Lecture 4 5 carbohydrates


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Lecture 4 5 carbohydrates

  1. 3. <ul><li>In plants: </li></ul><ul><ul><li>Carbohydrates are synthesized from CO 2 and H 2 O in presence of light (photosynthesis). </li></ul></ul><ul><ul><li>CO 2 + H 2 O (CH 2 O) n + O 2 </li></ul></ul><ul><ul><li>Plants synthesize glucose, the most important carbohydrate and store it as “Starch” or use to build “Cellulose”, which is used in the plant’s structural framework </li></ul></ul><ul><li>In animals: </li></ul><ul><ul><li>Animals can synthesize carbohydrates from non-carbohydrate sources: </li></ul></ul><ul><ul><ul><li>Examples: lipid glycerol and amino acids </li></ul></ul></ul><ul><ul><li>The process of synthesizing glucose from non-carbohydrate sources is called “Gluconeogenesis”. </li></ul></ul><ul><ul><li>However, most animal carbohydrates are derived from plants. </li></ul></ul>Light
  2. 4. <ul><li>Most dietary carbohydrate is absorbed into bloodstream as glucose </li></ul><ul><li>Other sugars are converted into glucose in the liver </li></ul><ul><li>Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus. </li></ul><ul><li>Glucose is the precursor for synthesis of other carbohydrates of biological importance: </li></ul><ul><ul><li>Examples </li></ul></ul><ul><ul><ul><li>Starch and Glycogen (the storage form of carbohydrates in plants and animals, respectively.) </li></ul></ul></ul><ul><ul><ul><li>Ribose and deoxyribose (used in nucleic acids synthesis) </li></ul></ul></ul><ul><ul><ul><li>Galactose (a sugar used to make milk sugar, lactose, in glycolipids and in combination with proteins in glycoproteins and proteoglycans) </li></ul></ul></ul>
  3. 5. <ul><li>Diseases associated with abnormal carbohydrate metabolism include “Diabetes mellitus” and “Lactose intolerance”. </li></ul>
  4. 7. <ul><li>Monosaccharides are the simplest forms of carbohydrate. They can’t be hydrolyzed into simpler forms. </li></ul><ul><li>Classification </li></ul><ul><ul><li>They are classified into 3-, 4-, 5-, 6- or 7- carbon sugars called, trioses, tetroses, pentoses, hexoses and heptoses, respectively </li></ul></ul><ul><ul><li>They can also be classified as aldoses or ketoses, depending on whether they have an aldehyde or a ketone group. </li></ul></ul>
  5. 8. <ul><li>Disaccharides, are condensation products of 2 monosaccharide units </li></ul><ul><li>Examples </li></ul><ul><ul><li>Maltose </li></ul></ul><ul><ul><li>Sucrose </li></ul></ul><ul><ul><li>Lactose </li></ul></ul>
  6. 9. <ul><li>Oligosaccharides are condensation products of 3-10 monosaccharide units </li></ul>
  7. 10. <ul><li>Polysaccharides are condensation products of more than 10 monosaccharide units </li></ul><ul><li>Examples </li></ul><ul><ul><li>Starch </li></ul></ul><ul><ul><li>Cellulose </li></ul></ul><ul><ul><li>Dextrins </li></ul></ul><ul><ul><li>Glycogen </li></ul></ul>
  8. 11. <ul><li>Structure of glucose can be represented in 3 ways: </li></ul><ul><ul><li>Straight chain </li></ul></ul><ul><ul><li>Cyclic or simple ring (Haworth projection) </li></ul></ul><ul><ul><li>Chair form. </li></ul></ul>O=C-H | H-C-OH | HO-C-H | H-C-OH | H-C-OH | CH 2 OH D-Glucose 1 2 3
  9. 13. <ul><li>Isomerism of carbohydrates </li></ul><ul><ul><li>Example </li></ul></ul><ul><ul><ul><li>Glucose, for example, has 4 asymmetric carbons which can make 16 isomers (4 2 ). Different configurations of OH groups about these asymmetric carbon atoms give rise to different types of isomerism and isomers. </li></ul></ul></ul>O=C-H | H-C-OH | HO-C-H | H-C-OH | H-C-OH | CH 2 OH 1 2 3 4 5 6
  10. 14. <ul><li>D and L isomerism </li></ul><ul><ul><li>D and L isomers are mirror images to each other </li></ul></ul><ul><ul><li>It is determined by the orientation of the OH group (to the right is D and to the left is L) of the asymmetric carbon atom as compared to that of the parent sugar “Glycerose or Glyceraldehyde” </li></ul></ul><ul><ul><li>Most monosaccharides present in animals are of D- configuration </li></ul></ul><ul><ul><li>Example </li></ul></ul><ul><ul><ul><li>D-glucose is called dextrose in clinical practice </li></ul></ul></ul>D-Glycerose L-Glycerose D-Glucose O=C-H | H-C-OH | HO-C-H | H-C-OH | H-C-OH | CH 2 OH 1 2 3 4 5 6 O=C-H | HO-C-H | CH 2 OH 1 2 3 O=C-H | H-C-OH | CH 2 OH 1 2 3
  11. 15. <ul><li>Pyranose (6-membered ring) or furanose (5-membered ring) ring structures are based on the pyran and furan ring structure. </li></ul>Glucose (Pyranose) Fructose (Furanose) Pyran Furan
  12. 16. <ul><li>Alpha- and beta- anomers </li></ul><ul><ul><li>It is the isomerism that occurs about the carbonyl group (called anomeric carbon atom), position 1 in glucose or 2 in fructose, which forms alcohol in the ring structure, where the sugar is called alpha- if the OH group of its anomeric carbon is to the right (downward in ring structure) or beta- if it’s to the left (upward in ring structure). </li></ul></ul> -D-Glucose  -D-Glucose
