Carbohydrate Chemistry Lecture Notes PDF

Structure and Function Lecture-3 Topic title: Carbohydrate Chemistry Department of Biochemistry Khalid, VD  Carbohydrates Polyhydroxyalcohols with an aldehyde or keto group. Represented with general formulae Cn(H2O)n Called as...

Structure and Function Lecture-3 Topic title: Carbohydrate Chemistry Department of Biochemistry Khalid, VD  Carbohydrates Polyhydroxyalcohols with an aldehyde or keto group. Represented with general formulae Cn(H2O)n Called as hydrates of carbons. Biological molecules made up of carbon, hydrogen, and oxygen This composition gives carbohydrates their name: they are made up of carbon (carbo-) plus water (-hydrate). Khalid, VD  Most abundant molecule in nature. Widely distributed in plant and animals. They have a wide range of functions, which includes :- Providing a significant fraction of the dietary calories for most organisms. Storage form of energy in the body. Serving as cell membrane components that mediate some forms of intercellular communication. serve as important raw materials for synthesis of other molecules. Provides cell with fibrous structural material. important for an organism's fertilization, development, blood clotting, and immune system function. Khalid, VD  Glucose is the most important carbohydrate. In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch. In animals, it is stored as glycogen. Glucose Structure A: straight chain form. B: α-D-glucose; Haworth projection. C: α-D-glucose;chair form. Khalid, VD  Classification of carbohydrates Classified according to the hydrolysis products into four main groups as follows: 1- Monosaccharides: contain one monosaccharide unit. 2- Disaccharides: contain 2 monosaccharide units per molecule. 3- Oligosaccharides: contain from 3 to 10 monosaccharide units per molecule. 4- Polysaccharides: contain more than 10 monosaccharide units per molecule. Khalid, VD  1- MONOSACCHARIDES Simplest group, having the general formula (CH2O)n. Classified according to:- A) The number of carbons in the molecule into trioses (3 carbons), tetroses (4 carbons), pentoses (5 carbons), hexoses (6 carbons) and heptoses (7 carbons). B) The presence of aldehyde or ketone group into aldoses and ketoses. Khalid, VD 2-Disaccharides are condensation products of two monosaccharide units united together by glycosidic linkage. Examples are maltose (Glucose+Glucose),sucrose (Glucose+Fructose) and lactose(Galactose+Glucose) 3- Oligosaccharides polymers of 3-10 monosaccharide units. not an important source of carbohydrates in diet for humans as most of them are nondigestible. 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. Eg:Raffinose, Stachyose Khalid, VD 4- POLYSACCHARIDES are condensation products of more than ten monosaccharide units. They can be homopolysaccharides-made up of one type of monosaccharide unit.Eg- Starch,dextrin,glycogen,cellulose. Heteropolysaccharides-formed of more than one type of monosaccharide unit. They include glycosaminoglycans (GAGs) formerly called mucopolysaccharides. Khalid, VD Biomedical Importance of Monosaccharides Smallest monosaccharides are trioses (n = 3) 1.Aldoses:-mother compound of all aldoses is the aldotriose glyceraldehyde.Other aldoses can be derived from this. Most of the naturally occurring monosaccharides are of the D type. Khalid, VD 2:-Ketoses: have two terminal primary alcohol groups (CH2OH) and have one ketone group (C=O). simplest ketose is dihydroxyacetone (C3). Other ketoses are formed from this. Khalid, VD  Importance of Monosaccharides 1- Trioses: Glyceraldehyde 3-phosphate and dihydroxyacetonephosphate are intermediates during glucose oxidation in living cells. 2- Tetroses : Erythrose 4-phosphate is formed during glucose oxidation in living cells. 3- Pentoses : - D-ribose is a component of many nucleosides and nucleotides and ribonucleic acids (RNA).Deoxyribose is a component of deoxyribonucleic acid (DNA). Khalid, VD Glucose - main sugar present in blood and is present in all animal and plant cells, honey and fruits. Major metabolic fuel of mammals. Other sugars are converted into glucose in the liver. Glucose is the precursor for synthesis of all the other carbohydrates in the body, including: - Glycogen for storage - Ribose and deoxyribose in nucleic acids - Galactose in lactose of milk - Glycolipids - In combination with protein in glycoproteins and proteoglycans Khalid, VD Fructose (fruit sugar) is present in honey, fruits and semen. It is a component of sucrose and inulin. D-galactose is a component of lactose which is present in milk. It is also found in glycosaminoglycans (GAGS), glycolipids and glycoproteins. Khalid, VD Biomedical Importance of Disaccharides Two monosaccharides join via condensation reaction/Dehydration reaction The hydroxyl group of one monosaccharide combined with the hydrogen of another. A molecule of water is released and a glycosidic linkage is formed. Khalid, VD Disaccharides: Sucrose (table sugar) Khalid, VD Khalid, VD  Biomedical Importance of Polysaccharides Polysaccharides: Storage and Structural function Starch It is a polymer of glucose. Mixture of two polysaccharides :amylose(20–30% ) and amylopectin ( 70–80%) Excess glucose is stored as starch. Khalid, VD Polysaccharides: Glycogen (branch points) Khalid, VD Khalid, VD Glycosaminoglycans (mucopolysaccharides) are complex carbohydrates containing amino sugars and uronic acids. They may be attached to a protein molecule to form a proteoglycan Khalid, VD  Monosaccharide derivatives Many derivatives of the monosaccharides are found in the systems, which include sugar phosphate, deoxy and amino sugars, sugar alcohol, and sugar acids. 