Carbohdrates 2013


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Carbohdrates 2013

  1. 1. CarbohydratesDr Imran Siddiqui PhD, MPhil, MBBS
  2. 2. Objectives• Define Carbohydrates• List the biomedical importance ofcarbohydrates• Write down the Empirical formula ofCarbohydrates• Classify with examples various types ofcarbohydrates• Explain isomerism in carbohydrates
  3. 3. Significance of Carbohydrates• Most abundant organic compound in nature• A major source of energy from our diet• Storage form of energy in plants and animals• Cell membrane component• Structural component of many organisms• Composed of the elements C, H and O andempiric formula is C(H2O)n• Also called hydrates of carbon OR saccharides,which means “sugars”
  4. 4. DefinitionCarbohydrates are polyhydric alcohols (-OH)with aldehyde or ketone.
  5. 5. Figure 7.1. Examples of monosaccharides found in humans, classifiedaccording to the number of carbons they contain.
  6. 6. Isomers and epimers• Compounds that have same chemical formula but havedifferent structures = ISOMERS• e.g., fructose, glucose, mannose, & galactose are all isomersof each other, having same formula C6H12O6• If 2 monosacch differ in configuration around only onespecific C atom (with exception of carbonyl C), they aredefined as epimers of each other (of course they are alsoisomers) e.g.,– glucose & galactose are C-4 epimers, their structuresdiffer only in the position of –OH group at C 4.– Glucose & mannose are C-2 epimersNote: carbons in sugars are numbered beginning at endcontaining the carbonyl C i.e., aldehyde or keto group
  7. 7. Figure 7.4C-2 and C-4 epimers and an isomer ofglucose.
  8. 8. B. Enantiomers- A special type of isomerism is found in the pairs of structuresthat are mirror images of each other. These mirror images =enantiomers, & the 2 members of the pair are designated asD- & L-sugar. Vast majority of sugars in humans are D-sugarsFigure 7.5Enantiomers (mirrorimages) of glucose.
  9. 9. C. Cyclization of monosaccharides- Less than 1% of each of the monosacch with 5 ormore C’s exist in the open-chain (acyclic) form.Rather, they are predominantly found in ring form,in which aldehyde (or ketone) group has reactedwith an alcohol group on the same sugar
  10. 10. Two Cyclic Forms of Glucose
  11. 11. Figure 7.6. The interconversion of the α and β anomeric forms ofglucose (mutarotation).
  12. 12. • Anomers are isomers which differ in placement ofhydroxyl on C1• The –OH is drawn down for the -anomer, and up forthe -anomer-D-Glucose -D-GlucoseOCH2OHOHOHOHOHOCH2OHOHOHOHOHand Anomers for D-Glucose
  13. 13. Types of Carbohydrates Monosaccharides are the simplest carbohydrates Empirical formula = (CH2O)n Disaccharides consist of two monosaccharides Polysaccharides contain many monosaccharides
  14. 14. MonosaccharideUnbranched chain of 3-8 C atomsOne is carbonyl; others attached to -OHAldosescontain an aldehyde group (carbon 1)Ketosescontain a ketone group (carbon 2)CHOHO HCH2OHCHOHO HOHHCH2OHCH2OHOHHOOHHOHHOHCH2OHClassification of Monosaccharides
  15. 15. Structural representation of sugars• Fischer projection: straight chainrepresentation• Haworth projection: simple ring in perspective• Conformational representation: chair and boatconfigurations
  16. 16. Rules for drawing Haworth projections• draw either a six or 5-membered ring includingoxygen as one atom• most aldohexoses are six-membered• aldotetroses, aldopentoses, ketohexoses are 5-memberedO O
