Presentation on theme: "Lecture 5 Carbohydrates and Glycobiology"— Presentation transcript:
1 Lecture 5 Carbohydrates and Glycobiology Life Is Sweet
2 Outline Definitions Classification Structure & Function of Carbohydrates
3 1. IntroductionCarbohydrate, as the name implies, consist of carbon, hydrogen, and oxygen.Hydrate=(water) hydrogen and oxygen.The basic formula for carbohydrates is CH2O, meaning that there is one carbon atom, two hydrogen atoms, and one oxygen atom as the ratio in the structure of carbohydratesWhat would be the formula for a carbohydrate that has 3 carbons?
4 (1) Where Do Carbohydrates Come From? Plants take in Carbon dioxide (CO2), water (H2O) and energy from the sun and make glucose– photosynthesis.6CO2+6H2O+energy (from sunlight) C6H12O6+6O2
5 (2) Functions Provide Energy structural components cellulose in plants and chitin in arthropods.Transfer to other life molecules such as lipids and proteinInformational molecules for recognition
6 (3) NomenclatureCarbohydrates are polyhydroxy aldehydes or ketones, or their derivatives.
7 (4) Classifying Carbohydrates Scientist use the word saccharides to describe sugars.Monosaccharides (simple sugers):only one sugar molecule, cannot be hydrolyzed to simpler sugars,eg：glucose, fructose and galactoseOligosaccharides:'a few' ( from two to ten monosaccharide units )covalently linked monosaccharideseg：sucrose, maltose, lactosepolysaccharides:'many' covalently linked monosaccharideseg：Starch, Glycogen, Cellulose (a form of fiber)
8 Glycoconjugates Also called complex saccharide Polysaccharide linked to proteins or lipids includingProteoglycansGlycoproteinsGlycolipids
9 2. Monosaccharide(1) Monosacchrides can be divided into two families: aldoses and ketoses.aldehyde groupketo groupD-Glucose,an aldohexoseD-Fructose,a ketohexose
10 Monosaccharides can be classified according to the number of carbon atoms they contain 3 carbons : triose.4 carbons : tetrose.5 carbons : pentose.6 carbons : hexose.7 carbons : heptose.
11 3 carbons : Triose The simplest aldose is glyceraldehyde The simplest ketose is dihydroxyacetone
13 6 carbons : HexoseThe most abundant monosaccharide in nature is the six-carbon sugar D-glucose.
14 (2) Configuration of monosaccharide Monosaccharides have asymmetric centers: -chiral carbon atom except dihydroxyacetoneOpticity: D- (Dextrorotatory) and L- (Levorotatory)Glyceraldehyde is conventionally used as the standard for defining D and L configurationsenantiomers
15 D- and L-glucoseD- and L-fructoseEnantiomers: mirror images of each other, D- and L-sugars.
16 D-Sugars predominate in nature (in living organisms) (e. g D-Sugars predominate in nature (in living organisms) (e.g., D-ribose, D-glucose, D-galactose, D-mannose, D-fructose)Each stereoisomer has a different conventional name, ending with “-ose” suffixKetoses are often named by inserting an “ul” into the name of the corresponding aldoses (e.g., aldopentose is named as ribose, the ketopentose is named as ribulose
17 Stereoisomers of D-family of aldose Epimer: Two sugars that differ only by the configuration around one carbon.
19 (3) Cyclization of monosaccharide Less than 1% of each of the monosaccharides exists in open-chain (acyclic) form (Fischer Projection).Rather, they are predominantly found in a ring form (Haworth Projection).
20 The aldehyde or ketone group can react with a hydroxyl group to form a covalent bond. An aldehyde reacts with a hydroxyl group creates a hemiacetal.A ketone reacts with a hydroxyl group to form a hemiketal.
21 Conversion of a linear form to a Haworth projection anomeric carbonanomer-OH group of anomeric carbon (C1) and –CH2OH group (C6) :On the contrary-αSame side - β
22 D-GlucoseFuranose form(five-membered ring)Pyranose form(six-membered ring)D-Glucose can cyclize in two ways, forming either furanose or pyranose structures.
