Carbohydrates Lecture Notes PDF

BASIC CHEMISTRY OF BIOMOLECULES Dr. Shaista Zafar  Biomolecules Any of numerous substances that are produced by cells and living organisms is called Biological molecule or Biomolecules They have a wide range of sizes and structures and perf...

BASIC CHEMISTRY OF BIOMOLECULES Dr. Shaista Zafar  Biomolecules Any of numerous substances that are produced by cells and living organisms is called Biological molecule or Biomolecules They have a wide range of sizes and structures and perform a vast array of functions. Small biomolecules are sugars, fatty acids, amino acids, nucleotides The major types of biomolecules (macromolecules) are: Carbohydrates Lipids, Nucleic acids Proteins and amino acids Vitamins Harmons Enzymes  CARBOHYDRATES Chemistry: A carbohydrate is a naturally occurring compound, or a derivative of such a compound, with the general chemical formula Cn(H2O)n, made up of molecules of carbon (C), hydrogen (H), and oxygen (O). Carbohydrates are the most widespread organic substances The chemical formula Cn(H2O)n, denotes some carbons (C) with some water molecules (H2O) attached—hence the word carbohydrate, which means “hydrated carbon.” Chemically carbohydrates are polyhydroxy aldehydes or ketones, their simple derivatives or their polymers. OR Chemically, carbohydrates are defined as “optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis” Sources of Carbohydrates We know carbohydrates are an important part of any human’s diet. Some common sources of carbohydrates are: Potatoes Maze Milk Popcorn Bread  Classification and nomenclature Carbohydrates in grains are classified based on their chemical structures or their digestibility when consumed by humans as food or by livestock as feed. Carbohydrates are divided into four types: Monosaccharides Disaccharides Oligosaccharides Polysaccharides. Monosaccharides consist of a simple sugar; that is, they have the chemical formula C6H12O6. Disaccharides are two simple sugars. Oligosaccharides are three to six monosaccharide units polysaccharides are more than six. Classification on the basis of functional group Carbohydrates are divided into 2 types on the basis of functional group If aldehyde group(CHO) present, aldoses e.g. glucose CH2OH If ketone group (C=O) present, it is ketoses e.g. fructose C=O CH2OH Classification on the basis number of carbon atoms Dihydroxyaceton Trioses contain 3C e.g. glyceralaldehyde Tetroses contain 4C e.g. erythrose Pentoses contain 5C e.g. Ribose Hexoses contain 6C e.g. glucose , fructose, galactose Heptoses contain 7C e.g. glucoheptose Monosaccharide carbohydrates are those carbohydrates that cannot be hydrolyzed further to give simpler units of polyhydroxy aldehyde or ketone. Glucose is also called aldohexose and dextrose Glucose  Disaccharide Composed of two units of sugars joined by O-glycosidic bonds OR Two molecules of a simple sugar that are linked to each other form a disaccharide, or double sugar. The disaccharide sucrose, or table sugar, consists of one molecule of glucose and one molecule of fructose Examples The most familiar sources of sucrose are sugar beets and cane sugar. Milk sugar, or lactose, and maltose. The molecules must be broken down into their respective monosaccharides. Oligosaccharides consist of three to six monosaccharide units, are rather infrequently found in natural sources, although a few plant derivatives have been identified. Polysaccharides (many sugars) represent most of the structural and energy- reserve carbohydrates found in nature Large molecules that may consist of as many as 10,000 monosaccharide units linked together, polysaccharides vary considerably in size, in structural complexity, and in sugar content Examples cellulose The starch found in plants and the glycogen found in animals also are complex glucose polysaccharides. Glycogen, which consists of branching chains of glucose molecules, is formed in the liver and muscles of higher animals and is stored as an energy source.  