Carbohydrates. TOPICS: Role & Significance of Carbohydrates Monosacharides Oligosacharides Polysacharides Glyconoconjugates

Transcription

1 Carbohydrates TOPICS: Role & Significance of Carbohydrates Monosacharides Oligosacharides Polysacharides Glyconoconjugates

2 ROLE OF CARBOHYDRATES As a major energy source for living organisms (glucose is a principal energy source in animal and plants) As a means of transporting energy ( exp: sucrose in plant tissues) As a structural material ( cellulose in plants, chitin in insects, building blocks of nucleotides). As a precursor for other biomolecules (purine, pyrimide)

3 SIGNIFICANCE OF CARBOHYDRATES Carbohydrates are the most abundant biomolecules in nature, having a direct link between solar energy and the chemical bond energy in living organisms. Source of rapid energy production Structural building blocks of cells Components of several metabolic pathways Recognition of cellular phenomena, such as cell recognition and binding (e.g., by other cells, hormones, and viruses)

4 Carbohydrates Carbohydrate : compounds contains H, C & O with the comp : (CH2O)n (Hydrate of carbon) Carbohydrates : Consist of sugar (saccharum) Sugars : compound that contains alcohol & carbonyl functional group Carbonyl func.group : >C=o Adehyde aldose Ketone ketose

5 Classification Carbohydrate Mono saccharide Oligo saccharide Poly saccharide Glyconoconjugates Glucose, fructose Ribose (aldopentose) Deoxy ribose disaccharides Glycoproteins (bacterial cell walls cellulose, chitin, starch, glycogen, glucoaminoglycans glycoproteins and proteoglycans

6 MONOSACHARIDES

7 Classification of Carbohydrates. I. Number of carbohydrate units monosaccharides: one carbohydrate unit (simple carbohydrates) disaccharides: two carbohydrate units (complex carbohydrates) trisaccharides: three carbohydrate units polysaccharides: many carbohydrate units 7

8 II. Position of carbonyl group at C1, carbonyl is an aldehyde: aldose at any other carbon, carbonyl is a ketone: ketose III. Number of carbons three carbons: triose four carbons: tetrose five carbons: pentose six carbons: hexose seven carbons: heptose etc. 8

9 Monosacharides Sub sections : Properties & classification Stereoisomers Cyclic structure Important Reactions Important monosach glycoproteins and proteoglycans Monosaccharide derivatives

10 Monosach Properties& classification Colorless, crystalline solids Soluble in water but insoluble in nonpolar solvents One of the carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group. Carbohydrates with an aldehyde (-CHO) functional group are called aldoses e.g. glyceraldehyde (CH 2 OH-CHOH-CHO) Those with a keto group (-C=O) are ketoses e.g.dihydroxyacetone (CH2OH-C=O-CH2OH) Classified according to the number of carbon atoms they contain

11 Monosacharides : Exp. aldoses & ketoses Aldotriose Aldotetrose Aldopentoses Aldohexose Ketotriose Ketotetrose Ketopentose Ketohexose

12 MONOSACCHARIDES STEREOISOMERS Isomers: same chemical formulas, different structures Conformation : the spatial arrangement of substituent groups chiral centers: asymmetric carbons, i.e carbon atom with four different substituents Enantiomers : mirror images Stereoisomers The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and has two different optical isomers, or enantiomers the projection in which the carbohydrate backbone is drawn vertically with the carbonyl shown on the top.

13 D vs L Designation D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde. The lower representations are Fischer Projections. CHO H C OH CH 2 OH D-glyceraldehyde CHO H C OH CH 2 OH D-glyceraldehyde CHO HO C H CH 2 OH L-glyceraldehyde CHO HO C H CH 2 OH L-glyceraldehyde

14 D vs L Designation For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. O H O H C C H C OH HO C H HO C H H C OH H C OH HO C H H C OH HO C H CH 2 OH CH 2 OH D-glucose L-glucose