23 In general, the pyranose form is favored over the furanose ring for aldohexose sugars.
27 Fructose also forms pyranose rings. The pyranose form predominates in fructose free in solution, whereas the furanose form is the major one in most of its derivatives.
28 Notice Terminology describing sugar structure Enantiomers Epimers D- and L-Epimersdiffer only by the configuration around one carbonAnomersα and β configuration
29 brick-red precipitates (4) Reducing sugarSugars that can reduce Fe3+ or Cu2+ ion are called reducing sugars. The carbonyl group is oxidized to carboxyl group.Monosaccharides are reducing sugars.In disaccharides or polysaccharides, the end of a chain with a free anomeric carbon is called the reducing end.brick-red precipitates
30 Fructose is a ketose that changes to aldose in a basic solution. a ketone cannot be oxidized directly, a keto sugar can be converted to an aldehyde to migrate the carbonyl to the end of the chain.
31 Benedict's reagent and Fehling's reagent are used to test for the presence of a reducing sugar. Fehling reagents: copper(II) sulfate (CuSO4)、sodium hydroxide(NaOH)、potassium sodium tartrateBenedict reagents: CuSO4、sodium carbonate (Na2CO3)、sodium citrate (Na3C6H5O7∙2H2O).The red precipitate is copper(I) oxide, Cu2O.These reagents can be used to measure the presence of reducing sugars qualitatively and so can monitor the concentration of blood glucose for diabetes.
32 (5) GlycosidesGlycosides are molecules in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule.glycosidic bondGlucoseaglyconeglyconeGlycosides can be linked by an O-, N-, S- or C- glycosidic bond.
33 (6) The three important dietary monosaccharides Glucose, which is produced in plants during photosynthesis.Fructose, which is also produced in plants during photosynthesis and found in fruit juices and honey.Galactose, which is found in milk.
34 3. OligosaccharidesDefinition: 'a few' ( from two to ten monosaccharide units )covalently linked monosaccharides3 disaccharidesSucrose = glucose-fructoseMaltose = glucose-glucoseLactose = glucose-galactose
35 Sucrose is known as common table sugar. Composed of D-glucoses and D-fructose linked by α-1-β-2 glycosidic bondsugarcaneThe hydrolysis of sucrose, will yield both glucose and fructose.This chemical reaction is achieved by honeybees which use invertase enzymes.Glc(α β2)Fruhoney - a mixture of glucose and fructose
36 Sucrose is nonreducing sugar The anomeric carbonatom for glucoseis carbon 1The anomeric carbonatom for fructoseis carbon 2Since the anomeric carbon is involved in a glycosidic bond, sucrose is classified as a nonreducing sugar.
37 2). Maltose reducing sugar Maltose syrupComposed of two D-glucoses linked by α-1, 4 glycosidic bondreducing sugarGlc(α1 4)Glc
38 lactose also referred to as milk sugar. Composed of D-galactose and D-glucose linked by β-1,4 glycosidic bond.reducing sugar
39 Digestion of lactose Lactose intolerance The intestinal villi secrete an enzyme called lactase (β-D-galactosidase) to digest lactose, and produce glucose and galactose, which can be absorbed.Lactose intoleranceMore than half of the world’s adults are lactose intolerance.Lactose intolerance is the inability to metabolize lactose, because the lactase is absent in the intestinal system or its availability is lowered.
40 Lactose intoleranceIn the absence of lactase, lactose remains uncleaved and passes intact into the colon.The operons of enteric bacteria quickly switch over to lactose metabolism, and produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane).This, in turn, may cause a range of abdominal symptoms, including stomach cramps, bloating, and flatulence.Treatment for this disorder is simple to remove lactose from diet.
