Carbohydrates of physiological significance - PowerPoint PPT Presentation

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Carbohydrates of physiological significance
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Carbohydrates of physiological significance

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  1. Carbohydrates of physiological significance

  2. Definition of carbohydrates • Carbohydrates may be defined as polyhydroxyaldehydes or ketones or compounds which produce them on hydrolysis

  3. Functions of Carbohydrates • Abundant dietary source of energy • Precursors for many organic compounds • Participate in the structure of cell membrane & cellular functions (as glycoproteins & glycolipids) • Structural components (cell wall in plants, exoskeleton in insects etc) • Storage energy to meet immediate demand

  4. Carbohydrates- widely distributed in plants and animals; having important structural and metabolic roles. In plants- glucose synthesized from carbon dioxide and water by photosynthesis and stored as starch or converted to the cellulose of the plant framework. In animals- can synthesize carbohydrate from lipid glycerol and amino acids, but most animal carbohydrate is derived ultimately from plants.

  5. CLASSIFICATION OF CARBOHYDRATES • Monosaccharides – simpler unit of carbohydrate eg.glucose, fructose, sucrose etc… • (2) Disaccharides - condensation products of two monosaccharide units e.g. maltose and sucrose. • (3) Oligosaccharides - condensation products of three to ten monosaccharides e.g. maltotriose, raffinose • (4) Polysaccharides - condensation products of more than ten monosaccharide units e.g. starch, glycogen, cellulose, dextrin etc, which may be linear or branched polymers.

  6. MONOSACHARIDES • Monosaccharides– those carbohydrates that cannot be hydrolyzed into simpler carbohydrates: aldehyde or ketones that have two or more hydroxyl groups. • Empirical formula-(C-H2O)n .literally ‘ CARBON HYDRATE’

  7. Structure of a simple aldose and a simple ketose

  8. Glucose -the most important carbohydrate: • the major metabolic fuel of mammals • the precursor for synthesis of other carbohydrates in the body, including a universal fuel of the fetus. • glycogen for storage; • ribose and deoxyribose in nucleic acids; • galactose in lactose of milk, • in glycolipids, in combination with protein in glycoproteins Diseases associated with carbohydrate metabolism - diabetes mellitus, glucosuria, glycogen storage diseases, and lactose intolerance.

  9. BIOMEDICALLY, GLUCOSE IS THE MOST IMPORTANT MONOSACCHARIDE A- Fischer projections-H and OH groups attached to the carbon atoms in a straight chain. B- Haworth projections- if the molecule is viewed from the side and above the plane of the ring. By convention, bonds nearest to the viewer are bold and thickened. C- Chair conformation: The six-member ring containing one oxygen atom is in the form of a chair

  10. Sugars Exhibit Various Forms of Isomerism- Glucose, with four asymmetric carbon atoms ( 2n ),can form 16 isomers. D and L isomerism: the D form or of its mirror image L form (enantiomers) is determined by it spatial relationship to the parent compound of the carbohydrates- Glyceraldehyde. Tetroses, pentoses, hexoses having multiple asymetric carbons exist as diastreoisomers- isomers that are not mirror images of each other.

  11. OPTICAL ACTIVITY Optical activity occurs due to asymmetric carbon atoms (chiral carbon): those bonded to four different atoms or groups of atoms When a beam of plane-polarized light is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+); or to the left, levorotatory (−). When equal amounts of D and L isomer - no optical activity – mixture is called racemic mixture .

  12. Pyranose and furanose ring structures:The stable ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a 5-membered ring) For glucose in solution, > 99% is in the pyranose form.

  13. Pyranose and furanose forms of fructose

  14. ANOMERIC FORMS: (α and β anomers) hemiacetal - formed by combination of an aldehyde and an alcohol group. Similarly the ring structure of a ketose is a hemiketal. Crystalline glucose is α-D-glucopyranose. The cyclic structure is retained in solution, but isomerism occurs about position 1, the carbonyl or anomeric carbon atom, to give a mixture of α-glucopyranose (38%) and β-glucopyranose (62%).

  15. Epimers: Isomers differing as a result of variations in configuration of the -OH and -H on carbon atoms 2, 3, and 4 of glucose e.g. mannose and galactose, formed by epimerization at carbons 2 and 4, respectively

  16. Oxidation reactions • Aldoses may be oxidized to 3 types of acids • Aldonic acids: aldehyde group is converted to a carboxyl group ( glucose – gluconic acid) • Uronic acids: aldehyde is left intact and primary alcohol at the other end is oxidized to COOH • Glucose --- glucuronic acid • Galactose --- galacturonic acid • Saccharic acids (glycaric acids) – oxidation at both ends of monosaccharide) • Glucose ---- saccharic acid • Galactose --- mucic acid • Mannose --- mannaric acid

  17. Reduction reactions • either done catalytically (hydrogen and a catalyst) or enzymatically • the resultant product is a polyol or sugar alcohol • glucose form sorbitol • mannose forms mannitol • fructose forms a mixture of mannitol and sorbitol

  18. Some important carbohydrates Trioses of physiological importance : - both D-glyceraldehyde and dihydroxyacetone (in phosphate esters form)-intermediate in glycolysis Tetroses of physiological importance: - erythrose-4-P ; an intermediate in HMP shunt

  19. Pentoses of physiological importance

  20. Hexoses of physiological importance

  21. Sugars as reducing agents- Oxidation of the anomeric carbon of glucose and other sugars is the basis for Fehling’s reaction. The cuprous ion (Cu) produced under alkaline conditions forms a red cuprous oxide precipitate. In the hemiacetal (ring) form, C-1 of glucose cannot be oxidized by Cu2+. However, the open-chain form is in equilibrium with the ring form, and eventually the oxidation reaction goes to completion.

