Lec 5 level 3-de(chemistry of carbohydrates)


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Lec 5 level 3-de(chemistry of carbohydrates)

  1. 1. Dental Biochemistry 1- (5) Chemistry and digestion of carbohydrates1
  2. 2. DEFINITION Carbohydrates are organic substances composed of carbon, hydrogen and oxygen.2
  3. 3. Function of carbohydrates:• Carbohydrates are the main sources of energy in the body. Brain cells and RBCs are almost wholly dependent on carbohydrates as the energy source. Energy production from carbohydrates will be 4 Kcal/g.• Storage form of energy (starch and glycogen).• Excess carbohydrate is converted to fat.• Glycoproteins and glycolipids are components of cell membranes and receptors, 3
  4. 4. • Structural unit of many organisms: Cellulose of plants; exoskeleton of insects, cell wall of microorganisms, mucopolysaccharides as ground substance in higher organisms.• Important part of nucleic acids and free nucleotides and coenzyme.• Major antigens are carbohydrates in nature, e.g., blood group substance.• Has a biological role as a part of hormones and their receptors and enzymes. 4
  5. 5. Nomenclature• Molecules having only one actual or potential sugar group [(CH2O)n] are called monosaccharides (e.g. C6H12O6); they cannot be further hydrolyzed into smaller units.• When two monosaccharides are combined together with elimination of a water molecule, it is called a disaccharide (e.g. C12H22O11).• Trisaccharides contain three sugar groups. Further addition of sugar groups will correspondingly produce tetrasaccharides, pentasaccharides and so on, commonly known as oligosaccharides. 5
  6. 6. • When more than 10 sugar units are combined, they are generally named as polysaccharides.• Polysaccharides having only one type of monosaccharide units are called homopolysaccharides and those having different monosaccharide units are called heteropolysaccharides. 6
  7. 7. MonosaccharidesThey have the common formula(CH2O)n, where n = 3 or somelarger number7
  8. 8. CLASSIFICATION OF MONOSACCHARIDES Can be carried out by one of two methods:1) According to the number of carbon atoms : Trioses, Tetroses, Pentoses, Hexoses, Heptoses, Octoses. 8
  9. 9. • 2) According to the characteristic carbonyl group:Aldehyde group or ketone group a) Aldo sugars: Aldoses :Monosaccharides containing aldehyde group e.g. glucose, ribose, erythrose and glyceraldehyde. b) Keto sugars: Ketoses :Monosaccharides containing ketone group e.g. fructose, ribulose and dihydroxyacetone. 9
  10. 10. 10
  11. 11. STEREOISOMERS• Compounds having same structural formula, but differ in spatial configuration are known as stereoisomers.• All monosaccharides can be considered as molecules derived from glyceraldehyde.• Depending on the configuration of Hand OH around the reference carbon atom, the two mirror forms are designated as Land D forms.• All naturally occurring sugars are D sugars. 11
  12. 12. 12
  13. 13. Epimerism of Aldoses• When sugars are different from one another, only in configuration with regard to a single carbon atom (other than the reference carbon atom), they are called epimers.• For example, glucose and mannose13
  14. 14. Anomerism of Sugars• This is explained by the fact that D-glucose has two anomers, alpha and beta varieties. 14
  15. 15. Monosaccharides of physiologic importance1-Pentoses:• They are sugars containing 5 carbon atoms. Ribose is a constituent of RNA while deoxyribose is seen in DNA.• Ribose is also seen in co-enzymes such as ATP and NAD.15
  16. 16. 16
  17. 17. Hexoses Glucose, Mannose and Galactose• They are the common aldohexoses.• Glucose is the sugar in human blood. It is the major source of energy.• Mannose is a constituent of many glycoproteins. Mannose was isolated from plant mannans; hence the name. 17
  18. 18. b) D-Fructose :( Fruit sugar = Levulose). It is found in fruits, honey and obtained from sucrose and inulin by hydrolysis.c)D-Galactose :It is obtained from hydrolysis of lactose (milk sugar). .It is a constituent of galactolipids, glycoprotein. 18
  19. 19. 19
  20. 20. DlSACCHARIDES• When two monosaccharides are combined together by glycosidic linkage, a disaccharide is formed. The important disaccharides are sucrose, maltose, isomaltose and lactose.20
  21. 21. 1. Sucrose• It is the sweetening agent known as cane sugar. It is present in sugarcane and various fruits. Hydrolysis of sucrose will produce one molecule of glucose and one molecule of fructose.• ii. The enzyme producing hydrolysis of sucrose is called sucrase.• Sucrose is not a reducing sugar. 21
  22. 22. 2. Lactose• Reducing disaccharide.• Capable of forming osazone.• It is only found in milk.• By acid or lactase enzyme in the intestine, it yields D-galactose and D-glucose 22
  23. 23. 3. Maltose• Maltose contains two glucose residues with alpha1,4 linkage.• It is found in germinating cereals and malt.• Intermediate product of the action of amylases on starch. It is reducing disaccharide• Hydrolysis: By acid or by maltase enzyme intotwo D-glucose units. 23
