Bio I Carbohydrates Chemistry (1) PDF

Carbohydrates Dr. Mohamed Bakr  Carbohydrates and Biochemistry Carbohydrates are compounds of tremendous biological importance: ✓ they provide energy through oxidation ✓ they supply carbon for the synthesis of cell components e.g Ribose in nucleic acids...

Carbohydrates Dr. Mohamed Bakr  Carbohydrates and Biochemistry Carbohydrates are compounds of tremendous biological importance: ✓ they provide energy through oxidation ✓ they supply carbon for the synthesis of cell components e.g Ribose in nucleic acids ✓ they serve as a form of stored chemical energy ✓ they form part of the structures of some cells and tissues Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules because they are closely associated with living organisms  Carbohydrates Carbohydrates include not only sugar, but also the starches that we find in foods, such as bread, pasta, and rice The term “carbohydrate” comes from the observation that when you heat sugars, you get carbon and water (hence, hydrate of carbon) Carbohydrates are organic compounds which contain carbon, (H & O in the ratio of 2 : 1 ) Carbohydrates have the general formula (C H2O)n and thus are named as “Hydrates of Carbon Carbohydrates Carbo – Hydrate C H2O Aldehyde or ketone derivative of polyhydric alcohols, or any substances derived from them. Poly hydroxy aldehydes or poly hydroxy ketones derivatives. They called Saccharides Biological Importance of Carbohydrates Carbohydrates are widely distributed in plants and animals. Carbohydrates constitute about 60% of our diet. They are important because:- 1. Primary source of energy e.g., glucose. 2. Storage form of energy e.g., glycogen 3. Biosynthesis of : Energy compounds as ATP DNA & RNA Receptors Vitamins 4. Carbohydrates may combine with lipids (glycolipids) or protein (glycoproteins), both enter in the structure of the cell membrane. Classification of Carbohydrates According to no. of sugar units Monosaccharaides: 1 sugar unit → sugars that are sweet and soluble Disaccharides: 2 sugar units Oligosaccharides: 3 – 9 units Polysaccharides: ≥ 10 units → polymers that are not sweet nor soluble Glucoconjugates: carbohydrate part + non CHO part Monosaccharidse They are the simplest units of carbohydrates. The general formula is Cn(H2O)n. Classified according to number of carbon atom & carbonyl group. Triose (n=3) Tetrose Pentose (n=5) Hexose (n=6) Heptose (n=7) (n=4) - Ribose→ Structure of - Glucose → major - Sedoheptulose No. of RNA, ATP, GTP, source of energy → formed in Carbon coenzymes - Fructose → main Hexose atoms (NAD,NADP). sugar of semen monophosphate - Lyxose→ lyxoflavin - Galactose → shunt Type of isolated from heart synthesis of lactose carbonyl muscle. (milk sugar) in group - Arabinose & Xylose lactating mammary → glycoprotein in plant gland. & animal cell. - Mannose → glycoprotein Aldoses (contain Glyceraldehyde Erythrose Ribose, Arabinose, Glucose, Mannose, aldehyde group) Xylose, Lyxose Galactose. (Sufix- ose) Ketoses (contain Dihydroxy- Erythrulose Ribulose, Xylulose. Fructose (Fruit Sedoheptulose ketone group) acetone Sugar) (Sufix- ulose) Aldoses Ketoses Structure of Monosaccharide Haworth Projection The aldehyde or ketone will react with OH group of C4 or C5 to form hemiacetal or hemiketal. This creates a new asymmetric carbon. This forms a cyclic ring either five- membered ring called furanose or six-membered ring called pyranose. If the remaining – OH is on the right side, it is α – sugar while if the remaining – OH is on the left side, it is β- sugar. Asymmetric carbon atom (chiral carbon): It is the carbon atom which attached to 4 different groups or atoms. Any substances containing one or more asymmetric carbon atom shows two properties optical activity & stereoisomerism. Optical Activity: The ability of substance to rotate plane polarized light either to the right or to the left. If the substance rotate plane polarized light to the right, so it is called dextrorotatory or d- or (+). If rotate it to the left, so it is called levorotatory or (l) or (-). Racemic mixture: It is the mixture containing equal number of molecule of two optically active sugars; one is dextro- and the other is levo- →No optical activity Isomerism The compounds possessing identical molecular formula, but Structural Isomers Glucose / Fructose different structures or configuration are called Isomerism isomers. Enantiomers D/L Number of isomers = 2n where n = the number of asymmetric carbon atoms. Stereoisomers Epimers e.g. glucose has 4 asymmetric carbon atoms, so, the number of isomer Anomers α- / β- is 24 = 16 Structural Isomers (Aldose-Ketose Isomerism) Fructose has the same molecular formula as glucose but differs in structure formula. One contain keto group and the other contains an aldehyde group. Both are isomers. Enantiomers They are two stereoisomers that are mirror images of each other and non-superposable (not identical). They are classified into D & L forms according to the position of – OH group attached to the carbon atom adjacent to the last –CH2OH (i.e.; carbon atom number 5 in glucose). Most of the monosaccharides occurring in mammals are of D-form. Epimers They are sugar molecules that differ in configuration at only one of chiral centers other than anomeric carbon. D-glucose and D-mannose are C2-epimers D-glucose and D-galactose are C4-epimers