  17. 17. Rules for drawing Haworth projections• for D-sugars the last alcohal group (farthestfrom the carbonyl) is drawn up.• For L-sugars, it is drawn down• for D-sugars, the OH group at the anomericposition is drawn down for and up for .• For L-sugars is up and is down
  18. 18. Chair and boat conformations of a pyranose sugar2 possible chair conformationsof -D-glucose
  19. 19. Oxidation reactions• Aldoses may be oxidized to 3 types of acids– Aldonic acids: aldehyde group is converted to a carboxylgroup ( glucose – gluconic acid)– Uronic acids: aldehyde is left intact and primary alcoholat the other end is oxidized to COOH• Glucose --- glucuronic acid• Galactose --- galacturonic acid– Saccharic acids (glycaric acids) – oxidation at both endsof monosaccharide)• Glucose ---- saccharic acid• Galactose --- mucic acid• Mannose --- mannaric acid
  20. 20. Special monosaccharides: deoxy sugars• These are monosaccharides which lack one ormore hydroxyl groups on the molecule• deoxy sugar is 2’-deoxy ribose which is thesugar found in DNA
  21. 21. Special monosaccharides: amino sugarsConstituents of mucopolysaccharides
  22. 22. D-Glucose• Most common hexose• Found in fruits, corn syrup,and honey• An aldohexose with theformula C6H12O6• Known as blood sugar in thebody• Building block for manydisaccharides andpolysaccharides
  23. 23. D-Fructose• Ketohexose C6H12O6• Differ from glucose at C1 and C2 (locationof carbonyl)• The sweetest carbohydrate (2x sucrose)• Found in fruit juices and honey• Formed from hydrolysis of sucrose• Converts to glucose in the bodyD-FructoseCH2OHCCOCHOHHHOCCH2OHOHH
  24. 24. D-Galactose• Aldohexose• Differ from D-glucose at C4• Not found in the free form innature• Obtained from lactose, adisaccharide (milk products)• Important in cellular membranesin CNSCHOCCOHCHHHOHOCCH2OHOHHHD-Galactose
  25. 25. Memorize!
  26. 26. Memorize!
  27. 27. •Most common disaccharides are•Sucrose, lactose, and maltose•Maltose hydrolyzes to 2 molecules of D-glucose•Lactose hydrolyzes to a molecule of glucose and amolecule of galactose•Sucrose hydrolyzes to a molecules of glucose and amolecule of fructoseDisaccharides
  28. 28. •galactose- -(1,4)-fructose•a semi-synthetic disaccharide (not naturallyoccurring)•not absorbed in the GI tract•used either as a laxative (Chronulac) or in themanagement of portal systemic encephalopathy(Cephulac)•metabolized in distal ileum and colon by bacteria tolactic acid, formic acid and acetic acid (removeammonia)Lactulose
  29. 29. Less common glucose disaccharidesIsomaltose (alpha 1,6)Gentiobiose (beta 1,6)Laminaribiose (beta 1,3)Cellobiose (beta 1,4)
  30. 30. CellobioseCellobiose consists of 2 molecules of glucose linked by a beta-1,4 glycosidic bondIt is usually obtained by the partial hydrolysis of cellulose
  31. 31. Trehalose is a disaccharide that occurs naturally in insects, plants, fungi, and bacteria. Themajor dietary source is mushrooms. Trehalose is used in bakery goods, beverages,confectionery, fruit jam, breakfast cereals, rice, and noodles as a texturizer, stabilizer with a lowsweetening intensityOCH2OHHO OHHHHOHOHOHHOH HHOHOHHHOH2CTREHALOSETrehalose
  32. 32. Sucralose (Splenda)About 600 times more sweet than sucrose
  33. 33. Figure 7.11Abnormal lactose metabolism.