15 D & L sugars are mirror images of one another. They have the same name, e.g., D-glucose & L-glucose. Other stereoisomers have unique names, e.g., glucose, mannose, galactose, etc. O H O H The number of stereoisomers is 2 n, where n is the number of asymmetric centers. The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L- sugars). C H C OH HO C H H C OH H C OH CH 2 OH D-glucose C HO C H H C OH HO C H HO C H CH 2 OH L-glucose

16 Pyranoses& Furanoses Pyranoses: six-membered ring compounds ( resemble pyran ) Furanoses : fivemembered rings, (resemble furan) The structure systematic names glucose & fructose become HAWORTH STRUCTURES An English chemist W.N. Haworth gave a more accurate picture of carbohydrate structure. (Ref. P.205 of textbook)

17 Cyclic structure of monosacharides in aqueous solution, monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. The new chiral center in cyclic (c1) is called anomeric carbon

18 Mutarotation. The - and -anomers are in equilibrium, and interconvert through the open form. The pure anomers can be isolated by crystallization. When the pure anomers are dissolved in water they undergo mutarotation, the process by which they return to an equilibrium mixture of the anomer. -D-Glucopyranose (64%) ( -anomer: C1-OH and CH 2 OH are cis) [ ] D acid-catalyzed mechanism: p D-Glucopyranose (36%) ( -anomer: C1-OH and CH 2 OH are trans) [ ] D

19 FISHER AND HAWORTH FORMS OF SUGAR

20 Epimers Sugars that differ only in their stereochemistry at a single carbon. => Chapter 23 20

21 SUMMARY OF SUGAR STRUCTURES ISOMERS- compounds that have the same chemical formula e.g. fructose, glucose, mannose, and galactose are isomers of each other having formula C 6 H 12 O 6. EPIMERS- refer to sugars whose configuration differ around one specific carbon atom e.g. glucose and galactose are C-4 epimers and glucose and mannose are C-2 epimers. ENANTIOMERS- a special type of isomerism found in pairs of structures that are mirror images of each other. The mirror images are termed as enantiomers and the two members are designated as D- and L- sugar. The vast majority of sugars in humans are D-sugars. CYCLIZATION OF SUGARS- most monosaccharides with 5 or more carbon atoms are predominately found in a ring form, where the aldehyde or ketone group has reacted with an alcoholic group on the same sugar group to form a hemiacetal or hemiketal ring. Pyranose ring- if the ring has 5 carbons and 1 oxygen. Furanose ring- if the ring is 5-membered (4 carbons and 1 oxygen

22 2.4.IMPORTANT REACTIONS IN MONOSACCHARIDES Monosaccharides undergo the following reactions : 1. Oxidation 2. Reduction 3. Isomerization 4. Esterification 5. Glycoside formation

23 Oxidation in presence of oxidising agents, metal ions (Cu2+) and enzymes,monosacchs undergo several oxidation reactions e.g. Oxidation of aldehyde group (R-CHO) yields aldonic acid; of terminal CH2OH (alcohol) yields uronic acid; and of both the aldehyde and CH2OH gives aldaric acid. The carbonyl groups in both aldonic and uronic can react with an OH group in the sam

24 Oxidation of Monosaccharides. C1 of aldoses can be selectively oxidized to the carboxylic acid (aldonic acids) with Br 2 or Ag(I) (Tollen s test), Cu(OH) 2 (Trommer s test), Cu(OH) 2 and tartaric acid (Trommer s test) Reducing sugars: carbohydrates that can be oxidized to aldonic acids. 24

25 Oxidation by Bromine Bromine water oxidizes aldehyde, but not ketone or alcohol; forms aldonic acid.

26 Oxidation by Tollens Reagent Tollens reagent reacts with aldehyde, but the base promotes enediol rearrangements, so ketoses react too. Sugars that give a silver mirror with Tollens are called reducing sugars. Chapter 23

27 Problem with Tollens 2-Ketoses (e.g. fructose) are also oxidized to aldonic acids in basic solution (Tollens). CH 2 OH CHOH CHO C= O (1) C-OH (2) CHOH ( CHOH) n ( CHOH) n ( CHOH) n (3) CH 2 OH CH 2 OH CH 2 OH A 2-ketose An enediol An aldose COOH CHOH ( CHOH) n CH 2 OH An aldonic acid Ketose to aldose conversion via keto enol tautomerism Oxidation Reducing sugar

