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CHAPTER 1
- Structure and Biological importance of carbohydrates Monosaccharides (Glucose, fructose, ribose, deoxyribose) Disaccharides (Lactose, Maltose, Sucrose, Trehalose) Polysaccharides(Homopolysaccharides–Starch, Glycogen, Chitin) Heteropolysaccharides – Heparin, Hyaluronic acid, Keratan sulphate).
- Lipids saturated (Palmitic acid), unsaturated Fatty acids (Omega 3 and omega 6 fatty acid), Tri-acylglycerols, Phospholipids, Glycolipids and Steroids)
- Proteins Structure, Classification and General Properties of a-amino acids; Essential and non-essential amino acids, Levels of organization in proteins; Simple and conjugate proteins. derived Proteins, and Peptide linkages 1. Structure and biological importance of carbohydrates Carbohydrates are the most abundant organic molecules in nature. They are primarily composed of the elements carbon , hydrogen and oxygen. The name carbohydrate literally means ‘ hydrates of carbon ’. Some of the carbohydrates possess the empirical formula Cn(H 2 O)n where n ≤3, satisfying that these carbohydrates are in fact hydrates of carbon. However, some of the carbohydrates (e.g. rhamnohexose, C 6 H 12 O 5 ; deoxyribose, C 5 H 10 O 4 ) do not satisfy the general formula. Hence carbohydrates cannot be always considered as hydrates of carbon. Similarly , there are several non-carbohydrate compounds (e.g. acetic acid, C 2 H 4 O 2 ; lactic acid, C 3 H 6 O 3 ) which also appear as hydrates of carbon. Carbohydrates are widely distributed in both plant and animal tissues. They occur as food reserves in the storage organs of plants and animals, as they are the important sources of energy required for the various metabolic activities of the living organisms. In plants they are formed by the process of photosynthesis where carbon dioxide of the atmosphere and water combine to produce the sugar molecule. Plants use carbohydrates as the precursor for the synthesis of proteins, lipids and other organic compounds. Animals obtain their carbohydrates from plants.
Definition Carbohydrates are defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis. General functions of carbohydrates
- They are the most abundant dietary source of energy (4 Cal/g) for all organisms.
- Carbohydrates are precursors for many organic compounds (fats, amino acids). 3.Ribose and deoxyribose sugars forms the structural frame of the genetic material, RNA and DNA.
- Carbohydrates (as glycoproteins and glycolipids) participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization.
- They are structural components of many organisms. These include the fibre (cellulose) of plants, exoskeleton of some insects and the cell wall of microorganisms.
- Carbohydrates also serve as the storage form of energy (glycogen often called animal starch) to meet the immediate energy demands in animals. Physical Properties of Carbohydrates Stereoisomers;- The presence of asymmetric carbon atoms in a compound give rise to the formation of isomers of that compound. Such compound which are identical in composition and differs only in spatial configuration are called “stereo isomers’’. Stereoisomers are mirror images of each other. For example, glucose can exist in two forms as D-glucose and L-glucose are mirror images of each other.
Alpha and beta anomers Alpha(α) and beta(ᵦ) differ only in the direction that - H and - OH groups on carbon 1 in the cyclical form. These forms do not differ in their chemical properties such isomers are known as anomers. Classification Carbohydrates are often referred to as saccharides (Greek: sakcharon–sugar). They are broadly classified into three major groups based on the number of sugar units as —
- monosaccharides,
- disaccharides and
- polysaccharides Carbohydrates Monosaccharides Disaccharides Polysaccharides Oligosaccharides e.g.; e.g.; e.g.; Glucose Maltose Starch Fructose Sucrose Glycogen Galactose Lactose
Monosaccharides (Greek : mono-one, sakcharon - sugar) Monosaccharide s are the simplest group of carbohydrates with the general formula Cn(H 2 O)n, and are often referred to as simple sugars, as they cannot be further hydrolysed. Classification of monosaccharides based on the number of carbon atoms and the fictional groups They are also classified based on the number of carbon atoms present in the monosaccharides as Based on the number of carbon atoms, the monosaccharides are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). also subdivided as aldoses and ketoses depending upon the presence of aldehyde or ketone groups as shown in the table The monosaccharides are divided into different categories, based on the functional group and the number of carbon atoms as Aldoses : When the functional group in monosaccharides is an aldehyde group they are known as aldoses e.g. glyceraldehyde, glucose. Ketoses : When the functional group is a keto group, they are referred to as ketoses e.g. dihydroxyacetone, fructose
- Aldotriose eg: Glycerose 1. Ketotriose :eg.Dihydroxy acetone
- Aldotetrose eg : Erythrose 2. Ketotetrose : eg : Erythrulose
- Aldopentose eg : Ribose 3. Ketopentose : eg. Ribulose
- Aldohexose eg: Glucose 4. Ketohexose : eg. Fructose Aldoses Triose- C 3 H 6 O 3 Tetrose- C 4 H 8 O 4 Pentose-C 5 H 10 O 5 Hexoses
Ribose Ribose is a simple sugar a monosaccharide with molecular formula C 5 H 10 O 5. Ribose was discovered by Emil Fischer in the year 1891. It is made up of five carbon atoms with aldehyde functional group in the linear form (Fischer projection formula) thus referred as an aldopentose. Ribose is the most important monosaccharide in the life of living organisms after glucose. It abundantly occurs in nucleic acids of living organisms. The structural formula of ribose can be represented in two forms; Linear chain and
Closed ring form. They are readily interconvertible. The closed ring form includes five (furanose ring) or six-membered ring (pyranose ring). Properties
- It is sweet in taste, but less sweet than glucose
- It is also a polar compound, thus can readily dissolve in water
- It cannot by hydrolysed further to smaller units. Functions
- Ribose is the most important monosaccharide in the life of living organisms after glucose.
