sugar unit.

II- Disaccharides: contain 2 monosaccharide units per molecule.

III- Oligosaccharides: contain from 3 to 10 monosaccharide units per molecule.

IV- Polysaccharides: contain more than 10 monosaccharide units per molecule.

 

I- MONOSACCHARIDES Monosaccharides are the simplest sugars, they

are crystalline water soluble sweet carbohydrates.

Monosaccharides are classified according to the presence of aldehyde or ketone group into:

aldoses. ketoses.Each class is further subclassified according to

the number of carbons in the molecule into trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons) and hexoses (6 carbons).

D-Glyceraldehyde

H – C = O

H – C – OH

CH2OH

Aldotrioses

H – C = O

H – C – OH

H – C – OH CH2OH

D-Erythrose

H – C = O

H – C – OH

H – C – OH

H – C – OH CH2OH

D-Ribose

H – C = O

H – C – OH

HO – C – H

H – C – OH CH2OH

D-Xylose

H – C = O

H – C – OH

HO – C – H

H – C – OH H – C – OH CH2OH

D-Glucose

H – C = O

HO – C – H

HO – C – H

H – C – OH H – C – OH CH2OH

D-Mannose

H – C = O

H – C – OH

HO – C – H

HO – C – H H – C – OH CH2OH

D-Galactose

CH2OH

C = O

H – C – OH

CH2OH

D-Erythrulose

CH2OH

C = O

HO – C – H H – C – OH CH2OH D-Xylulose

CH2OH

C = O

H – C – OH H – C – OH CH2OH D-Ribulose

CH2OH

C = O

HO – C – H H – C – OH H – C – OH CH2OH D-Fructose

CH2OH

C = O

HO – C – H H – C – OH H – C – OH

H – C – OH CH2OH D-Sedoheptulose

Dihydroxyacetone

CH2OH

C = O

CH2OH

Characters of Monosaccharides1-Stereochemical Isomers of Monosaccharides:

Isomerism: Isomers are compounds having the same molecular formula i.e. same number of carbon, hydrogen , oxygen ,etc atoms per molecule, but different structural or steric formulae.

Asymmetric carbon atoms are those attached to four different groups. All monosaccharides contain one or more asymmetric carbon atoms except dihydroxyacetone.

 

2-Optical Activity

Plane Polarized Light (PPL): It is light waves vibrating in one plane. It is obtained by passing ordinary light (vibrating in all planes) through a prism of pure CaCO3 ( Nicol prism) , which reflects all vibrations except those which are parallel to the base of the prism.

W

α

Unpolarizedlight Polarizer Polarized light

Sample tube containing optically active compound

Analyzer Observer

Light source

Polarimeter

Optical Activity: It is the ability of a substance to rotate the plane polarized light.

If the rotation is to the right (clockwise), it is termed dextrorotation.For example, D-glucose which is dextrorotatory.

If the rotation is to the left (anti-clockwise), it is termed levorotation. For example, D-fructose which is levorotatory.

Optical activity depends on the presence of at least one asymmetric carbon atom in the molecule.

Accordingly, all monosaccharides are optically active except, dihydroxyacetone (has no asymmetric carbon atoms).

Specific Rotation:

Optical activity can be measured by a polarimeter.

Each substance has its specific rotation, which is the degree of rotation measured under specific conditions e.g. D-glucose (+52.5°) and D-fructose (-92.3°).

3-Ring Structures of MonosaccharidesFor example, in solution, the functional aldehyde

group of glucose combines with hydroxyl group of 5th carbon atom. As a result a 6 numbered heterocyclic pyranose ring structure containing 5 carbons and one oxygen is formed. Similarly, a 5 numbered furanose ring structure is formed from fructose when its keto group combines with hydroxyl group on 5th carbon atom.

Furanose

O

13 2

45

Haworth FormulaPyranose

O

13 2

4

Haworth Formula

-C-C--C-

-C-C-

1

4

O

-C-C--C--C

-C--C-

1

5

O

In either case, a new asymmetric carbon is created by the reaction and we refer to the carbon as the anomeric carbon and the two possible configurations as anomers.

Cyclization creates a carbon with two possible orientations of the hydroxyl around it (-form and β-form). The -form (-OH is to the right of the anomeric carbon) and the β- form (-OH is to the left of the anomeric carbon)

-D-Fructofuranose

H

H

HOH2C

H

OH

OH

CH2OH1

OH

6 O

2

4 3

5

-D-Fructopyranose

CH2OH

OHOH

OH

OH

H

H

H

H

1

H

O

2

4 3

5

6

Mutarotation

It is the spontaneous change in specific rotation of an optically active substance when a freshly prepared solution is left to stand.

When or β forms of D-glucose are dissolved in water, the optical rotation of each gradually changes with time and reaches a final value of +52.5°. This change is called mutarotation , which is due to the formation of an equilibrium mixture consisting of about one third of -D-glucose (+110°) and two thirds of β-D-glucose (+17.5°) at 20°C.

 

β-D-Glucose

+(17.5°)

α-D-Glucose

+(110°)

D-Glucose

4-Reducing power:

All monosaccharides are reducing agents in vitro.

They reduce metallic ions in alkaline media.

Cupric ions of Fehling or Benedicts solutions are reduced to cuprous compound in alkaline media.

The reducing property of sugars is strictly related to existence of free carbonyl group.

Monosaccharide Derivatives1- Sugar acids:a- Aldonic acids:The aldehyde group of aldoses is oxidised to form

the corresponding aldonic acid. Glucose is oxidised to form gluconic acid

b- Uronic acids :The primary alcohol group of monosaccharides is

oxidised to form the corresponding uronic acid. Glucose is oxidised to form glucuronic acid (GlcUA).

c- Aldaric acids:These are monosaccharides in which both the

aldehyde and primary alcohol groups are oxidised to form the corresponding aldaric acid e.g. glucose gives glucaric acid.

