Carbohydrate

Carbohydrate

Carbohydrates can be defined as polyhydroxyaldehyde or polyhydoxyketones and their derivatives of compounds which produce them on hydrolysis. The word ‘saccharides’ is derived from Greek language ‘sacckharon’, meaning sugar. The term sugar is applied to carbohydrates which are soluble in water and sweet to taste. Only lower carbohydrates have sweet taste whereas higher carbohydrates such as starch, cellulose and glycogen are tasteless.

Carbohydrates are the most abundant organic molecules found in the nature. These are biological molecule consisting of carbon (C), hydrogen(H) and oxygen(O) atoms, usually with a hydrogen-oxygen atom ratio of 2:1(as in water). They are primarily composed of the elements carbon, hydrogen and oxygen and ave the empirical formula [CH₂O]_n. However the carbohydrates which contains besides C, H and O have not this formula. The name carbohydrate literally means hydrates of carbon though there are several non-carbohydrate compounds which are hydrates of carbon i.e. acetic acid, lactic acid etc. Some complex carbohydrates polymers which are covalently attached to proteins or lipid are glycol-conjugates.

Functions Of Carbohydrate

1. They are the most abundant dietary source of energy (4.1kcal/gram) for all living organisms.
2. Carbohydrates are the structural components of many organisms. These include cellulose of plants, exo-skeleton (chitin) of insects, cell wall of microorganisms (n-acetyl glucosamine and n-acetyl muramic acid), ground substance of cartilages and bone i.e. synovial fluid contains hyaluronic acid which helps in lubrications.
3. Carbohydrate serves as the storage form of energy i.e. glycogen in animal and starch in plants) to meet the immediate energy demands.
4. Carbohydrate plays important role in defense mechanisms i.e. immunoglobins formed in response to the antigens in the body.
5. Glucose in indispensable for the maintenance of the integrity of nervous tissue (some central nervous system areas are able to use only glucose for energy production) and red blood cells.
6. Two sugars, ribose and deoxyribose, are part of the bearing structure, respectively of the RNA & DNA and obviously find themselves in the nucleotide structure as well.
7. They exert a protein saving action: if present in adequate amount in daily nourishment, the body does not utilize proteins for energy purpose, an anti-economic and ‘polluting’ fuel because it will need to eliminate nitrogen and sulfur present in some amino acids.
8. Carbohydrates are utilized as raw materials for several industries i.e. paper, plastics alcohols etc.
9. Carbohydrate as glyco-proteins and glyco-lipids participate in structure of cell membrane and cell function such as cell growth, adhesion and fertilization (i.e. follicles stimulating hormones(FSH), leutinizing hormones(LH), thyroid stimulating hormones(TSH), lacta-albumin; these all are glycol-protein. Lipopolysaccharides of bacterial(gram negative) cell wall, gangliosides, membrane lipids which contains carbohydrates moiety Sialic acid.
10. Blodd grouping is based on the carbohydrates molecule of blood (RBC) which is clinically very important during blood transfusion.
11. Carbohydrates are the precursors of many organic compounds i.e. fats, amino acids etc.
12. Every cell Carbohydrates on its surface which help to recognize other substance. In the gram negative bacteria, the outer membrane is made up of LPS, which is again composed of core-polysaccharide and O-antigens. Polysaccharides contain different sugar which confers cell specificity.

Classification Of Carbohydrate

Carbohydrate can be classified into following three groups;

1) Monosaccharide 2) Oligosaccharide 3) polysaccharide

1) Monosaccharide

Monosaccharide is the smallest group of carbohydrates and is often referred to as sugars. They cannot be further hydrolyzed into smaller units under mild conditions. They are compounds with 2-10 carbons per molecule. Monosaccharide containing 3 carbons is called triose. Similarly tetrose, pentose, hexose, heptones and nonoses for 4,5,6, 7 and 9 carbon atoms respectively. They are colorless crystalline solids that are freely soluble in water but insoluble in non-polar solvents. In an open-chain form, one of the carbon atoms is double bonded to an oxygen atom to from a carbonyl group; each of the other carbon atoms has a hydroxyl group. If the carbonyl group is at an end of te carbon chain, the monosaccharide is an aldose; if the carbonyl group is at any other position the monosaccharide is a ketose.

Some important monosaccharide

1)Trioses: Monosaccharide containing 3-carbon atoms are known as trioses. Trioses may be aldo-triose or keto-trioses.

• Aldotriose: Glyceraldehyde
• Ketotriose: Dihyroxyacetone

They form the metabolic intermediates.

2)Tetroses: Monosaccharide containing 4-carbon atoms are called tetroses.

• Aldotetrose: Erythrose
• Ketotetrose: Erythrulose

They also form the metabolic intermediates.

3) Pentose: Monosaccharide containing 5-carbon atoms are called pentose.

• Aldopentose Ribose, Arabinose
• Ketopentose Ribulose, Xylulose

4)Hexoses: Monosaccharide containing 6-carbon atoms are called hexoses.

1. Aldohexose: glucose, galactose, mannose
2. Ketohexoses: Fructose

2) Oligosaccharide

Oligosaccharides are the polymers of monosaccharide consisting of 2-10 monosaccharide units. Upon hydrolysis of oligosaccharide, 2-10 monosaccharide units are released. Oligosaccharides are composed of hexose, pentose or the mixture of hexose and pentoses. The monosaccharide units in oligosaccharides are joined by the covalent bond known as glycosidic bond or glycosidic linkage. During glycosidic linkage formation, if hydroxyl group at α-position is involved, it is called α-glycosidic linkage.

