Carbohydrates

The most abundant organic molecules in nature contains (C,H,O)

with formula (CH2O)n where n > or = 3. They may be defined as aldehyde or ketone compounds with multiple hydroxy groups (OH).

Biological importance of carbohydrates

• Provide a significant fraction of the energy in the diet of most organisms

•Act as a storage form of energy

• Serve as structural components of many organisms (cell wall, exoskeleton of insects,…….

• Act as components of cell-membrane mediating intercellular communication

•Cell-surface antigens

•Associated with proteins and lipids

Classification of Carbohydrates: 1- Monosaccharides or simple sugars:

They are the simplest units of carbohydrates. They cannot give simpler forms on hydrolysis.

2- Disaccharide.

They yield 2 monosaccharide residues upon hydrolysis (Lactose yields 1 mol of glucose and 1 mol of galactose, maltose yields 2 mol of glucose, sucrose yields 1 mol fructose, 1 mol of glucose).

3- Oligosaccharides: Sugars composed of 3:10 monosaccharide residues united together by glycosidic linkages (HC-O-CH

4- Polysaccharides: They are carbohydrate polymers composed of more than 10 monosaccharide residues, attached by glycosidic linkages.

They are formed by polymerization of a number of molecules of simple sugars to form one.

Monosaccharides

Classified according to:

• The number of carbon atoms.

• Chemical nature of carbonyl group (aldoses or Ketoses).

Examples of monosaccharide:

1- Trioses (3 carbon atoms) e.g. Glyceraldehyde (aldose).

Dihydroxy acetone (ketose).

2- Tetroses (4 carbon atoms) e.g. Erythrose.

Erythrulose.

3- Pentoses (5 carbon atoms) e.g. Ribose, Xylose.

Ribulose, Xylulose

4- Hexoses (6 carbon atoms) e.g. Glucose, Galactose, Mannose.

Fructose.

• D-Glucose (Grape sugar) is present in all animal and plant cells, blood, fruits and honey. It is called dextrose because it is dextrorotatory.

• D- Galactose: is a component of lactose, agar, mucopolysaccharides and galactolipids.

• D- Mannose: is present in mucopolysaccharides and in some plant polysaccharides.

• D- Fructose (Fruit sugar): is present in fruits, honey and semen. It is called levulose because it is levorotatory. It is a very sweety sugar even more than sucrose.

Cyclic structure:

• In natural state, aldehyde & ketone groups exist in a condensed form as “hemiacetal” by combination with one of the alcoholic hydroxyl group

• In aldohexoses, the cyclic forms results from condensation between the aldehyde group on C1 and the OH group of C5 or C4. If condensation occurs with OH group of C5, we get a six-membered ring, forming a pyran ring, so the sugar described as pyranose.

• If condensation occurs with OH group of C4, we get a five membered ring forming a furan ring, so the sugar is described as furanose.

• Haworth suggested the ring structure like a ring. In the Haworth formula, the groups written above the ring correspond to the groups written to the left in the straight chain structure, and vice versa.

• In aldohexoses, C1 is is the anomeric C atom, in case of ketoses, the anomeric C atom is C2. According to the position of-OH in C1 in aldoses or C2 in ketoses, there are two forms (anomers):

• - form: in which -OH is to the right. - form: in which -OH is to the left.

Isomerism

• Monosaccharides are optically active compounds and can rotate plane of polarized light either to right or to left.

• Such a property is a characteristic of the substances having a asymmetric carbon, that has 4 different atoms or groups attached to the same carbon atom. Any compound having one or more asymmetric carbon atom shows 2 properties:

1-Stereoisomerism

2-Optical isomerism

1- Stereoisomerism:

• They are compounds that have the same structural formula but differ in the arrangement of groups (spatial configuration).

1. The presence of assymetric C atom allows the formation of isomers.

Enantiomers:

• They are “mirror images” of each other. They can rotate plane polarized light to the same extent but in opposite direction. The D-form of sugars are majority forms utilised in the body. The L-form is exerted in urine.

Epimers:

• These are isomers that differ in position of OH groups attached to a single C atom other than the anomeric C. e.g. galactose is an epimer of glucose at the 4 carbon.

Anomers (alpha and beta isomerism(:

• These are isomers that differ in position of OH group at the anomeric C atom (C1 in glucose). If the OH group is on the right, the sugar is -glucose, and if towards the left, it is -glucose.

D and L isomerism:

• When the OH group attached to the carbon atom which is next to the last CH2OH group is on the right, the sugar is described as D-sugar. If the OH group is on the left, it is described as L-sugar.

2- Optical isomerism:

• An isomer which rotates plane of the polarized light to the right, is designated as dextrorotatory (d or +) while its isomer which rotates plane of the polarized light to the left is designated as levorotatory (l or - ).

• A chiral molecule with the D configuration thus, can either be dextrorotatory [D(+)] or levorotatory [D(-)] indicating its structural relationship to the D or L glyceraldehyde but exhibiting different optical rotations.

• A mixture containing equal amounts of the 2 isomers, i.e. a mixture of the dextrorotatory and the levorotatory isomers of a chiral molecule is called a racemic.

Sugar derivatives:

• Several derivatives of monosaccharides, suc as acids, alcohols, amino sugars and methyl glycosides are found in the body.