  34. 34. Oligosaccharides• Trisaccharide: raffinose (glucose, galactoseand fructose)• Tetrasaccharide: stachyose (2 galactoses,glucose and fructose)• Pentasaccharide: verbascose (3 galactoses,glucose and fructose)• Hexasaccharide: ajugose (4 galactoses, glucoseand fructose)
  35. 35. Honey also contains glucose and fructose along withsome volatile oils
  36. 36. Polysaccharides or glycansTypes• homoglycans / homopolysaccharides (starch,cellulose, glycogen, inulin)• heteroglycans / heteropolysaccharides (gums,mucopolysaccharides)Characteristics• Polymers (MW from 200,000)• White and amorphous products (glossy)• not sweet• form colloidal solutions or suspensions
  37. 37. Tertiary structure - sterical/geometricalconformations• Rule-of-thumb: Overall shape of the chain isdetermined by geometrical relationship within eachmonosaccharide unit 1 4) - zig-zag - ribbon like 1 3) & 4) - U-turn - hollow helix 1 2) - twisted - crumpled (1 6) - no ordered conformation
  38. 38. Starch• most common storage polysaccharide inplants• composed of 10 – 30% amylose and 70-90% amylopectin depending on the source• the chains are of varying length, havingmolecular weights from several thousands tohalf a million
  39. 39. Starch• Main sources of starch are rice, corn, wheat,potatoes• A storage polysaccharide• Starch is used as an excipient, a binder inmedications to aid the formation of tablets.• Industrially it has many applications such asin adhesives, paper making, biofuel, textiles
  40. 40. Cellulose• Polymer of -D-glucose attached by (1-->4) linkages• Only digested and utilized by ruminants (cows, deers,giraffes, camels)• A structural polysaccharide• Yields glucose upon complete hydrolysis• Partial hydrolysis yields cellobiose• Most abundant of all carbohydrates• Cotton flax: 97-99% cellulose• Wood: ~ 50% cellulose• Gives no color with iodine
  41. 41. Products obtained from cellulose• Microcrystalline cellulose : used as binder-disintegrant in tablets• Methylcellulose: suspending agent and laxative• Oxidized cellulose: hemostat• Sodium carboxymethyl cellulose: laxative• Cellulose acetate: rayon; photographic film; plastics• Cellulose acetate phthalate: enteric coating• Nitrocellulose: explosives;
  42. 42. Glycogen• also known as animal starch• stored in muscle and liver (mostly)• present in cells as granules (high MW)• contains both (1,4) links and (1,6) branches atevery 8 to 12 glucose unit (more frequent than instarch)• complete hydrolysis yields glucose• hydrolyzed by both and -amylases and byglycogen phosphorylase
  43. 43. Inulin• -(1,2) linked fructofuranoses• linear only; no branching• lower molecular weight than starch• colors yellow with iodine• hydrolysis yields fructose• sources include onions,and garlic, dandelions andjerusalem artichokes• used as diagnostic agent for the renal function testJerusalem artichokes
  44. 44. Chitin• Chitin is the second mostabundant carbohydratepolymer• Present in the cell wall offungi and in theexoskeletons of crustaceans,insects and spiders• Chitin is used commerciallyin coatings (extends theshelf life of fruits and meats)
  45. 45. Dextrans• products of the reaction of glucose and the enzymetransglucosidase• contains (1,4), (1,6) and (1,3) linkages• MW: 40,000; 70,000; 75,000• used as plasma extenders (treatment of shock)• components of dental plaques
  46. 46. Dextrins• produced by the partial hydrolysis of starchalong with maltose and glucose• dextrins are often referred to as eitheramylodextrins or erythrodextrins• used as mucilages (glues)• also used in infant formulas (prevent thecurdling of milk in baby’s stomach)
  47. 47. Glycoproteins and proteoglycans• Glycoproteins are proteins conjugated tosaccharides lacking a serial repeating unit• In glycoprotein the protein>>>carbohydrate• Example include enzymes, immunoglobulins or antibodies,certain hormones• In Proteoglycans proteins are conjugated topolysaccharides with serial repeating units• Here carbohydrate>>> protein• Proteoglycans modulate cell processes and make cartilageflexible and resilient
  48. 48. Glycosaminoglycans• they are the polysaccharide chains of proteoglycans• they are linked to the protein core via a serine orthreonine (O-linked)• the chains are linear (unbranched)• the glycosaminoglycan chains are long (over 100monosaccharides)• they are composed of repeating disaccharides