On the basis of number of monosaccharide units on the oligosaccharide, it is further classified as disaccharides, trisaccharides, tetrasaccharides and etc.

1)Diasaccharides

It consists of two monosacharides units joined by glycosidic linkage. Disaccharides may be reducing and non-reducing. Disaccharides may compose of same monosaccharides i.e. maltose or may composed of different monosaccharides i.e. sucrose or galactose.

• Maltose: It is also known as malt sugar. Maltose is composed of 2α-D glucose units held together by α[1,4] glycosidic bond. It is not found in free form in the body. It is produced during the course of digestion of starch by the enzyme amylase. It is also involved in the formation of sun-flower shaped osazone. It can be hydrolyzed by dilute acid or enzyme maltase to liberate 2 molecules of α-D glucose.
• Lactose: Lactose is more commonly known as milk sugar, it is disaccharide found in milk. Lactose is composed of β-D galactose and β-D glucose held together by β[1,4] glycosidic bond. The anomeric carbon of C-1 glucose is free hence lactose exhibit reducing properties and forms osazone. It is solely of animal origin and found in mammary gland. It is hydrolyzed by the intestinal enzymes lactase to glucose and galactose.
• Sucrose: Sucrose is the sugar of commerce and kitchen, mostly produced by sugarcane is made up of α-D glucose and β-D fructose. The two monosaccharides are held together by a glycosidic bond [α1 β1] between C-1 of α-glucose and C-2 of β-fructose. Sucrose is non-reducing sugar, as it contains no free anomeric carbon atom. It is the second sweetest sugar after fructose. The hydrolyzed mixture of sucrose, scontaining glucose and fructose is known as invert sugar.
• Cellulose: It is made up of two β-D glucose units and has β(1,4) glycosidic linkage. It is formed during the hydrolysis of cellulose. It is a reducing sugar and it is hydrolyzed by β-glycosidase and dilute acids.
• Mellibiose: It is a compound of α-D galactose and α-D glucose linked by α(1,6) glycosidic linkage. It is a reducing sugar.
• Trehalose: It is similar to maltose except that it has an α(1,6) glycosidic linkage. It is a non reducing sugar and is a major constituent of circulation fluid of insects, in which it serves as an energy storage compound.
• Isomaltose: It is similar to maltose except that it has an α(1,6) glycosidic linkage.

2)Trisaccharides

Trisaccharides are composed of 3-manosaccharides units. The most commonly occurring .Trisaccharides are raffinose found in sugar beets. It is composed of galactose, glucose and fructose.

3)Tetrasaccharides

Tetraccharides are composed of 4 monosaccharide units. Stachyose is composed of 2 molecules of galactose, 1 molecule of glucose and fructose unit each. It is non reducing sugar and partially fermented by yeast.

4)Pentasaccharides: It has five monosaccharides units i.e. Verbascose.

2)Polysachharides

Polysaccharides consist of repeating unit of monosaccharide or their derivatives held by glycosidic linkage. Most carbohydrates found in nature occur as polysaccharide.They are primarily concerned with two important functions i.e. structural and storage of energy. Polysaccharides are of two types:

1)Homosaccharides: These are the polysaccharides, which on hydrolysis yield only a single type of monosaccharide. They are named according to the nature of monosaccharide units.

e.g.

Glucans [polymer of galactose]: starch, glycogen

Galactans [polymer of mannose]: Agar, pectin.

Mannans [ polymer of mannose]: Yeast mannan

Fructans [polymer of fructose]: Insulin.

• Starch: Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals, including humans. Starch id found in cereals, roots, tubers, vegetables etc. Starch is a homopolymer composed of D-glucose units held by α-glycosidic bonds. Starch consists of two polysaccharides components i.e. water soluble amylose [15-20%] and water insoluble amylopectin [80-85%].
• Glycogen: Glycogen is the carbohydrate reserve in animals, hence often referred to as animal starch. It is present in high concentration in liver, followed by muscle, brain etc. Glucogen is also found in yeast, fungi and parasites. Branching in glycogen is abundant e. each branch per 8-10 glucose residues. The linkage is α(1,4) glycosidic linkage and during branching it forms α(1,6) glycosidic linkage. Glucogen plays an important role to maintain blood glucose level either by its hydrolysis or synthesis.

2)Heterosaccharide:When polysaccharides are composed of different types of sugars of their derivatives, they are referred to as heteropolysaccharides. Heteropolysaccharides on hydrolysis yields a mixture of few monosaccharide or their derivatives.

e.g. Peptidoglycan, hyaluronic, chondroitin sulfate, heparin, dermatan sulfate etc.

• Heparin:It is composed of N-acetyl-6-sulfonic glucosamine and sulfate esters of glucuronic acid held byα(1,4) ester linkage. It is an anti-coagulant that occurs in blood, lung, liver, kidney, spleen. It prevents blood clotting by inhibiting convertion and eliminating the effect of thrombin on fibrinogen.
• Dermatan sulphate:It is composed of L-iduronic acid and N-acetyl galactosamine 4-sulfate. It is found in skin, blood vessels and heart valves.

Bibliography

• Nelson DL and Cox MM (2004). Lehininger Principles of Biochemistry 5^th Freeman Publication.
• Stever I (1995). Biochemistry, 4^th Wall Freeman Company, New York.
• Creuger and Creuger A (2000). Biotechnology, A textbook of industrial Microbiology. Sinaeur Associates.