28 REDUCING SUGARS All monosacchs are reducing sugars. They can be oxidised by weak oxidising agent such as Benedict s reagent Benedict's reagent is a solution of copper sulfate, sodium hydroxide, and tartaric acid. Aqueous glucose is mixed with Benedict's reagent and heated. The reaction reduces the blue copper (II) ion to form a brick red precipitate of copper (I) oxide. Because of this, glucose is classified as a reducing sugar.

29 Benedict s Test Benedict s reagent undergoes a complex colour change when it is reduced The intensity of the colour change is proportional to the concentration of reducing sugar present The colour change sequence is: Blue green yellow orange brick red

30 Nonreducing Sugars Glycosides are acetals, stable in base, so they do not react with Tollens reagent. Disaccharides and polysaccharides are also acetals, nonreducing sugars.

31 Oxidation of aldoses to aldaric acids with HNO 3. Uronic Acid: Carbohydrate in which only the terminal -CH 2 OH is oxidized to a carboxylic acid. CHO H OH HO H H OH H OH CH 2 OH D-Glucose enzyme-catalyzed oxidation H HO H H CHO OH COOH H O HO OH HO OH OH OH COOH D-Glucuronic acid (a uronic acid)

32 . Reduction of Monosaccharides. C1 of aldoses are reduced with sodium borohydride to the 1 alcohol (alditols)

33 Other alditols common in the biological world are: CH 2 OH H OH H OH CH 2 OH Erythritol CH 2 OH HO H HO H H OH H OH CH 2 OH D-Mannitol CH 2 OH H OH HO H H OH CH 2 OH Xylitol 34

34 Epimerization In base, H on C2 may be removed to form enolate ion. Reprotonation may change the stereochemistry of C2. =>

35 IMPORTANT REACTIONS (Cont) ESTERIFICATION Free OH groups of carbohydrates react with acids to form esters. This reaction an change the physical and chemical propteries of sugar.

36 Ester Formation Acetic anhydride with pyridine catalyst converts all the oxygens to acetate esters. => Chapter 23 37

37 IMPORTANT REACTIONS (Cont) GLYCOSIDE FORMATION Glycosidic Bonds The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond: R-OH + HO-R' R-O-R' + H 2 O E.g., methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose). H OH H OH HO HO H H O OH H + CH 3 -OH H 2 O HO HO H H O OH H H OH -D-glucopyranose methanol H OCH 3 methyl- -D-glucopyranose

38 Glycosidic binds are between two sugars They can either be in the or configuration and can be linked through the 1-2, 1-4 or 1-6 linkage Alpha Beta

39 Fermentation. Sugars undergo a chemical change known as fermentation in the presence of enzymes secreted by certain microorganisms. Alcoholic fermentation is commercially used in making wines, beers, and other alcoholic beverages. Lactose is the only sugar that cannot be fermented, since yeast does not contain the proper enzyme.

40 IMPORTANT MONOSACCHARIDES GLUCOSE FRUCTOSE GALACTOSE D-Glucose: D-glucose (dextrose) is the primary fuel in living cells especially in brain cells that have few or no mitochondria. Cells such as eyeballs have limited oxygen supply and use large amount of glucose to generate energy Dietary sources include plant starch, and the disaccharides lactose, maltose, and sucrose

41 DIABETES (diabetes mellitus) Characterized by high blood glucose levels that splills over into the urine These high glucose levels impairs the insulin-stimulated glucose entry into cells and starve the cells of insulin. This leads to ketosis or high levels of ketone bodies (acids) that hinders the buffering capacity of the blood in the kidney, which controls blood ph (by excreting excess H+ ions into the urine). The H+ excretion is accompanied by the excretion ammonia, sodium, potassium, and phosphate ions causing severe dehydration This leads to excessive thirst symptom of diabetes and lifethreatening decrease in blood volume.