  24. 24. 4. Isomaltose• It is also a reducing sugar. It contains 2 glucose units combined in alpha -1, 6 linkage.• Partial hydrolysis of glycogen and starch produces isomaltose.24
  25. 25. POLYSACCHARIDES• These are polymerized products of many monosaccharide units.• They may be homoglycans composed of single kind of monosaccharides, e.g. starch. glycogen and cellulose.• Heteroglycans are composed of two or more different monosaccharides.25
  26. 26. 1. Starch• It is the reserve carbohydrate of plant as in potatoes, rice, wheat. 10-20% is soluble part called amylose. The insoluble part is called amylopectin.• Amylose is made up of glucose units with alpha- 1,4 glycosidic linkages to form an unbranched long chain.• Amylopectin is also made up of glucose units, but is highly branched with molecular weigh more than 1 million. The branching points are made by alpha-1,6 linkage (similar to isomaltose) . 26
  27. 27. 2. Glycogen• It is the reserve carbohydrate in animals. It is stored in liver and muscle. About 5% of weight of liver is made up by glycogen.• Glycogen is composed of glucose units joined by alpha-1 ,4 and alpha-1 ,6 glycosidic linkages.3. Cellulose• It is the chief carbohydrate in plants.• The enzyme act on hydrolysis of cellulose is absent in animal and human digestive system, and hence cellulose cannot be digested. 27
  28. 28. 4. Inulin• It is the reserve carbohydrate present in onion, garlic, etc.• 5. Chitin• It is present in exoskeletons of crustacea and insects 28
  29. 29. B- Heteroglycan:These are polysaccharides containing more than one type of sugar residues.1.Agar:• It is prepared from sea weeds and contains galactose, glucose and other sugars.• It is used as a supporting medium for electrophoresis.• 2. Mucopolysaccharides:• Mucopolysaccharides or glycosaminoglycans (GAG) are carbohydrates containing uronic acid and amino sugars. 29
  30. 30. 3. Hyaluronic acid:• It is present in connective tissues, tendons,4. Heparin:• It is an anticoagulant• 5. Keratan sulphate:• It is found in cornea and tendons. 30
  31. 31. Glycoproteins and Mucoproteins• When the carbohydrate chains are attached to a polypeptide chain it is called a proteoglycan.• If the carbohydrate content is less than 10%, it is generally named as a glycoprotein.• If the carbohydrate content is more than 10% it is a mucoprotein.• They are seen in almost all tissues and cell membranes. 31
  32. 32. Digestion of carbohydrates• Cooking helps in breaking of glycosidic linkages in polysaccharides and thus makes the digestion process easier.• In the diet carbohydrates are available as polysaccharides (starch, glycogen), and to a minor extent, as disaccharides (sucrose and lactose). 32
  33. 33. • This process of digestion starts in mouth by the salivary alpha-amylase. However, the time available for digestion in the mouth is limited. The gastric hydrochloric acid will inhibit the action of salivary amylase.• In the pancreatic juice another alpha-amylase is available which will hydrolyze the alpha-1,4 glycosidic linkages randomly, so as to produce smaller subunits like maltose, isomaltose, dextrin and oligosaccharides. 33
  34. 34. • The intestinal juice (succus entericus) and brush border of intestinal cells contain enzymes, which will hydrolyze disaccharides into component monosaccharaides.• These enzymes are sucrase, maltase, isomaltase and lactase.• The monosaccharides are then absorbed.34
  35. 35. Lactose intolerance• This is produced by the deficiency of lactase. This enzyme hydrolyses lactose to glucose and galactose.• In this condition, lactose accumulate in the gut produce irritant diarrhea.• If milk is withdrawn temporarily, the diarrhea will be limited. Curd is also an effective treatment, because the lactobacilli present in curd contains the enzyme lactase.• Lactase activity is high during infancy and it decreases to adult levels by 5-7 years of age. 35
  36. 36. Absorption of carbohydrates• Only monosaccharides are absorbed by the intestine. Minute quantities of disaccharides that may be absorbed, are immediately eliminated through kidneys.• The duodenum and upper jejunum absorb the bulk of the dietary sugars.• Insulin is not required for the uptake of glucose by intestinal cells. 36
  37. 37. • However, different sugars have different mechanisms of absorption.1. galactose and glucose are transported into the mucosal cells by an active, energy-requiring process that involves a specific transport protein and requires a concurrent uptake of sodium ions (sodium-dependent monosaccharide transporter SGluT). 2. Fructose uptake requires a sodium-independentmonosaccharide transporter (GLUT-5) for itsabsorption.3. All three monosaccharides are transported fromthe intestinal mucosal cell into the portal circulationby yet another transporter, GLUT-2.37
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