  22. Disacharides • Disacharides are the molecules which consist of two monosacharaides units held together by a glycosidic bond • These are of two types: • Reducing disacharides (having free aldehyde or keto group) e.g. maltose, lactose • Non-reducing disacharides (having no free aldehyde or keto group) e.g. sucrose

  23. An -OH (alcohol) of one glucose (right) condenses with intramolecular hemiacetal of the other glucose (left), with elimination of H2O and formation of an O-glycosidic bond. The reversal of this reaction is hydrolysis—attack by H2O on the glycosidic bond. The maltose molecule retains a reducing hemiacetal at the C-1 not involved in the glycosidic bond.

  24. Malt-sugar

  25. Milk-sugar

  26. Table-sugar

  27. MALTOSE, SUCROSE, & LACTOSE ARE IMPORTANT DISACCHARIDES Lactase and sucrase deficiencies- malabsorption leads to diarrhea and flatulence.

  28. Sugars Form Glycosides With Other Compounds & With Each Other • Glycosides – • formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. • If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an -OH group) and an-other -OH group. • If the hemiacetal portion is glucose, the resulting compound is a glucoside; if galactose, a galactoside.

  29. Sugars Form Glycosides With Other Compounds & each Other • If the second group is an amine, an N-glycosidic bond is formed, e.g. between adenine and ribose in nucleotides such as ATP . • Glycosides are widely distributed in nature; the aglycone may be methanol, glycerol, a sterol, a phenol, or a base such as adenine. • The glycosides that are important in medicine because of their action on the heart (cardiac glycosides) all contain steroids as the aglycone. • These include derivatives of digitalis and strophanthus such as ouabain, an inhibitor of the Na+-K+ ATPase of cell membranes and antibiotics such as streptomycin.

  30. Deoxy Sugars Lack an Oxygen Atom a hydroxyl group has been replaced by hydrogen--deoxyribose in DNA.

  31. Oligosaccharides • Trisaccharide: raffinose (glucose, galactose and fructose) • Tetrasaccharide: stachyose (2 galactoses, glucose and fructose) • Pentasaccharide: verbascose (3 galactoses, glucose and fructose) • Hexasaccharide: ajugose (4 galactoses, glucose and fructose)

  32. Structures of some oligosaccharides starch

  33. Structures of some oligosaccharides

  34. Structures of some oligosaccharides

  35. Oligosaccharides occur widely as components of antibiotics derived from various sources


  37. Definition & Classification • Polysacharides are linear as well as branched chain polymers of monosacharides or their derivatives, held together by glycosidic bonds • Polysacharides are of two types • Homopolysacharides: which on hydrolysis yield only a single type of monosacharide • Heteropolysacharides: which on hydrolysis yield a mixture of a few monosacharides or their derivatives

  38. Polysaccharides or glycans • homoglycans (starch, cellulose, glycogen, inulin) • heteroglycans (gums, mucopolysaccharides) • functions: serve storage and structural function • characteristics: • polymers (MW from 200,000) • White and amorphous products (glassy) • not sweet • not reducing; do not give the typical aldose or ketose reactions) • form colloidal solutions or suspensions

  39. HOMOPOLYSACHARIDES • When the polysacharides are composed of same types of monosacharides or their derivatives, they are referred to as homopolysacharides or homoglycans

  40. Starch • most common storage polysaccharide in plants • composed of 10 – 30% a-amylose and 70-90% amylopectin depending on the source • the chains are of varying length, having molecular weights from several thousands to half a million

  41. Amylose Amylosehas a non-branching helical structure composed of glucose residues

  42. Amylopectin amylopectin consists of branched chains composed of 24–30 glucose residues united by 1 → 4 linkages in the chains and by 1 → 6 linkages at the branch points.

  43. Amylose and amylopectin are the 2 forms of starch. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues

  44. suspensions of amylose in water adopt a helical conformation iodine (I2) can insert in the middle of the amylose helix to give a blue color that is characteristic and diagnostic for starch

  45. Glycogen • also known as animal starch • stored in muscle and liver • present in cells as granules (high MW) • contains both a(1,4) links and a(1,6) branches at every 8 to 12 glucose unit • complete hydrolysis yields glucose • glycogen and iodine gives a red-violet color • hydrolyzed by both a and b-amylases and by glycogen phosphorylase

  46. Glycogen is highly branched structure with chains of 12–14 α-D-glucopyranose residues (in α[1 → 4]-glucosidic linkage), with branching by α(1 → 6)-glucosidic bonds. A: General structure. B: Enlargement of structure at a branch point.

  47. Inulin • b-(1,2) linked fructofuranoses • linear only; no branching • lower molecular weight than starch • colors yellow with iodine • hydrolysis yields fructose • sources include onions, garlic, dandelions and jerusalem artichokes • used as diagnostic agent for the evaluation of glomerular filtration rate (renal function test) Jerusalem artichokes

  48. Chitin • Chitin is the second most abundant carbohydrate polymer • consists of N-acetyl-D-glucosamine units joined by β (1 →4)-glycosidic linkages • Present in the cell wall of fungi and in the exoskeletons of crustaceans, insects and spiders • Chitin is used commercially in coatings (extends the shelf life of fruits and meats)

  49. Dextrins • produced by the partial hydrolysis of starch along with maltose and glucose • dextrins are often referred to as either amylodextrins, erythrodextrins or achrodextrins • used as mucilages (glues) • also used in infant formulas (prevent the curdling of milk in baby’s stomach)