42 Important monosaccharides. Cont FRUCTOSE D-fructose (levulose) is often referred as fruit sugar and is found in some vegetables and honey This molecule is an important member of ketose member of sugars It is twice as sweet as sucrose (per gram basis) and is used as sweeting agent in processed food products

43 Important monosaccharides. Cont... GALACTOSE is necessary to synthesize a variety of biomolecules (lactose-in mammalary glands, glycolipids, certain phospholipids, proteoglycans, and glycoproteins) Galactose and glucose are epimers at carbon 4 and interconversion is catalysed by enzyme epimerase. Medical problems galactosemia (genetic disorder) where enzyme to metabolize galactose is missing; accumulation of galactose in the body can cause liver damage, cataracts, and severe mental retardation

44 MONOSACCHARIDE DERVATIVES URONIC ACIDS formed when terminal CH 2 OH group of a mono sugar is oxidised Important acids in animals D- glucuronic acid and its epimer L- iduronic acid In liver cells glucuronic acid combines with steroids, certain drugs, and bilirubin to improve water solubility therby helping the removal of waste products from the body These acids are abundant in the connective tissue carbohydrate components.

45 Mono sugar derivatives AMINO SUGARS Sugars in which a hydroxyl group (common on carbon 2) is replaced by an amino group e.g. D-glucosamine and D-galactosamine common constituents of complex carbohydrate molecule found attached to cellular proteins and lipids Amino acids are often acetylated e.g. N-acetyl-glucosamine.

46 Mono sugar derivatives DEOXYSUGARS monosaccharides in which an - H has replaced an OH group Important sugars: L-fucose (formed from D-mannose by reduction reactions) and 2- deoxy-d-ribose L-fucose found among carbohydrate components of glycoproteins, such as those of the ABO blood group determinates on the surface of red blood cells 2-deoxyribose is the pentose sugar component of DNA.

47 GLYCOSIDES Monosaccharides can be linked by glycosidic bonds (joining of 2 hydroxyl groups of sugars by splitting out water molecule) to create larger structures. Disaccharides contain 2 monosaccharides e.g. lactose (galactose+glucose); maltose (glucose+glucose); sucrose (glucose+fructose) Oligosaccharides 3 to 12 monosaccharides units e.g. glycoproteins Polysaccharides more than 12 monosaccharides units e.g. glycogen (homopolysaccharide) having hundreds of sugar units; glycosaminoglycans (heteropolysaccharides) containing a number of different monosaccharides species.

48 DISACCHARIDES AND OLIGOSACCHARIDES

49 DISACCHARIDES AND OLIGOSACCHARIDES Cnfigurations: alfa or beta ( 1,4, glycosidic bonds or linkages; other linkages 1,1; 1,2; 1,3; 1,6) Digestion aided by enzymes. Defficiency of any one enzyme causes unpleasant symptoms (fermentation) in colon produces gas [bloating of cramps]. Most common defficiency, an ancestoral disorder, lactose intolerance caused by reduced synthesis of lactase

50 Important sugars of Disaccharides LACTOSE (milk sugar) disaccharide found in milk; composed of one molecule of galactose and glucose linked through beta(1,4) glycosidic linkage; because of the hemiacetal group of the glucose component, lactose is a reducing sugar

51 Lactose intolerance Lactose (milk sugar) in infants is hydrolyzed by intestinal enzyme lactase to its component monosacch for absorption into the bloodstream (galactose epimerized to glucose). Most adult mammals have low levels of betagalactosidase. Hence, much of the lactose they ingest moves to the colon, where bacterial fermentation generates large quantities of CO2, H2 and irritating organic acids. These products cause painful digestive upset known as lactose intolerance and is common in the African and Asian decent.

52 MALTOSE ( malt sugar) An intermediate product of starch hydrolysis; it is a disaccharide with an alfa(1,4) glycosidic linkage between two D-glucose molecules; in solution the free anomeric carbon undergoes mutarotation resulting in an equilibrium mixture of alfa and beta maltoses; it does not occur freely in nature

53 SUCROSE common table sugar: cane sugar or beet sugar produced in the leaves and stems of plants; it is a disaccharide containing both alfa-glucose and betafructose residues linked by alfa,beta(1,2)glycosidic bond.