41 Two kind disaccharides (1)reducing disaccharide(2) nonreducing disaccharideKey factor: free aldehyde or ketone group ( a free reducing group)
42 REDUCING SUGARSWhen Benedicts test is performed with the disaccharides maltose and sucrose, the following result is obtained:Sucrose is anon-reducing sugarMaltose is areducing sugarSUCROSERESULTMALTOSE
43 4. PolysaccharidesHomoglycans- homopolysaccharides containing only one type of monosaccharideHeteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharideLengths and compositions of a polysaccharide may vary within a population of these molecules
44 Polysaccharides may be composed of one, two, or several different monosaccharides, in straight or branched chains of varying length
46 (1). StarchStarch is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.Function: starch is produced by all green plants as an energy store and is a major food source for humansStarch is a mixture of unbranched amylose (α1-4 bonds) and branched amylopectin (α 1-4 and α1-6 branchpoints).AmyloseAmylopectin
48 (2) Glycogen The glucose storage device in animals D-Glucoses linked byα-1,4 and α-1,6 glycosidic bondsGlycogen has the same overall structure as amylopectin but there is significantly more branching in this molecule
49 GlycogenGlycogen is synthesized and stored mainly in the liver and the muscles.
50 (3). CelluloseCellulose is the structural component of the primary cell wall of green plants.Cellulose is the most common organic compound on Earth. About 33% of all plant matter is cellulose.Cellulose is a polymer of β-glucose units.
51 Cellulose structure Cellulose is a straight chain polymer. Intra- and interchain H-bonding gives strength.hydrogen bonds between parallel chains of beta glucose
52 In cellulose, sugar units are joined by beta linkages. The straight chains formed by beta linkages is optimal for structural function.
53 The multiple hydroxyl groups on the glucose residues form hydrogen bonds, holding the chains firmly together side-by-side and forming microfibrils.
54 CelluloseCellulosa can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bond.The functions of dietary fiberDecrease the absorption of glucose and cholesterol from the intestine, increase the bulk of feces, prevent constipation.
55 (4). ChitinChitin is a polysaccharide forming the outer skeleton of arthropods (such as insects, crabs, shrimps, and lobsters).It is a polymer of N-acetylglucosamine(NAG) in β-1 to 4 glycosidic linkage.Chitin is the main source of production of chitosan, which is used in a number of applications, such as a wound healing agent, surgical thread, flocculating agent and a delivery vehicle for pharmaceuticals and genes.
56 (5). Testing for polysaccharides: the Iodine Test When iodine solution is added to a suspension of starch, the iodine molecules pack inside the amylose helix to give a blue-black colour.All monosaccharides and all disaccharides give negative Iodine Tests.
57 5. Glycoconjugate (complex saccharide) Glycoconjugates: carbohydrates covalently linked with other chemical species.GlycoconjugateSugar + lipidSugar + proteinGlycolipidLipopolysaccharide (LPS)GlycoproteinProteoglycanGlycoconjugates are very important compounds. They are involved in cell-cell interactions, including cell-cell recognition, and cell-matrix interactions.
58 (1). GlycoproteinGlycoproteins: proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains.O-Glycosidic and N-glycosidic linkagesOligosaccharide chains exhibit great variability in sugar sequence and composition.
59 O-Glycosidic and N-glycosidic linkages N-acetylgalactosamine (GalNAc)N-acetylglucosamine (GlcNAc)
60 Glycoproteins play essential roles in the body Glycoproteins play essential roles in the body. For instance, in the immune system almost all of the key molecules involved in the immune response are glycoproteins.Glycoprotein in cell membrane
61 (2). ProteoglycanA special type of glycoprotein with sugar weighing about 95%On cell surface or Extracellular matrixEssential components of tissue (particularly connective tissue) structure.
62 (3) Glycolipids Glycolipids are carbohydrate-attached lipids. Their role is to provide energy and also serve as markers for cellular recognition.
63 Points Definition, function and classification of carbohydrates MonosaccharidesAldoses and ketosesFischer projections and Haworth structuresReducing sugarTerminology describing sugar structureEnantiomers,Epimers,AnomersOligosaccharidesSucrose, maltose, lactosePolysaccharidesStarch, Glycogen, CelluloseIodine TestGlycoconjugate (complex saccharide)