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Lecture 5 Carbohydrates and Glycobiology

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1 Lecture 5 Carbohydrates and Glycobiology
Life Is Sweet

2 Outline Definitions Classification
Structure & Function of Carbohydrates

3 1. Introduction Carbohydrate, 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 carbohydrates What 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 protein Informational molecules for recognition

6 (3) Nomenclature Carbohydrates 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 galactose Oligosaccharides: 'a few' ( from two to ten monosaccharide units )covalently linked monosaccharides eg:sucrose, maltose, lactose polysaccharides: 'many' covalently linked monosaccharides eg:Starch, Glycogen, Cellulose (a form of fiber)

8 Glycoconjugates Also called complex saccharide
Polysaccharide linked to proteins or lipids including Proteoglycans Glycoproteins Glycolipids

9 2. Monosaccharide (1) Monosacchrides can be divided into two families: aldoses and ketoses. aldehyde group keto group D-Glucose, an aldohexose D-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

12 5 carbons : Pentose

13 6 carbons : Hexose The 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 dihydroxyacetone Opticity: D- (Dextrorotatory) and L- (Levorotatory) Glyceraldehyde is conventionally used as the standard for defining D and L configurations enantiomers

15 D- and L-glucose D- and L-fructose Enantiomers: 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” suffix Ketoses 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.

18 Stereoisomers of D-family of ketose

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 carbon anomer -OH group of anomeric carbon (C1) and –CH2OH group (C6) : On the contrary-α Same side - β

22 D-Glucose Furanose 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.

24 Glucose: linear form Magic words: right, left, right, right; (top down)

25 Glucose: Ring form Magic words: down, up, down (for carbons 2, 3, and 4)

26 Fructose furanose ring

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- Epimers differ only by the configuration around one carbon Anomers α and β configuration

29 brick-red precipitates
(4) Reducing sugar Sugars 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 tartrate Benedict 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) Glycosides Glycosides are molecules in which a sugar is bound to a non-carbohydrate moiety, usually a small organic molecule. glycosidic bond Glucose aglycone glycone Glycosides 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. Oligosaccharides Definition: 'a few' ( from two to ten monosaccharide units )covalently linked monosaccharides 3 disaccharides Sucrose = glucose-fructose Maltose = glucose-glucose Lactose = glucose-galactose

35 Sucrose is known as common table sugar.
Composed of D-glucoses and D-fructose linked by α-1-β-2 glycosidic bond sugarcane The hydrolysis of sucrose, will yield both glucose and fructose. This chemical reaction is achieved by honeybees which use invertase enzymes. Glc(α β2)Fru honey - a mixture of glucose and fructose

36 Sucrose is nonreducing sugar
The anomeric carbon atom for glucose is carbon 1 The anomeric carbon atom for fructose is carbon 2 Since the anomeric carbon is involved in a glycosidic bond, sucrose is classified as a nonreducing sugar.

37 2). Maltose reducing sugar
Maltose syrup Composed of two D-glucoses linked by α-1, 4 glycosidic bond reducing sugar Glc(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 intolerance More 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 intolerance In 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 disaccharide Key factor: free aldehyde or ketone group ( a free reducing group)

42 REDUCING SUGARS When Benedicts test is performed with the disaccharides maltose and sucrose, the following result is obtained: Sucrose is a non-reducing sugar Maltose is a reducing sugar SUCROSE RESULT MALTOSE

43 4. Polysaccharides Homoglycans- homopolysaccharides containing only one type of monosaccharide Heteroglycans - heteropolysaccharides containing residues of more than one type of monosaccharide Lengths 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). Starch Starch 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 humans Starch is a mixture of unbranched amylose (α1-4 bonds) and branched amylopectin (α 1-4 and α1-6 branchpoints). Amylose Amylopectin

47 Starch structure

48 (2) Glycogen The glucose storage device in animals
D-Glucoses linked byα-1,4 and α-1,6 glycosidic bonds Glycogen has the same overall structure as amylopectin but there is significantly more branching in this molecule

49 Glycogen Glycogen is synthesized and stored mainly in the liver and the muscles.

50 (3). Cellulose Cellulose 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 Cellulose Cellulosa can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bond. The functions of dietary fiber Decrease the absorption of glucose and cholesterol from the intestine, increase the bulk of feces, prevent constipation.

55 (4). Chitin Chitin 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. Glycoconjugate Sugar + lipid Sugar + protein Glycolipid Lipopolysaccharide (LPS) Glycoprotein Proteoglycan Glycoconjugates are very important compounds. They are involved in cell-cell interactions, including cell-cell recognition, and cell-matrix interactions.

58 (1). Glycoprotein Glycoproteins: proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains. O-Glycosidic and N-glycosidic linkages Oligosaccharide 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). Proteoglycan A special type of glycoprotein with sugar weighing about 95% On cell surface or Extracellular matrix Essential 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
Monosaccharides Aldoses and ketoses Fischer projections and Haworth structures Reducing sugar Terminology describing sugar structure Enantiomers,Epimers,Anomers Oligosaccharides Sucrose, maltose, lactose Polysaccharides Starch, Glycogen, Cellulose Iodine Test Glycoconjugate (complex saccharide)

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