54 CELLOBIOSE degradation product of cellulose containing two molecules of glucose linked by a beta (1,4) glycosidic bond; it does not occur freely in nature

55 Reducing sugars: carbohydrates that can be oxidized to aldonic acids. cellobiose and maltose are reducing sugar lactose is a reducing sugar sucrose is not a reducing sugar 58

56 OLIGOSACCHARIDE SUGARS Oligosaccharides are small polymers often found attached to polypeptides in and some glycolipids. They are attached to membrane and secretory proteins found in endoplasmic reticulum and Golgi complex of various cells Two classes: N-linked and O-linked

57 POLYSACCHARIDES Intro to Polysaccharides Classification of Polisacharides Homosacharides Heteropolysacharides

58 Intro to Polysaccharides Composed of large number of monosaccharide units connected by glycosidic linkages Classified on the basis of their main monosaccharide components and the sequences and linkages between them, as well as the anomeric configuration of linkages, the ring size (furanose or pyranose), the absolute configuration (D- or L-) and any other substituents present. Polysaccharides are more hydrophobic if they have a greater number of internal hydrogen bonds, and as their hydrophobicity increases there is less direct interaction with water Divided into homopolysaccharides (e.g.starch, glycogen, cellulose, and chitin) & heteropolysaccharides (glycoaminoglycans or GAGs, murein).

59 Classification of Polisacharides

60 HOMOSACCHARIDES Found in abundance in nature Important examples: starch, glycogen, cellulose, and chitin Starch, glycogen, and cellulose all yield D- glucose when they are hydrolyzed Cellulose - primary component of plant cells Chitin principal structural component of exoskeletons of arthropods and cell walls of many fungi; yield glucose derivative N-acetyl glucosamine when it is hydrolyzed.

61 STARCH (Homosaccharide) A naturally abundant nutrient carbohydrate, (C 6 H 10 O 5 )n, found chiefly in the seeds, fruits, tubers, roots, and stem pith of plants, notably in corn, potatoes, wheat, and rice, and varying widely in appearance according to source but commonly prepared as a white amorphous tasteless powder. Any of various substances, such as natural starch, used to stiffen cloth, as in laundering. Two polysaccharides occur together in starch: amylose and amylopectin

62 H OH CH 2 OH H OH H O H OH 6CH 2 OH CH 2 OH 5 H H O H H O H H 1 4 OH H 1 OH H O O 3 2 H OH H OH amylose CH 2 OH CH 2 OH H H O H H O H H OH H OH H O O H OH H OH H OH Polysaccharides: Plants store glucose as amylose or amylopectin, glucose polymers collectively called starch. Glucose storage in polymeric form minimizes osmotic effects. Amylose is a glucose polymer with (1 4) linkages. The end of the polysaccharide with an anomeric C1 not involved in a glycosidic bond is called the reducing end.

63 -amylose Amylose is poorly soluble in water, but forms micellar suspensions In these suspensions, amylose is helical

64 H OH CH 2 OH H OH H O H OH CH 2 OH H H O H OH H O H OH H 1 O amylopectin H OH CH 2 OH H OH H O H OH CH 2 OH 6CH 2 H H O H H 5 O H H OH H 4 OH H O O 3 2 H OH H OH CH 2 OH CH 2 OH H H O H H O H H 1 4 OH H OH H O O H OH H OH H OH Amylopectin is a glucose polymer with mainly (1 4) linkages, but it also has branches formed by (1 6) linkages. Branches are generally longer than shown above. The branches produce a compact structure & provide multiple chain ends at which enzymatic cleavage can occur.

