Lecture 3: Carbohydrate Biochemistry PDF

Lecture 3 Carbohydrate Biochemistry Carbohydrates Carbohydrates or Saccharides (Greek Sakcharon meaning "Sugar") Nature’s most abundant organic substance. Organic compounds composed of Carbon, Hydrogen, and oxygen. Hydrogen and Oxygen in Carbo...

Lecture 3 Carbohydrate Biochemistry Carbohydrates Carbohydrates or Saccharides (Greek Sakcharon meaning "Sugar") Nature’s most abundant organic substance. Organic compounds composed of Carbon, Hydrogen, and oxygen. Hydrogen and Oxygen in Carbohydrates were found to be present in the same proportion as in water. (2:1). (E.g., Glucose C6H12O6 or C6 (H2O)6). It is due to this fact that compounds derived their name “Carbon Hydrate”. Chemical Definition of Carbohydrates Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds upon hydrolysis. What are the five main biological significance of carbohydrates? Carbohydrates have important structural and metabolic roles. 1. Energy source for plants and animals (glucose). 2. Cell membrane components: e.g. Glycolipid and glycoprotein 3. Glucose is the precursor for synthesis of all other carbohydrates in the body, including Galactose in lactose of milk. 4. Provide energy storage: Animals store energy in a complex carbohydrate in their muscle tissue and liver in the called glycogen. 5. Structural framework of genetic material: The sugars ribose and deoxyribose are a part of the structural framework of genetic material RNA and DNA. Why does the body not use lipids as a major energy source instead of CHO? Carbohydrates are used as main source of energy because these are much easily made available as respiratory substrate in spite of the fact that lipids are much richer source of energy as compared to the carbohydrates. Lipids are used in respiration only when carbohydrates are not available. Why does the cell store excess glucose as glycogen? The glucose cannot be stored in the cells of the animal body due to the high osmotic pressure which glucose exerts inside the cell, causing the entry of water in the cell by endosmosis. This is the reason that the glucose is converted to glycogen in the liver for the storage because it is osmotically more stable. Polymer Vs. Monomer Monomer: the building block that is used to make polymers Ex. Glucose Polymer: A large molecule made up of identical or similar building blocks Ex. Polysaccharide (Starches). What are the four main types of carbohydrates? 1. Monosaccharides: (saccharides means sugars) that cannot be hydrolyzed into simpler carbohydrates, (Exp. Glucose). 2. Disaccharides: condensation products of two monosaccharide units (Exp. Lactose). Note: All mono and disaccharides have the ending ose (glucose, galactose, ribose, lactose, etc…). 3. Oligosaccharides are condensation products of three to ten monosaccharides; Maltotriose is an example. 4. Polysaccharides are condensation products of more than ten monosaccharide units (Exp. Glycogen & Starch). Structural representation of sugars Fisher projection: straight chain form. Haworth projection: simple ring. What is the mechanism of cyclization of monosaccharides? In aldopyransoses, such as glucose and mannose, the C5 hydroxyl group attacks the C1 aldehyde to form a six-membered ring bearing a C1 hemiacetal. Glucose Hemiacetal Form Glucose (Chain form) (Ring form) Fisher Projection Haworth projection What are the naturally occurring monosaccharides and how are they classified? Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are basic units of Carbohydrates. The general chemical formula of an unmodified monosaccharide is (C H2O)n. In this formula, the “n” varies from 3-6 and rarely seven. This implies that in nature no. of carbon atoms found in monosaccharide varies from minimum 3 to maximum 7. Chemical Classification of Monosaccharides 1. According to the position or type of carbonyl group Aldose is a type of monosaccharide which contains aldehydes on its carbon skeleton. Aldehydes are denoted as. (-CH=O). Ketose is a type of monosaccharide which contains a ketone group on the carbon skeleton. Ketones are denoted as (-C=O). Aldehydes are terminal carbonyls while ketones are carbonyls surrounded on each side by a carbon. 2. According to number of carbon atoms and active site group Number of carbon Type of carbonyl group atom Aldoses Ketoses Trioses Glyceraldehyde Dihydroxyacetone Tetroses Erythrose Erythulose Pentoses Ribose, Xylose Ribulose, Xylulose Hexoses Glucose, Galactose, Mannose Fructose Heptoses Heptose Sedoheptulose Classify this carbohydrate according to Its functional group and number of carbons. Many Thanks Dr. Hussein Nassar