1. Sugar acids: Sugar acids are carboxylic acids derived from aldoses, either by the oxidation of C-1 (the aldehyde carbon or by the oxidation of carbon bearing the primary alcohol). Sugar acids are important components of many polysaccharides. Eg-Gluconic acid,galacturonic acid,galactonic acid,ascorbic acid etc. # Ascorbic acid or Vitamin C is an essential cofactor for many hydroxylation processes. Khalid, VD 2.Sugar Alcohols: carbonyl group is reduced to alcohol group. Eg-Sorbitol:alcohol of glucose. Diabetic retinopathy and neuropathy may be related to excess sorbitol in the cells of the eyes and nerves. The source of this sorbitol in diabetics is excess glucose. Mannitol:alcohol of mannose; is used clinically to reduce acutely raised intracranial pressure, Dulcitol:alcohol of galactose. Ribitol: alcohol of ribose.It is a component of riboflavin (vitamin B2),is a component of FMN and FAD. Glycerol: It is the alcohol of glyceraldehyde or dihydroxyacetone; a component of triacylglycerol and most phospholipids. 3.De-oxy sugars:The sugars in which the hydroxyl group is replaced by a hydrogen atom. Eg-: a- 2-deoxy ribose: It is present in the structure of DNA. b- L-Fucose (6-deoxy-L-galactose): It is present in some cell membrane ,glycoproteins and blood group antigens. c-L-rhamonose (6-deoxy-L-mannose) is also found as components of cell wall. Khalid, VD  4.Amino sugars: The sugars in which an amino group (NH2) replaces the hydroxyl group on the second carbon e.g. glucosamine (GluN), galactosamine (GlaN) and mannosamine (ManN) Significant amounts of glucosamine have been found in the intestinal mucin, which binds cholesterol, thereby limiting its absorption. It has proven to decrease insulin secretion without suppressing liver glucose production. -Aminosugars are important constituents of glycosaminoglycans (GAGs) and some types of glycolipids and glycoproteins. -Several antibiotics contain aminosugars which are important for their antibiotic activity. Khalid, VD 5. Aminosugar acids : formed by the addition of acids to aminosugars. Addition of pyruvic acid to mannosamine gives neuraminic acid. 6- Ester formation: The hydroxyl groups of monosaccharides can form esters with acids. a- Phosphate esters: For example glucose 1-P and glucose 6-P. b- Sulfate esters : They are present in certain types of polysaccharides and glycolipids (sulfolipids) e.g. β-D galactose 3- sulfate. Khalid, VD  Glycoproteins and Cell identity Glycoproteins –type of protein molecule that have carbohydrate attached to it. The oligosachharide chains are covalently attached to amino acid side chain. Categorized according to the attachment site of the carbohydrate to an amino acid in the protein. O-linked glycoproteins are ones in which the carbohydrate bonds to the oxygen atom (O) of the hydroxyl group (-OH) of amino acid N-linked glycoproteins have a carbohydrate bonded to the nitrogen (N) of the amino group (-NH2) of the amino acid. Khalid, VD  Although polysaccharides are not able to store information, they can display information on the outer surface of cells in the form of glycoproteins. Glycoproteins function in the structure, reproduction, immune system, hormones, and protection of cells and organisms. They provide strength and stability to the tissues by cross-linking cells and proteins. Glycoproteins are found on the surface of the lipid bilayer of cell membranes. Their hydrophilic nature allows them to function in the aqueous environment, where they act in cell-cell recognition and binding of other molecules. They also help in cell-cell signaling and communication. Some hormones may be glycoproteins like human chorionic gonadotropin (HCG) and erythropoietin (EPO). Khalid, VD  Important for reproduction because they allow the binding of the sperm cell to the surface of the egg. Mucins are glycoproteins found in mucus. They protect sensitive epithelial surfaces, including the respiratory, urinary, digestive, and reproductive tracts. Glycoproteins have a important role in immunity. The carbohydrate of antibodies (which are glycoproteins) determines the specific antigen it can bind. B cells and T cells have surface glycoproteins which bind antigens, as well. Khalid, VD Glycoproteins: Identification badge for the cells Glycoprotein Outside of cell Inside of cell  Carbohydrates occur in cell membranes Approximately 5% is carbohydrate in glycoproteins of cell membrane. Their presence on the outer surface of the plasma membrane (the glycocalyx) has been shown with the use of plant lectins, protein agglutinins that bind with specific glycosyl residues. Glycophorin is a major integral membrane glycoprotein of human erythrocytes.