  13. 17. <ul><li>Epimers or epimerization </li></ul><ul><ul><li>It is the isomerism that occurs about the asymmetric carbon atoms other than the anomeric carbon and the last (which determine the D / L and alpha- / beta- isomerism), e.g. carbon # 2, 3 and 4 in glucose. </li></ul></ul><ul><ul><li>It gives rise to different sugars, which are epimers to the parent sugar </li></ul></ul><ul><ul><ul><li>Examples </li></ul></ul></ul><ul><ul><ul><ul><li>Mannose and galactose are epimers of glucose formed by epimerization of glucose at carbon 2 and 4, respectively </li></ul></ul></ul></ul>
  14. 18. Epimers
  15. 19. Isomers of D-Aldoses
  16. 20. Isomers of D-Ketoses
  17. 21. <ul><li>Glycosides , </li></ul><ul><ul><li>They are formed by condensation (removal of water molecule) between the (-OH) group of the anomeric carbon of a monosaccharide and a second compound that may or may not be another monosaccharide, e.g. maltose is formed between 2 glucose units, lactose is a disaccharide between glucose and galactose, sucrose is formed between glucose and fructose units. </li></ul></ul><ul><ul><li>If the participating group of the 2 nd compound is (-OH), the bond is called O-glycosidic bond, while it’s called N-glycosidic bond if the 2 nd group is an amine (-NH 2 ). </li></ul></ul><ul><ul><li>If the participating monosaccharide is glucose, then the resulting compound is called “Glucoside” and the formed linkage is glucosidic linkage. If this monosaccharide is galactose, the resulting compound is a galactoside, and so on. </li></ul></ul>
  18. 22. <ul><li>Disaccharides are glycosides , </li></ul><ul><ul><li>Since Glycosides are formed by a bond between the (-OH) gp of the anomeric carbon and another compound, the anomeric carbon loses its capacity to convert to the acetal group (-C=O), and hence it loses it’s reducing properties. </li></ul></ul><ul><ul><li>If the 2 nd compound (another monosaccharide) participates with (-OH) group other than that of the anomeric carbon, the resulting disaccharide still possesses reducing properties due to presence of free anomeric carbon that can convert into (-C=O). </li></ul></ul><ul><ul><li>The formed bond would be called by the type and the location of the participating groups, e.g. the bond in maltose is  -1-4 glucosidic bond because it’s formed between an (-OH) group of  -glucose (carbon no. 1) and an (-OH) group of carbon no. 4 of a second glucose. </li></ul></ul>
  19. 23.  -D-Glucose  -D-Glucose +  -D-Maltose  -1-4 G-linkage  -D-Galactose  -D-Glucose +  -D-Lactose  -1-4 G-linkage Sucrose  -D-Glucose  -D-Fructose  -1-2 G-linkage Disaccharides are Glycosides
  20. 24. Disaccharides are Glycosides
  21. 25. <ul><li>Polysaccharides Serve Storage and Structural Functions , </li></ul><ul><ul><li>Starch is a homopolymer (same consisting monosaccharides) of glucose. It’s the most abundant dietary carbohydrate. It’s the storage form of carbohydrate in plants. Two main constituents constitute starch, </li></ul></ul><ul><ul><ul><li>Amylose (15-20%), which has a non-branching helical structure and consists of about 250-300 glucose units linked together with  -1-4 glucosidic linkages; AND </li></ul></ul></ul><ul><ul><ul><li>Amylopectin (80-85%), which consists of branched chains composed of 24-30 glucose units linked together with  -1-4 glucosidic linkages in the chain and  -1-6 at the branch points. </li></ul></ul></ul><ul><ul><li>Glycogen is the storage polysaccharide in animals. It’s more highly branched than amylopectin, with chains of 12 to 14 linked with  -1-4 linkages in the chain and  -1-6 at the branching points. </li></ul></ul>
  22. 27. <ul><li>Polysaccharides Serve Storage and Structural Functions , </li></ul><ul><ul><li>Chitin is a structural polysaccharide in the exoskeleton of insects and crustaceans as well as in mushroom. It consists of a homopolymer of N-acetyl glucoseamine units joined by  -1-4 linkages. </li></ul></ul><ul><ul><li>Cellulose is the main constituent of plant structure. It’s an insoluble homopolymer of glucose, linked with  -1-4 linkages. Cellulose can’t be digested by mammals because of absence of cellulase enzyme that catalyzes the cleavage of the  -1-4 linkages. Microorganisms in the gut of ruminants and other herbivores secrete cellulase enzyme that hydrolyze the  -1-4 linkages. </li></ul></ul>HN-CO-CH 3  -D-N-acetyl glucoseamine
  23. 28. <ul><li>Glycoproteins , </li></ul><ul><ul><li>Also called Mucoproteins , occur in many fluids and tissues, including cell membranes. They are proteins containing branched or unbranched oligosaccharide chains. Carbohydrate derivatives are linked to proteins through O-glycosidic or N-glycosidic linkages with (-OH) or (-NH 2 ) containing amino acid side chains, respectively. </li></ul></ul><ul><ul><li>The human ABO blood groups are examples of glycoproteins or glycosides that have same oligosaccharide foundation (called O or more commonly, H antigen). Different blood groups result from the addition of one extra monosaccharide unit, either N-acetylgalactosamine (for Blood group A) or just galactose (for blood group B) through an  -1-3 linkage to a galactose moiety of the O-antigen. </li></ul></ul>