- Ribose is only rarely present in nature in its free form. The combination of ribose sugar and nitrogenous base forms ribonucleoside.
- Ribose is a component of nucleic acids and nucleotides, the structural units of RNA
- Ribose sugar also plays an important role in the synthesis of DNA.
- It is a component of nucleotides like NADH, NADPH, that act as co- enzymes in various biochemical reactions.
- It is a component of ATP, the main energy currency of living cells. Deoxyribose Deoxyribose is a simple pentose sugar with molecular formula C 5 H 10 O4. It was discovered by Phoebus Levene in 1929. It is an aldopentose sugar with an aldehyde group attached to it. It is a deoxy sugar, derived from ribose by the loss of one oxygen atom from the 2’ hydroxyl group. Thus, it is also called 2-deoxyribose. The only difference between the deoxyribose and ribose is the presence or absence of a hydroxyl group at the second carbon of the molecule.
Structure Glucose can exist as a linear molecule or as a five or six-membered ring. Functions
- It is the source of energy in cell function.
- It is found in fruits and honey and is the major free sugar circulating in the blood of higher animals.
- Molecules of starch, the major energy-reserve carbohydrate of plants, consist of thousands of linear units of glucose.
- It is also a major component of disaccharides, lactose maltose and sucrose
- The cellulose, structural component of cell wall is also a made up of linear units of D-glucose. Fructose Fructose is a is a monosaccharide and a ketohexose (made up of 6-carbon compound with ketone group on C-2, and has molecular formula C 6 H 12 O 6. Fructose is also called “fruit sugar,” since it is more abundant in fruits. It is the sweetest of all the sugars. Naturally occurring fructose is laevorotatory thus it is also known as laevulose It is also found in honey, sugar beets sugar cane and vegetables. it is often bonded to glucose to form the disaccharide sucrose. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in
- It is found in free form in the seminal fluids and provide energy for the motility of sperms.
Structure The open chain and furanose forms of fructose is shown below. Disaccharides Sugars that yield two molecules of same or different monosaccharides when hydrolyzed are known as a disaccharide. The disaccharides are formed when two monosaccharides undergo condensation reaction with a loss of molecule of water by the formation of glycosidic bond. The linkage between two monosaccharides is called a glycosidic linkage. The general formula is CnH2nOn−1. Examples :
- Lactose → Galactose + Glucose (reducing sugar)
- Maltose → Glucose + Glucose (reducing sugar)
- Sucrose → Glucose + Fructose (non-reducing sugar)
- Trehalose→ Glucose + glucose (non-reducing sugar) The disaccharides are of two types 1. Reducing disaccharides with free aldehyde or keto group e.g., maltose, lactose. (Disaccharides with free hemiacetal or hemiketal groups break to form aldehyde or ketone group that can reduce Fehling’s solution )
Sucrose Sucrose or “table sugar” is also called as “cane sugar” or as it can be obtained from sugar cane. It is also found in sugar cane, beet root, pine apple, honey, carrot and ripe fruits. It is a disaccharide with molecular formula C 12 H 22 O11. Sucrose consists of one molecule of α D- glucose and one molecule of ᵦ D- fructose. In sucrose the glucose is present in pyranose form and fructose is present in furanose form. The glycosidic bond is formed between the C-1 of α D - glucose and C-4 of ᵦ D-fructose. The linkage between these molecules are formed between the aldehyde group of glucose and the ketone group of fructose. Thus, both the potential aldehyde group of glucose and the ketone group of fructose are blocked in the linkage and sucrose has no free reducing group, thus they are non-reducing sugar. Trehalose Trehalose is a nonreducing disaccharide in which the two α D - glucose units are linked in an α,α-1,1-glycosidic linkage. Similar to sucrose, trehalose is non reducing sugar. Trehalose is present in a wide variety of organisms,
including bacteria, yeast, fungi, insects, invertebrates, and lower and higher plants, where it serves as a source of energy. It is main constituent of circulating fluid (heamolymph) in insects where it serves in energy storage. Polysaccharides (Greek: poly-many) Polysaccharides are polymers of monosaccharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. The polysaccharides are of two types – homopolysaccharides and heteropolysaccharides Homopolysaccharides These on hydrolysis yield same type of monosaccharide units. They are named based on the nature of the monosaccharide. E.g. starch, glycogen, cellulose, inulin, pectin and hemicellulose yield only glucose on hydrolysis.