Amino sugars:

• These are sugars in which the hydroxyl group of the second carbon is replaced by an amino group e.g. glucosamine (chitosamine) found in the shells of insects, reparin and hyaluronic acid and galactosamine (chondrosamine) found in chondroitin

Sugar alcohols:

• These are sugars produced from reduction of monosaccharides, e.g. glycerol from glyceraldehyde, ribitol from ribose, mannitol from mannose and sorbitol or inositol from glucose.

Sugar acids:

• These are sugars containing COOH group. There are 3 types:

1- Aldonic acids: If the aldehyde group is oxidized to COOH group e.g. gluconic acid from glucose and glyceric acid from glyceraldehyde. L-ascorbic acid is also an aldonic acid.

2- Uronic acids: When the last CH2OH group is oxidized to COOH group e.g. glucuronic acid from glucose and galacturonic acid from galactose.

Glucuronic acid is important in detoxication reactions, excretion of steriods and mucopolysaccharides formation.

3- Saccharic acids: When oxidation occurs to both the aldehyde

group and the last – CH2 OH group e.g. aldaric (glucaric or saccharic) acid from glucose and mucic acid from galactose.

Disaccharides

• Contain 2 monosaccharide units. They are classified into:

1- Reducing disaccharides: as maltose, lactose.

Maltose:

1. It is a reducing sugar due to the presence of one free aldehyde group, gives characteristic osazone.

2. It can be hydrolyzed by: 1- acid, 2- maltose enzyme.

3. It is a sweet sugar, very soluble in water.

4. It is formed of 2 molecules of -glucose united together by -1,4-glycosidic linkage.

Lactose (Milk sugar):

1. It is a reducing sugar, gives osazone which is sun shaped.

2. It can be hydrolyzed by: 1- acid, 2- lactase enzyme.

3. It is not fermented by backer’s yeast.

4. It is formed of one molecule of -glucose and another molecule of -galactose united together by -1,4 glycosidic linkage.

2- Non-Reducing disaccharides:

Sucrose (Cane sugar):

• It is formed by condensation of one molecule of -glucose with one molecule of -fructose through 1,2-glycosidic linkage.

• It is non reducing, very sweety, very soluble, not form osazone.

• It is the ordinary sugar used in diet, it occurs free in most fruits and vegetables.

• Sucrose is dextro-rotatory, and on hydrolysis gives a levo-rotatory mixture of equal amounts of glucose and fructose. This mixture is called Invert sugar.

• This is because the levo-rotation of fructose is more than the dextro-rotation of glucose.

• The enzyme responsible for this inversion of rotation from dextro to levo is called invertase, and is found in yeast. Hydrolysis of sucrose also occurs by sucrase enzyme.

Polysaccharides

• They are complex substances composed of more than 10 monosaccharide units. They are classified into:

1- Homogenous polysaccharides (Homopolysaccharides): On hydrolysis they yield the same type of monosaccharide units.

2- Heterogenous polysaccharides (Heteropolysaccharides): On hydrolysis they give monosaccharides and other compounds.

Starch:

• It is the storage form of carbohydratyes in plants.

• It is the main carbohydrates in our diet e.g. cereals, potatoes.

• It is non reducing as there is no free aldehyde or ketone.

• It is composed of -glucose units linked by 2 types of linkages -1,4 and -1,6 glycosidic linkages.

• The starch granule consists of 2 components, amylose (inner part) and amylopectin (outer part).

• Amylose: Forms less than 25% of the starch granule straight chain polymers, formed from -glucose units combined together by -1,4 glycosidic linkage.

• Amylopectin: Forms about 75% of starch granule, branched polymer 24 to 30 glucose residues combined together by 1,4 glycosidic linkage, but at the points of branches, the linkage is α-1,6 glycosidic.

• Starch on acid hydrolysis gives D-glucose units while enzymatic hydrolysis with amylase enzyme gives maltose.

Glycogen:

• The storage form of carbohydrates in animals mainly in liver and muscles.

• It is a branched chain polymer with 8-12 glucose units for chain. Its structure is similar to that of amylopectin of starch, i.e. formed of glucose units linked by 1,4-glycosidic linkages and 1,6-bonds at the branching points, but it contains more numerous branches than amylopectin.

• On hydrolysis, it gives -D-glucose units. It gives red colour with iodine, it is non-reducing sugar

:Cellulose

• It constitutes the walls of plant cells, so it is an example of structural polysaccharide.

• It is a linear polymer, composed of -glucose units linked together by -1,4 ghycosidic linkages (No branches in cellulose).

• It is not digested (as its linkage is -glucosidic linkage), so it has no nutritive value.

• It is important in the intestine as it forms the bulk of stools, stimulate peristalsis and prevents constipation. It is also a medium for intestinal flora which is important for vitamin B formation.

• It is insoluble and not hydrolyzed readily by dilute acids. In partial hydrolysis with acids, cellobiose is produced, while with complete hydrolysis, -glucose is obtained.

Inulin:

• It is found in the tubers and roots of fruits.

• Linear polymer, on hydrolysis it gives -D-fructose units.

• Non digestable, and if injected I.V., it is not metabolised.

:Dextrans

• These are polysaccharides produced by certain bacteria when allowed to grow on sugar media.

• They differ from dextrins in that they are formed from -glucose units linked together by 1,6-; 1,4 and 1,3-glycosidic linkages (highly branched).

• Due to their high viscosity, low osmotic pressure and slow disintegration, they are used as plasma substitute in case of haemorrhage to increase the volume of circulating blood.