LECTURE 5: TYPES OF CARBOHYRATE LECTURE OUTCOMES

Transcription

1 LECTURE 5: TYPES OF CARBOHYRATE LECTURE OUTCOMES After completing this lecture and mastering the lecture materials, students are expected to be able to explain the origin of plants carbohydrates and their functions in plant metabolism to classify monosaccharides based on carbonyl location (aldoses or ketoses) and the number of carbons the monosaccharides contain to draw a Fischer projection of a monosaccha-ride, and to identify a D-sugar or L-sugar. to identify chiral carbons and determine the number of stereoisomers that are possible to identify four common types of monosaccharide derivatives 1

2 to predict the products when a monosaccharide reacts with a reducing agent or with Benedict s reagent. to define the term anomer and explain the difference between α and β anomers. to explain and describe mutarotation to classify cyclic monosaccharides as either pyranoses or furanoses. to identify the anomeric carbon in Haworth structures. WHAT WOULD YOU LIKE TO KNOW I. II. III. IV. Where does it come from? photosynthesis What is its function many What is carbohydrate? definition What are characteristics of carbohydrate Classification? Aldose & Ketose Sugars? Number of carbon atoms? Location of carbonyl group (C=O)? The chirality of carbohydrate & asymmetric carbon? Diastereoisomers & Epimers? Enantiomers & Epimers? D or L designation refers to the asymmetric carbon? Fischer Projections: D-isomer & L-isomer? Number of Stereoisomers? 2

3 LECTURE OUTLINE 1. INTRODUCTION 1. Source of Carbohydrate 2. Carbohydrate Function 3. Definition 4. Classification of Carbohydrates 2. MONOSACCHARIDES 1. Number of carbon atoms 2. Location of carbonyl group 3. Chirality of carbohydrates 4. Pyranose and Furanose 3. POLARIMETRY EXERCISE VOCABULARY Derivatives Precursor Intermediates Entities Recognition Attach Encounter Properties referred to Configuration Superimposable 3

4 1. INTRODUCTION Where does it come from? 1. Source of Carbohydrate Carbohydrates are the most abundant class of organic compounds found in living organisms The carbohydrates of plants are derived originally from atmospheric CO2 which is converted, through the process of photosynthesis, into PGA (3Phosphoglyceric acid) and then to F-6-P (D-Fructose 6-phosphate) A whole range of monosaccharides and monosaccharide derivatives are synthesized from F6-P. Some of these monosaccharide derivatives are the precursor of oligosaccharides and polysaccharides. If cytosol Pi is high, Pi exchanges for trioses; sucrose is made If cytosolic Pi is low, trioses stay in plastid; starch is made 4

5 2. Carbohydrate Function Major source of energy: Carbohydrates are main source of energy for living cell and others. Structural components: Carbohydrates constitute structural tissues in plants and in microorganisms (cellulose, lignin, murein), and form protective coat on the surface of cells in animal cells. Storage: The stored carbohydrate is starch in plants and glycogen in animals. Role in Metabolism: Carbohydrate plays a key role in the metabolism of amino acids and fatty acids. Special Function: i. Some glycoprotein acts as hormones ii. Glycoprotein on cell surface help in cell recognition and help in immune system of the body. iii. Heparin, a mucopolysaccharide acts as anticoagulant. 5

6 3. Definition What Is a Carbohydrate? 1. Carbohydrates may be defined as polyhydroxy aldehydes or ketones, or substances that yield one of these compounds on hydrolysis. aldehyde ketone Poly hydroxy = poly OH Examples aldehyde ketone Aldoses (e.g., glucose) have an aldehyde at one end (C1). Ketoses (e.g., fructose) have a keto group, usually at C2 6

7 4. The formula (CH2O)n where n 3 can be used to represent many carbohydrates which are regarded originally as the hydrates of carbon. C3H6O3: Dihydroxyacetone, Dimethyl carbonate & Glyceraldehyde C4H8O4: Tetrose (Erythrose, Erythrulose & Threose) 5. The formula is not suitable when other compounds were encountered that had the general properties of carbohydrates but contained N (nitrogen) or S (sulfur) in addition to carbon, hydrogen and oxygen. 6. Moreover, the important simple sugar deoxyribose, found in every cell as a component of deoxyribonucleic acid, has the molecular formula C5H10O4 rather than C5H10O 5 4. Carbohydrate Classification 1. The carbohydrates can be devided into groups according to the number of individual simple sugar units Monosaccharides Trioses, tetroses, pentoses, hexoses Oligosaccharides Di, tri, tetra, penta, up to 9 or 10 Most important are the disaccharides 2. Polysaccharides. Homopolysaccharides Heteropolysaccharides Complex carbohydrates In general, the monosaccharides and disaccharides are commonly referred to as sugars. 7