Chitin Chitin is composed of N-acetyl D-glucosamine units held together by ᵦ 1 - 4 glycosidic bonds. It is a structural polysaccharide found in the exoskeleton of some invertebrates e.g. insects, crustaceans. Heteropolysaccharides These on hydrolysis yield a mixture of different types of monosaccharides. The heteropolysaccharides situated in extra cellular matrix are called as mucopolysaccharides. They are made up of repeating units of sugar derivatives, namely amino sugars and uronic acids. These are more commonly known as glycosaminoglycans (GAG). (eg). hyaluronic acid, heparin, keratan sulphate and chondroitin sulphate. Hyaluronic acid Hyaluronic acid also called hyaluronan, is a non-sulphate glycosaminoglycan(GAG ), distributed widely throughout connective, epithelial, and neural tissues. Hyaluronic acid is also a major component of
skin, cartilage and synovial fluid of the joints (where it serves as a lubricant and shock absorbent in joints) and vitreous humour of eyes. It also forms a gel around the ovum. Hyaluronic acid is made up of repeating units of disaccharides, which are composed of D-glucuronic acid and N-acetyl-D-glucosamine, linked via alternating β-(1→4) and β-(1→3) glycosidic bonds. Heparin Heparin is an anticoagulant (prevents blood clotting) which inhibits thrombosis by interacting with antithrombin III. It is composed of alternating units of N- sulfo D-glucosamine 6-sulfate and glucuronate 2-sulfate. It occurs in blood, lung, liver, kidney, spleen etc. Keratan sulphate Keratan sulphate also known as kerato-sulphate is a structural polysaccharide consists of N-acetyl glucosamine and galactose. It is a β-1,3-linked poly-N- acetyllactosamine, with alternating units of D-galactosamine and N- acetylglucosamine 6 - sulfate. It is found in the cornea (keeps cornea transparent), cartilage, brain and bone.
- Define polysaccharides and describe the structure of 3 heteropolysaccharides.
- Write any 2 examples for the following (a) Aldohexose (b) common pentose sugar (c) disaccharides (d) Homopolysaccharides (e) heteropolysaccharides (f) Storage polysaccharides
- Write Short notes on (a) Epimers, (b) Monosaccharides, (c) Glycosidic bond, (d) Anomers, (e) (f)heteropolysaccharides (g) homopolysaccharides (h) stereoisomers
- Mention the functions of (a) Glucose (b) ribose (c) deoxy ribose (d) maltose/sucrose/ trehalose/lactose (e)Starch (f) glycogen (g) Chitin (h) Hyaluronic acid/ heparin/ keratin sulphate Alpha and beta structures are isomers with same molecular formula but differ only in the position of their - OH (hydroxyl) and - H (hydrogen) groups on carbon 1 (also referred as the anomeric carbon). Beta structures have - OH group above the ring. On the other hand, Alpha structure will have its - OH attached below the ring.
GLYCOCONJUGATES Polysaccharides and oligosaccharides also act as information carrier molecules. Such informational carbohydrates are covalently joined to a protein or lipid to form glycoconjugates. Functions:
- Glycoconjugates act as information carrier molecules.
- They serve as destination labels for some proteins
- They serve as mediators of specific cell-cell interaction
- They can act as mediators for interaction between cell and ECM
- They form Glycocalyx (carbohydrate layer formed by specific chains attached to components of the plasma membrane).
- Specific carbohydrate-containing molecules from the Glycocalyx helps in
- Cell-cell recognition
- Adhesion
- Cell migration during development
- Blood clotting
- Immune response
- Wound healing Three main types of Glycoconjugates:
- Proteoglycans
- Glycoproteins
- Glycolipids and lipopolysaccharides (LPS) Proteoglycans: In proteoglycans, one or more sulphated glycosaminoglycan chains are joined covalently to a membrane protein or a secreted protein. They are macromolecules of cell surface and extracellular matrix. Glycan moiety commonly forms the greater fraction of the mass. In proteoglycan, glycan forms the main part of biological activity. Glycan part provides provisions for hydrogen bonding due to the presence of many - OH groups. Glycan part provides electrostatic interactions (due to presence of sulfate groups in glycosaminoglycans) with other preoteins of ECM. Proteoglycans are a major component of connective tissue such as cartilage.