65

66 GLYCOGEN (Homosaccharide) Glycogen is the storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles. Structurally, glycogen is very similar to amylopectin with alpha acetal linkages, however, it has even more branching and more glucose units are present than in amylopectin. Various samples of glycogen have been measured at 1, ,000 units of glucose. The structure of glycogen consists of long polymer chains of glucose units connected by an alpha acetal linkage. The branches are formed by linking C1 to a C6 through an acetal linkages. In glycogen, the branches occur at intervals of 8-10 glucose units, while in amylopectin the branches are separated by glucose units.

67 STRUCTURE OF GLYCOGEN

68 GLYCOGEN (Homosaccharide)

69 CELLULOSE (Homosaccharide) Cellulose is found in plants as microfibrils (2-20 nm diameter and nm long). These form the structurally strong framework in the cell walls. Cellulose is mostly prepared from wood pulp Cellulose is a linear polymer of β-(1 4)-D-glucopyranose units in 4 C 1 conformation. The fully equatorial conformation of β-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility Cellulose has many uses as an anticake agent, emulsifier, stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water. Water cannot penetrate crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible. Purified cellulose is used as the base material for a number of water-soluble derivatives e.g. Methyl cellulose, carbomethycellulose

70 Cellulose as polymer of β-d-glucose

71 Cellulose as polymer of β-d-glucose

72 CHITIN (Homosaccharide) Chitin is a polymer that can be found in anything from the shells of beetlesto webs of spiders. It is present all around us, in plant and animal creatures. It is sometimes considered to be a spinoff of cellulose, because the two are very molecularly similar. Cellulose contains a hydroxy group, and chitin contains acetamide. Chitin is unusual because it is a "natural polymer," or a combination of elements that exists naturally on earth. Usually, polymers are man-made. Crabs, beetles, worms and mushrooms contain large amount of chitin. Chitin is a very firm material, and it help protect an insect against harm and pressure

73 Structure of the chitin molecule, showing two of the N- acetylglucosamine units that repeat to form long chains in beta- 1,4 linkage.

74 Chitin is a very firm material, and it help protect e.g. an insect against harm and pressure

75 CHITOSAN Chitin, the polysaccharide polymer from which chitosan is derived, is a cellulose-like polymer consisting mainly of unbranched chains of N-acetyl-D-glucosamine. Deacetylated chitin, or chitosan, is comprised of chains of D-glucosamine. When ingested, chitosan can be considered a dietary fiber. Chitosan can be used within the human body to regulate diet programs, and researchers are looking into ways in which it can sure diseases.

76 CHITOSAN

77 HETEROPOLYSACCHARIDES Are high-molecular-weight carbohydrate polymers more than one kind of monosaccharide Important examples include glycosaminoglycans (GAGs) the principle components of proteoglycans and murein, a major component of bacterial cell walls.

78 GAGs - high-molecular-weight carbohydrate polymers Glycosaminoglycans forming the proteoglycans are the most abundant heteropolysaccharides in the body. They are long unbranched molecules containing a repeating disaccharide unit. Usually one sugar is an uronic acid (either D-glucuronic or L- iduronic) and the other is either GlcNAc or GalNAc. One or both sugars contain sulfate groups (the only exception is hyaluronic acid). GAGs are highly negatively charged what is essential for their function.

79 THE SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE Hyaluronic acid Occurence : synovial fluid, ECM of loose connective tissue Hyaluronic acid is unique among the GAGs because it does not contain any sulfate and is not found covalently attached to proteins. It forms non-covalently linked complexes with proteoglycans in the ECM. Hyaluronic acid polymers are very large (100-10,000 kd) and can displace a large volume of water.

80 Hyaluronic acid (D-glucuronate + GlcNAc)

81 Dermatan sulfate (L-iduronate + GlcNAc sulfate) Occurence : skin, blood vessels, heart valves

82 Heparin and heparan sulfate (D-glucuronate sulfate + N-sulfo-D-glucosamine) Heparans have less sulfate groups than heparins Occurence : Heparin :component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin Heparan sulfate : basement membranes, component of cell surfaces

83 Keratan sulfate ( Gal + GlcNAc sulfate) Occurence : cornea, bone, cartilage ; Keratan sulfates are often aggregated with chondroitin sulfates.