8 Monosaccharides In general, the monosaccharides and disaccharides are commonly referred to as sugars. Monosaccharides also known as simple sugars are classified based on three characteristics: 1. Number of carbon atoms in the molecule 2. Location of the carbonyl group 3. The chirality of the carbohydrate 1. Number of carbon atoms - three carbons: triose - four carbons: tetrose - five carbons: pentose - six carbons: hexose - seven carbons: heptose, etc. Monosaccharides and other sugars are often represented by Fischer projections. The Fischer projection, devised by Hermann Emil Fischer in 1891, is a two-dimensional representation of a threedimensional organic molecule by projection. 8

9 1. Triose (3 C) 2. Terose (4C) 3. Pentose (5 C) 4. Hexose (6 C) 9

10 5. Heptoses (7C) Sedoheptulose has the same structure as fructose, but it has one extra carbon. Sedoheptulose is found in carrots. Mannoheptulose is a monosaccharide found in avocados. CARBOHYDRATE CLASSIFICATION 2. Location of carbonyl group (C=O) 1. The location of carbonyl group is used to define carbohydrates as mentioned previously leading to an aldose (aldehyde sugar) a ketose (ketone sugar) aldose ketose 10

11 2. In aldehydes, the carbonyl (C=O) group has a hydrogen atom (H) attached to it together with either a second hydrogen atom or, more commonly, a hydrocarbon group which might be an alkyl group (CnH2n+1, CH3) or one containing a benzene ring (C6H6). 3. In ketones, the carbonyl group has two hydrocarbon (HC) groups attached. Again, these can be either alkyl groups (CnH2n+1) or ones containing benzene rings (C6H6). Notice that ketones never have a hydrogen atom attached to the carbonyl group. For example glucose is an aldose; fructose, a structural isomer of glucose, is a ketose 11

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13 3. Chirality of carbohydrate Chirality is a term derived from the Greek word for hand (χειρ =cheir) or asymmetric carbon atoms This causes optical isomerism which is the type of isomerism commonly found in carbohydrate. Isomerism may be divided into structural isomerism and stereoisomerism. - Structural isomers have the same molecular formula but differ from each other by having different structures, and - Stereoisomers have the same molecular formula and the same structure, but they differ in configuration, that is, in the arrangement of atoms in space. Saccharides with identical functional groups but with different spatial configurations have different chemical and biological properties. Compounds that are mirror images of each other but are not identical, comparable to left and right shoes, are called enantiomers Any carbon atom which is connected to four different groups is called asymmetric carbon or a chiral carbon and will have two nonsuperimposable mirror images or a center of chirality Monosaccharides contain one or more asymmetric Catoms: may be present in the D- or L-forms - D-form: OH group is attached to the right of the asymmetric carbon - L-form: OH group is attached to the left of the asymmetric carbon 13

14 D and L designations are based on the configuration about the single asymmetric carbon in glyceraldehyde. Glyceraldehyde is a chiral molecule it cannot be superimposed on its mirror image. The two mirrorimage forms of glyceraldehyde are enantiomers of each other. Enantiomers are two carbohydrates that are complete mirror images of one another. An example of an enantiomer is the D and L isomers of glucose mirror images The blue indicates the D-isomer and the red indicates the L-isomer 14

15 CH2O C=O HO-C-H H-C-OH H-C-OH CH2O CH2O C=O Enantiomers H-C-OH Mirror image HO-C-H configurations HO-C-H CH2O These are not mirror images of one another. CH2O C=O H-C-OH HO-C-H HO-C-H CH2O Diastereoisomers. Two carbohydrates are said to be diastereoisomers if they have the opposite configuration at one or more of the chiral centers present in the carbohydrate but the two carbohydrates are not mirror images of one another. Epimers are a special type of diastereoisomers in that they only differ at one of the stereogenic centers. CH2O C=O H-C-OH HO-C-H HO-C-H CH2O The differing stereogenic centers. An example of two carbohydrates that are diastereoisomers are DGlucose and D-Altrose An example of epimers that differ at one stereogenic center is DGlucose and D-Mannose, 15

16 The chirality of the carbohydrate For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers. D & L sugars are mirror images of one another. They have the same name. For example, D-glucose and Lglucose are shown at right. Number of Stereoisomers: When a molecule has more than one chiral carbon, each carbon can possibly be arranged in either the right-hand or left-hand form, thus if there are n chiral carbons, there are 2n possible stereoisomers. D-Glucose have four asymmetric carbon atoms (No. 2, 3, 4 & 5) 2n = 24 = 16 D-Glucose (an aldose) 16

17 4. Pyranose and Furanose 1. If the carbon chain is long enough, the alcohol at one end of a monosaccharide can attack the carbonyl group at the other end to form a cyclic compound. When a six-membered ring is formed, the product of this reaction is called a pyranose. When a five-membered ring is formed, it is called a furanose 2. Therefore certain monosaccharide such as glucose can exist in both a straight-chain and ring form. Dglucose can cyclize in two ways forming either pyranose or furanose structures. O H 1C 2C OH HO 3C H H 4C OH H 5C OH H 6CH 2OH HO H 5C 4C 3C 2C OH OH H OH O H 1C H 6CH 2OH H 17