 

2- Sugar Alcohols:These are sugars in which the carbonyl group

is reduced to alcohol group. Sorbitol is the alcohol of glucose, Dulcitol is the alcohol of galactose and mannitol is the alcohol of mannose.

CH2OH

C = O

HO – C – H H – C – OH H – C – OH CH2OH

D-Fructose

CH2OH

H – C – OH

HO – C – H H – C – OH H – C – OH CH2OH

D-Sorbitol

CH2OH

HO – C - H

HO – C – H H – C – OH H – C – OH CH2OH

D-Mannitol

+

Reduction

3- Deoxysugars :These are sugars in which an –OH group is

replaced by a hydrogen atom e.g. 2-deoxy ribose: It is present in the structure of DNA.

4- Aminosugars :These are sugars in which an amino group (NH2)

replaces the –OH group on the second carbon e.g. glucosamine (GluN

 5- Aminosugar acids :These are formed by the addition of acids to

aminosugars. Addition of pyruvic acid to mannosamine gives neuraminic acid.

   

II- DISACCHARIDESDisaccharides consist of two monosaccharides united

together by glycosidic linkage. If the glycosidic linkage involves the carbonyl group

of both sugars (e.g. sucrose) the resulting disaccharide is non-reducing.

On the other hand, if the glycosidic linkage involves the carbonyl group of one of its two sugars (e.g. lactose and maltose ) the resulting disaccharide is reducing.

A- Reducing disaccharidesThese are disaccharides which have a free anomeric

carbon in the second sugar unit:1. Maltose ( Malt sugar): It is formed of two molecules of glucose united by

glycosidic linkage. 3. Cellobiose : It is similar to maltose but the linkage is β-glycosidic.

It results from the hydrolysis of cellulose.4. Lactose ( Milk sugar): It is formed of galactose and glucose united by β

glycosidic linkage.  

B- Non –Reducing DisaccharidesSucrose (Cane sugar) (Table sugar):It is present in plants as sugar cane and beets. It is formed of fructose and glucose united by β

glycosidic linkage Both anomeric carbons are involved in the linkage, so sucrose is non-reducing.

Sucrose is dextrorotatory (+66.5°), and on hydrolysis it gives a levorotatory mixture of equal amounts of D-glucose (+52.5°) and D-fructose (-92.3°).

This mixture is termed invert sugar due to the inversion of the rotation of the solution from dextro to levo, so the enzyme is termed invertase or sucrase.

 

III- OLIGOSACCHARIDESThese are polymers of 3-10 monosaccharide

units.

They are present as a constituent of many types of glycolipids and glycoproteins e.g. oligosaccharides which are constituent of ABO blood group substance, immunoglobulins and glycolipids and glycoproteins of cell membranes.

IV- POLYSACCHARIDESPolysaccharides are composed of more than

10 monosaccharide units linked by glycosidic bonds.

Since the condensation of the monosaccharide units involves the carbonyl groups of the sugars, leaving only one free carbonyl group at the end of a big molecule, polysaccharides are nonreducing.

Polysaccharides include homo-polysaccharides and heteropolysaccharides.

A- HomopolysaccharidesThese are polysaccharides which are entirely made up

of only one type of monosaccharide units. 1- Starch :Starch is the chief storage form of carbohydrates in

chlorophyll – containing plants. It is present in large amounts in cereals (rice and wheat), tubers (potatoes and sweet potatoes) and legumes (beans).

2- Dextrins:They are produced during the hydrolysis of starch by

salivary or pancreatic amylase.   

3- Glycogen:Glycogen is the storage form of carbohydrates in

animals (animal starch). It is mainly present in skeletal muscles and liver.

It is a highly branched homopolysaccharide.

4- Cellulose:Cellulose forms the principal part of the cell wall of

plants. It is formed of a long non-branched chain of β-D-

glucose units connected together by β1,4-glucosidic linkage.

The presence of cellulose in diet is important as it increases the bulk of food, which stimulates intestinal contractions and prevents constipation.

B – HeteropolysaccharidesThese are polysaccharides which are formed of

more than one type of monosaccharide unit.

They include glycosaminoglycans (GAGs) formly called mucopolysaccharide.

Glycosaminoglycans (GAGs)heteropolysaccharides composed of repeating

disaccharide units, usually made up of an amino sugar and a uronic acid.

Glycosaminoglycans are classified into:

I- Sulfate free glycosaminoglycans: e.g. hyaluronic Acid-[(D-glucuronic acid)—(N-acetyl-D-glucosamine)]n

II- Sulfate containing glycosaminoglycans: e.g. chondroitin sulphate, keratan sulphate, dermatan sulphate, heparin and heparan sulphate.

Functions and Importance of GAGs and proteoglycans:

GAGs present in proteoglycans are polyanions (highly negatively charged due to the presence of the carboxyl and /or sulfate groups) and can bind greatly with cations like Na+ and K+, which attract water by osmotic pressure into the extracellular matrix producing its swelling.

Both characters offers viscid and resilience capability of intracellular ground substance, synovial fluid of joints, and vitreous humor of the eye globe.

Glycoproteins

They are proteins to which oligosaccharide chains are covalently bound. The carbohydrate contents of glycoproteins are less relative to proteoglycans and deficient of uronic acids and the repeating disaccharide units of proteoglycans.

ABO blood group antigens

The human blood groups A, B, AB, and O depend on the oligosaccharide part of the a glycoprotein or a glycolipid on the surface of erythrocyte cells.