D-mannose and D-galactose are NOT epimers. Sugar Derivatives: A- Sugar Acids These are produced by oxidation of carbonyl carbon, last hydroxyl carbon or both. 1. Aldonic acids: Oxidation of carbonyl carbon to carboxylic group gives aldonic acid e.g.; glucose is oxidized into gluconic acid. 2. Uronic acid: Oxidation of last hydroxyl carbon(C6 in glucose) will give glucuronic acid. 3. Aldaric acid: These are dicarboxylic acids produced by oxidation of both carbonyl group and last hydroxyl group into carboxylic group e.g., glucose will be oxidized to glucaric acid. Sugar Derivatives: B- Sugar Alcohols Monosaccharides, both aldoses and ketoses may be reduced at carbonyl carbon to the corresponding alcohols. For example, glucose is reduced to sorbitol, mannose into mannitol, fructose into mannitol and sorbitol.  Sugar Derivatives: C- Deoxysugar Sugars in which one of the hydroxyl groups has been replaced by hydrogen atom. Examples, deoxyribose which occurs in nucleic acid DNA. Sugar Derivatives: D- Amino Sugars In these sugars, the hydroxyl group is replaced by an amino group. Disaccharides These are formed by condensation of 2 molecules of monosaccharides bounded together by glycosidic bond. Its general formula is Cn (H2O)n-1 The most important disaccharides: 1-Maltose ( 2 α-glucose molecules linked by α 1-4 glycosidic bond) 2- Isomaltose (2 α-glucose molecules linked by α 1-6 glycosidic bond) 3- Cellobiose (2 β- glucose linked by β 1 – 4 linkage) 4- Lactose (β- glucose + β-galactose linked by β 1 – 4 linkage) 5- Sucrose (α-glucose + β- Fructose linked by α1 –β2 glycosidic bond). 1- Maltose (Malt Sugar) Structure: It is formed of 2 α-glucose molecules linked by α 1-4 glycosidic bond. Source: Malt, in addition it is produced during digestion of starch by amylase enzyme. Properties: Maltose containing free carbonyl (aldehyde) group, so it is reducing sugar. 2- Isomaltose Structure: It is formed of 2 α-glucose molecules linked by α 1-6 glycosidic bond. Source: It is produced during digestion of starch & glycogen by amylase enzyme. Properties: Isomaltose containing free carbonyl (aldehyde) group, so it is reducing sugar. 3- Cellobiose Structure: It is formed of 2 β-glucose units linked together by β 1-4 glycosidic bond. Source: It is obtained by partial hydrolysis of cellulose present in plant. 4- Lactose Structure: It is formed of β- glucose + β-galactose linked by β 1 – 4 linkage. Source: It is the sugar present in milk. Properties: Lactose containing free carbonyl (aldehyde) group, so it is reducing sugar. 5- Sucrose Structure: It is formed of α-glucose + β- Fructose linked together by α1 –β2 glycosidic bond. Source: Cane and beet sugar. It is also present in pineapple and carrot. Properties: Sucrose contain no free carbonyl group (because both anomeric carbons are involved in glycosidic bond), so Sucrose is non-reducing sugar. Polysaccharides These are carbohydrates formed of more than 10 monosaccharides units linked by glycosidic linkages. They are classified into: Homopolysaccharides which are composed of one type of monosaccharides such as starch, glycogen & cellulose Heteropolysaccharides which are made of different types of monosaccharides units such as mucopolysaccharides Homopolysaccharides: 1- Starch Function: glucose storage in plants. It has 2 forms. Amylose: Inner layer formed of non branching helical structure of glucose units linked together by α (1-4) glycosidic bonds. Amylopectin: Outer layer formed of branched chains. Each chain is composed of 24 – 30 glucose units linked together by α 1-4 glycosidic bonds and α 1-6 glycosidic bond at the branching point. Homopolysaccharides: 2- Glycogen Function & source: Animal Starch. Glycogen is the storage form of carbohydrates in human and animals. It is synthesized and stored in liver, muscles. Structure: It is highly branched chain homopolysaccharides. Each branch is composed of 12 – 14 glucose units, linked together by 1-4 glycosidic bonds and by 1-6 glycosidic bond at branching point. Homopolysaccharides: 3- Cellulose Function & source: It is the major form of structural carbohydrates in plants. It occurs in plant cell walls, mainly in wood. Structure: It is a linear polymer of β-D-glucopyranose linked by β 1,4 linkage. Properties: Cellulose in diet cannot be digested by many mammals including humans due to absence of hydrolase enzymes that can attack β-linkage. Its presence in diet is important, because it cannot be digested, so it will increase the bulk of stool and stimulate the intestinal movement and prevent constipation. Heteropolysaccharides They are made of 2 or more different types of monosaccharides units. They include: 1. Glycosaminoglycans (GAGs): (known as mucopolysaccharides) are long unbranched polysaccharides containing a repeating disaccharide unit that contains either of two modified sugars: a) N-acetylgalactosamine (GalNAc) or N-acetylglucosamine (GlcNAc) b) Uronic acid such as glucuronate or iduronate. c) Some contain sulphate groups 2. Proteoglycan: These are chains of glycosaminoglycan attached to protein molecules. The carbohydrate part is present in very long unbranched chains (more than 50 monosaccharide molecules) attached to protein core. 3. Glycoprotein : It consists of protein core and short branched chain of carbohydrate (2 – 15 monosaccharide units), usually called oligosaccharide chain. Thank You