18 3. In reality, an aqueous sugar solution contains only 0.02% of the glucose in the chain form, the majority of the structure is in the cyclic chair form The ring form of ribose is a component ribonucleic acid (RNA). Deoxyribose, which is missing an oxygen at position 2, is a component of deoxyribonucleic acid (DNA). In nucleic acids, the hydroxyl group attached to carbon number 1 is replaced with nucleotide bases Ribose Deoxyribose Steps of cyclation 18

19 D-Glucose Cyclation of Glucose -D-Glucose D-Glucose There are two possible structures for the pyranose and furanose forms of a monosaccharide, which are called the a-anomers and b-anomers Cyclization of glucose produces a new asymmetric center at C1, and the 2 stereoisomers are called anomers, α & β. a-anomers b-anomers α (OH below the ring) & β (OH above the ring) 19

20 6. 7. The reactions that lead to the formation of a pyranose or a furanose are reversible. For example a-dglucopyranose and b-d-glucopyranose are interconvertable to give an equilibrium mixture that is 63.6% of the β-anomer and 36.4% of the α-anomer. The 2:1 preference for the β-anomer can be understood by comparing the structures of these molecules. 8. In the β-anomer, all of the bulky -OH or -CH2OH substituents lie more or less within the plane of the sixmembered ring. In the -anomer, one of the -OH groups is perpendicular to the plane of the six-membered ring, in a region where it feels strong repulsive forces from the hydrogen atoms that lie in similar positions around the ring. As a result, the β-anomer is slightly more stable than the -anomer. The six-membered pyranose ring is not planar due to the tetrahedral geometry of its saturated carbon atoms. Instead, pyranose rings adopt two classes of conformations, termed chair and boat. 20

21 9. Since carbohydrates contain both alcohol and aldehyde or ketone functional groups, the straight-chain form is easily converted into the chair form - hemiacetal ring structure. tetrahedral 10. The chair form is more stable because of less steric hindrance as the axial positions are occupied by hydrogen atoms. 11. Furanose rings, like pyranose rings, are not planar and can be puckered so that four atoms are nearly coplanar and the fifth is about 0.5 Å away from this plane. This conformation is called an envelope form that resembles an opened envelope with the back flap raised. In the ribose moiety of most biomolecules, either C-2 or C-3 is out of the plane on the same side as C-5. These conformations are called C2-endo and C3-endo, respectively. 12. The C2-endo and C3-endo forms of β-d-ribose The color indicates the four atoms that lie approximately in a plane Envelope Conformations of β-d ribose, 21

22 3. POLARIMETRY Measurement of optical activity in chiral or asymmetric molecules using plane polarized light Molecules may be chiral because of certain atoms or because of chiral axes or chiral planes Measurement uses an instrument called a polarimeter (Lippich type) Rotation is either (+) dextrorotatory or (-) levorotatory Polarimeter The D and L form of monosaccharide rotate plane of a polarized light in the opposite direction by the same amount. Prism Polarizes light vertically 22

23 When the sample tube is empty, the planes of polarization of the polarizing prism and the analyzing prisms are same and αobs is 0 When the sample tube has a solution of a chiral (optically active) substance, the plane of polarization of the emergent polarized light changes. One now needs to rotate the analyzer prism for its plane of polarization to coincide with the plane of the emergent light. This corresponds to the maximum intensity of the transmitted light. Magnitude of rotation depends upon: The nature of the compound The length of the tube (cell or sample container) usually expressed in decimeters (dm) The wavelength of the light source employed; usually either sodium D line at nm or mercury vapor lamp at nm Temperature of sample Concentration of analyte (g/100 ml) [ ] T D observed x 100 = lxc 23

24 D = Na D line, T = temperature (oc), obs : observed rotation (0, specify solvent), l = length of tube (dm, decimeter), c = concentration (g/100ml), and [ ] = specific rotation Specific rotation of various carbohydrates at 20 oc 1. D-glucose D-xylose D-fructose Lactose D-galactose Sucrose L-arabinose Maltose D-mannose Invert sugar D-arabinose Dextrin

25 EXERCISE How many isomers are there of How many chiral carbon atoms are there in 25

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27 Carbohydrate synthesis CHO s are made in the chloroplast and cytosol Starch = chloroplast Sucrose = cytosol Chloroplast organelles are found in plants and algae Enclosed by a double membrane Have their own small genome The inner membrane is impermeable to ions such as H+, and to polar and charged molecules Glyceraldehyde-3 Phosphate (G 3-P) Glyceraldehyde-3 Phosphate (3-phosphoglycerate) is the first product of photosynthesis through three rounds of the Calvin cycle to fix three CO2 molecules to produce one molecule of G 3-P G 3-P is converted to starch in the chloroplast, and to sucrose in the cytosol for export G 3-P synthesis is balanced by phosphate levels & triose phosphate levels in the compartments 27