L19 Simple Carbohydrates-Nomenclature and Classification PDF

L19 Carbohydrates: Nomenclature, Classification and Isomerism Dr. Nelofar Khan www.gmu.ac.ae COLLEGE OF MEDICINE At the end of the lecture, you should be able to • Define a carbohydrate. • Explain the basis for the nomenclature and classification of carbohydrates into Mono, Di and Polysaccharide...

L19 Carbohydrates: Nomenclature, Classification and Isomerism Dr. Nelofar Khan www.gmu.ac.ae COLLEGE OF MEDICINE At the end of the lecture, you should be able to • Define a carbohydrate. • Explain the basis for the nomenclature and classification of carbohydrates into Mono, Di and Polysaccharides. • Apply the system of classification to identify and name a monosaccharide. • Explain the isomers of monosaccharides i.e., the D and L isomers, Aldose -ketoses, Pyranose-Furanoses, Alpha-beta anomers and Epimers. • Define sugar acids, amino sugars, deoxy sugars and sugar alcohols with examples. • Reading: Satyanarayana U and Chakrapani U. Biochemistry; Elsevier; 5th Edition; 2020. ISBN- 978-8131262535. Chapter 2, pages 9 - 20. Carbohydrates: General Information • Widely distributed in plants and animals • Have important structural and metabolic roles • Plants synthesize glucose by photosynthesis and store it as starch or use it to synthesize cellulose • Animals can synthesize carbohydrates from (glucogenic) amino acids, but most are obtained from the diet • Dietary carbohydrates: ✔ mainly starch ✔ hydrolyzed to glucose ✔ absorbed into blood ✔ other sugars are converted to glucose in the liver • Glucose is the major metabolic fuel for all cells in our body • It is also the precursor for synthesis of other carbohydrates: ✔ glycogen for storage ✔ ribose and deoxyribose in nucleic acids ✔ galactose for synthesis of lactose in milk ✔ also found in glycolipids, glycoproteins and proteoglycans Functions of Carbohydrates • Supply the energy needs of the body. • Serve as storage forms of energy. Eg: Starch and Glycogen • Glycolipids and glycoproteins are important part of membrane structure and participate in functions like cell growth, adhesion and fertilization. • Carbohydrates also have special functions. For eg: ✔ Ribose sugar is found in RNA ✔ Hyaluronic acid lubricates skeletal joints • Carbohydrates serve as precursors for other biomolecules like fats and amino acids. Carbohydrate Classification Carbohydrates are classified according to the number of sugar units. Monosaccharides: They contain one molecule of sugar and cannot be hydrolyzed into simpler carbohydrates. Disaccharides: They contain two monosaccharides linked together. Bonds linking carbohydrates are called glycosidic linkages. Oligosaccharides: Contain three to ten monosaccharide units linked together. Polysaccharides: Polymers of >10 monosaccharides Classification of Monosaccharides • Monosaccharides: sugars that cannot be hydrolyzed into simpler carbohydrates. • They may be classified as: trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms aldoses or ketoses, depending on whether they have an aldehyde or ketone group. Glucose is an Aldohexose, and Fructose is a Ketohexose. • Aldehyde and ketone groups of monosaccharides can also be converted (reduced) to alcohols and these are called sugar alcohols The sugars are called aldoses if they contain an aldehyde group (-CHO) and ketoses if they contain keto groups (-C=O). The monosaccharides are classified according to the number of carbons they contain. # Carbons Class Examples 3 Triose Glyceraldehyde, Dihydroxyacetone 4 Tetrose 5 Pentose Xylulose Erythrose Ribose, Ribulose, 6 Hexose Glucose, Galactose, Mannose, Fructose 7 Heptose Sedoheptulose Some important Hexoses • Glucose Major fuel of life Sugar found in Blood Constituent of Many di and Polysaccharides • Fructose Found in fruits Can be converted to glucose in the body • Galactose Constituent of milk Sugar (Lactose) • Mannose Constituent of glycoproteins 11 Structure of Glucose Can Be Represented in Three Ways Fischer Projection Haworth Projection f Haworth Projection Chair form Sugars Exhibit Various Forms of Isomerism The more important types of isomerism found with glucose are as follows: 1. D and L isomerism (stereoisomerism, enantiomers, mirror images) 2. Optical isomerism (dextro and levorotatory) 3. Pyranose and Furanose ring structures 4. Alpha and beta anomers 5. Epimers (Glucose and Galactose) 6. Aldose-ketose isomers (Glucose and Fructose) Stereoisomerism Compounds with asymmetric carbon atoms (attached to four different groups) can show stereoisomerism. They have the same structural formula but exist in different spatial arrangements. 1. D and L isomerism (stereoisomerism, enantiomers, mirror images) All carbohydrates are called D or L isomers depending on whether they resemble D or L glyceraldehyde. OH group on last but one carbon dictates D or L isomer. All naturally occurring monosaccharides belong to the D enantiomer family. 2. Optical isomerism ∙ Monosaccharides have asymmetric carbon atoms and show “optical activity” i.e. rotating the plane of polarization of light. ∙ They can be dextrorotatory (+; d; Clockwise) or levorotatory (˗; l; Anticlockwise) ∙ For eg: Naturally occurring fructose is the D(-) isomer. ∙ Optical rotation of glucose in solution is dextrorotatory and so is also called “Dextrose”. 3. Pyranose and Furanose ring structures When these ring structures are similar to the pyran (6 member ring), they are called pyranoses; similarly furanose (5 member ring). 4. Anomers Anomers are cyclic monosaccharides differing from each other in the configuration of C-1 (if they are aldoses) or in the configuration at C-2 (if they are ketoses). α (OH down the plane) β (OH up the plane) 19 Derived sugars: Amino sugars • They are sugars where an -OH group is replaced by an -NH2 group. • This usually takes place at carbon atom 2. • Amino sugars and their derivatives are important constituents of Glycosaminoglycans and glycoproteins. Derived Sugars: Deoxy sugars • Removal of oxygen from sugars results in deoxy sugars. • 2-Deoxyribose is present in DNA • 6-Deoxy Galactose (Fucose) is found in many glycoproteins and glycolipids. Derived Sugar: Sugar alcohols Sugars when treated with reducing agents form corresponding alcohols. The aldehyde/keto group is reduced to alcohol group. Glucose and Fructose form Sorbitol (D-Glucitol): build up in eyes of diabetics. Mannose forms Mannitol (sweetens sugarless gum) Galactose forms Dulcitol Sugar alcohols • Maltitol, mannitol, sorbitol and xylitol are used in sugar-free chewing gum as they are Noncariogenic, sweet and cool taste. • Mannitol contains less calories. • Mannitol is slowly and incompletely absorbed from our small intestine. Its consumption (compared with an equal amount of sugar) reduces insulin secretion, which helps keep blood glucose levels lower. • Having too much mannitol may cause gastrointestinal discomfort including bloating, gas and diarrhea. Derived Sugar: Sugar acids 1. 2. Oxidation of aldehyde group (CHO to COOH) results in the formation of gluconic acid. Oxidation of terminal alcohol group (CH2OH to COOH) leads to the production of glucuronic acid. ▪ D-glucuronic acid is important for formation of glucuronides (such as with bilirubin) and glycosaminoglycans. ▪ L-Iduronic acid is also found in glycosaminoglycans α-D-Glucuronate (left) and L-Iduronate (right). 27 Let us summarize Thank You www.gmu.ac.ae 28 COLLEGE OF MEDICINE