Polysaccharides These consist of more than two sugar monomer units. They are also known as glycans. They are of two types: 1. Homopolysaccharides: They are composed of only a same type of sugar unit. Based on the function they perform, further divided into two groups: Structural polysaccharides: They provide mechanical stability to cells, organs, and organisms. Examples: chitin and cellulose. Chitin is involved in the construction of a fungal cell wall, while cellulose is an important constituent of the diet for ruminants Storage polysaccharides: They serve as carbohydrate stores that release sugar monomers when required by the body. Examples starch, glycogen, and inulin. Starch stores energy for plants. In animals, it is catalyzed by the enzyme amylase (found in saliva) to fulfill the energy requirement. Glycogen is a polysaccharide food reserve of animals, bacteria, and fungi 2. Heteropolysaccharides Contain two or more different types of sugar units. It includes glycosaminoglycans like hyaluronic acid, heparan sulfate, keratan sulfate, and murein. These polysaccharides have diverse functions. Examples Heparin is an anticoagulant (prevents blood clotting, it’s also known as blood thinners) Hyaluronic acid is a shock absorber and lubricant, while peptidoglycans or mureins are present in the bacterial cell wall.  Chemical Reactions of Carbohydrates Alkylation Through a simple reaction, the -OH groups in carbohydrates react quickly with alkylating agents to produce ethers. They react with alkylating halide such as diazomethane or alkyl iodide and with benzyl halides for benzylation. Benzylation can be accomplished with the help of benzyl halides (Cl, Br, I), C6H5CH2X and a base like NaH or Ag2O Acylation Esters are formed when the -OH groups in carbohydrates combine with acylating substances such as acid halides (carboxylic acid chloride, sulfonyl chloride) or acid anhydrides. An acid anhydride is a molecule that is capable of forming an acidic solution in water. Reduction It is a nucleophilic addition reaction where the C=O group is reduced to alcohol by sodium borohydride. The resulting product is known as alditols. If the carbohydrate is an aldehyde, it is reduced to primary alcohol (methanol ethano), but if it is a ketose, it forms secondary alcohol. Oxidation Sugars readily undergo oxidation to produce carboxylic acids and hence are termed reducing sugars. Aldehydes are easier to oxidize because they have an open C=O bond. Carbohydrate C=O can be converted to carboxylic acids via oxidation. Hydrolysis Carbohydrates undergo hydrolysis to produce 𝜶 and 𝜷 isomers. The -OR bond at the anomeric carbon hydrolyses to form a -OH bond. Glycoside Formation ❑ Carbohydrates form glycosides when the anomeric hydroxyl group undergoes condensation with the hydroxyl group of another carbohydrate molecule, eliminating a water molecule.  Optical Activity of Carbohydrates Optical activity of a molecule is its ability to rotate a plane-polarized (light vibrating in one direction) light that passes through it. The molecules which rotate the plane of polarized light are known as optically active. The molecules in which a carbon atom is connected to all the four different atoms or groups is known as a chiral carbon or center Carbohydrates are generally optically active because they have one or more chiral carbon atoms. The molecules of carbohydrates are chiral because of the presence of many asymmetric carbon atoms. Not all carbohydrates are optically active. The monosaccharides generally are optically active and it is because they contain chiral carbon atoms Carbohydrates which possess one or more such carbon chiral centers are mostly optically active. The molecules which rotate the plane-polarized light to the left are called laevorotatory, denoted by L or (–). Example L(-)glucose The molecules which rotate the plane-polarized light to the right are called dextrorotatory, denoted by d (+) Examples d(+)-glucose Molecules having more than one chiral carbons can exist in meso forms. Example of a meso compound is tartaric acid, etc. A meso compound is a non- optical mixture of two optically active isomers.  BIOLOGICAL AND PHARMACEUTICAL IMPORTANCE OF CARBOHYDRATES Instant source of energy. Each gram of carbohydrates provides 4 calories. Normal blood glucose level Deficiency of carbohydrates cause serious problems e.g. hypoglycemia Excessive intake can cause Diabetes mellitus Sucrose is most widely used in preparation of simple Syrups Lactose, starch and gums are used as diluents and binders in manufacturing of tablets Liquid glucose is used in sugar coated tablets Starch and dextrin used in preparation of infants food Pectin used in anti-diarrheal drugs Dextrose used in sterile IV solutions Mucilage and lactulose used as laxative Sucralfate is used as antacid or diuretic Mannitol and sorbitol both are used as diuretics Agar is used ass nutrient media for bacterial/tissue culture Streptomycin is used in tuberculosis  References Wilson and Walker's Principles and Techniques of Biochemistry and Molecular Biology Lippincott Illustrated Reviews: Biochemistry Edition: 8