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LECTURE 5:TYPES OF CARBOHYRATE

LECTURE OUTCOMESAfter completing this lecture and mastering thelecture materials, students are expected to be able1. to explain the origin of plants carbohydrates and their

functions in plant metabolism2. to classify monosaccharides based on carbonyl location

(aldoses or ketoses) and the number of carbons themonosaccharides contain

3. to draw a Fischer projection of a monosaccha-ride, andto identify a D-sugar or L-sugar .

4. to identify chiral carbons and determine the number ofstereoisomers that are possible

5. to identify four common types of monosaccharidederivatives

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6. to predict the products when a monosaccharide reactswith a reducing agent or with Benedict’s reagent .

7. to define the term anomer and explain the differencebetween α and β anomers .

8. to explain and describe mutarotation9. to classify cyclic monosaccharides as either pyranoses

or furanoses .10. to identify the anomeric carbon in Haworth structures.

WHAT WOULD YOU LIKE TO KNOWI. Where does it come from?photosynthesisII. What is its functionmanyIII. What is carbohydrate?definitionIV. What are characteristics of carbohydrate

1. Classification?2. Aldose & Ketose Sugars?3. Number of carbon atoms?4. Location of carbonyl group (C=O)?5. The chirality of carbohydrate & asymmetric carbon?6. Diastereoisomers & Epimers?7. Enantiomers & Epimers?8. D or L designation refers to the asymmetric carbon?9. Fischer Projections: D-isomer & L-isomer?10. Number of Stereoisomers ?

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LECTURE OUTLINE1. INTRODUCTION

1. Source of Carbohydrate2. Carbohydrate Function3. Definition4. Classification of Carbohydrates

2. MONOSACCHARIDES1. Number of carbon atoms2. Location of carbonyl group3. Chirality of carbohydrates4. Pyranose and Furanose

3. POLARIMETRYEXERCISE

VOCABULARY1. Derivatives2. Precursor3. Intermediates4. Entities5. Recognition6. Attach7. Encounter8. Properties9. referred to10. Configuration11. Superimposable

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1. INTRODUCTION

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, throughthe process of photosynthesis, into PGA (3-Phosphoglyceric acid) and then to F-6-P (D-Fructose6-phosphate)

A whole range of monosaccharides andmonosaccharide derivatives are synthesized from F-6-P.

Some of these monosaccharide derivatives are theprecursor of oligosaccharides and polysaccharides.

Where does it come from?

If cytosol Pi is high, Pi exchanges for trioses;sucrose is made

If cytosolic Piis low, triosesstay inplastid; starchis made

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2. Carbohydrate Function1. Major source of energy: Carbohydrates are main

source of energy for living cell and others.2. Structural components: Carbohydrates constitute

structural tissues in plants and in microorganisms(cellulose, lignin, murein), and form protective coaton the surface of cells in animal cells.

3. Storage: The stored carbohydrate is starch in plantsand glycogen in animals.

4. Role in Metabolism: Carbohydrate plays a key rolein the metabolism of amino acids and fatty acids.

5. Special Function:i. Some glycoprotein acts as hormonesii. Glycoprotein on cell surface help in cell recognition and help in immune

system of the body.iii. Heparin, a mucopolysaccharide acts as anticoagulant.

http://manet.illinois.edu/pathways.php

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3. DefinitionWhat Is a Carbohydrate?1. Carbohydrates may be defined as polyhydroxy

aldehydes or ketones, or substances that yield one ofthese compounds on hydrolysis.

aldehyde

ketonePolyhydroxy =poly OH

Aldoses (e.g., glucose)have an aldehyde atone end (C1).

Ketoses (e.g., fructose)have a keto group,usually at C2

aldehyde

ketone

Examples

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4. The formula (CH2O)n where n 3 can be used torepresent many carbohydrates which areregarded originally as the hydrates of carbon. C3H6O3: Dihydroxyacetone, Dimethyl carbonate &

Glyceraldehyde C4H8O4: Tetrose (Erythrose, Erythrulose & Threose)

5. The formula is not suitable when othercompounds were encountered that had thegeneral properties of carbohydrates butcontained N (nitrogen) or S (sulfur) in addition tocarbon, hydrogen and oxygen.

6. Moreover, the important simple sugardeoxyribose, found in every cell as a componentof deoxyribonucleic acid, has the molecularformula C5H10O4 rather than C5H10O 5

4. Carbohydrate Classification1. 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 10Most important are the disaccharides

Polysaccharides.HomopolysaccharidesHeteropolysaccharidesComplex carbohydrates

2. In general, the monosaccharides and disaccharides arecommonly referred to as sugars.

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Monosaccharides In general, the monosaccharides and disaccharides are

commonly referred to as sugars. Monosaccharides alsoknown as simple sugars are classified based on threecharacteristics:1. Number of carbon atoms in the molecule2. Location of the carbonyl group3. 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 oftenrepresented by Fischer projections. The Fischerprojection, devised by Hermann Emil Fischer in 1891,is a two-dimensional representation of a three-dimensional organic molecule by projection.

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1. Triose (3 C)

3. Pentose (5 C)

2. Terose (4C)

4. Hexose(6 C)

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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 inavocados.

2. Location of carbonyl group (C=O)1. The location of carbonyl group is used to

define carbohydrates as mentioned previouslyleading to an aldose (aldehyde sugar) a ketose (ketone sugar)

CARBOHYDRATE CLASSIFICATION

ketose

aldose

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2. In aldehydes, the carbonyl (C=O) group has a hydrogenatom (H) attached to it together with either a secondhydrogen atom or, more commonly, a hydrocarbon groupwhich might be an alkyl group (CnH2n+1, CH3) or onecontaining a benzene ring (C6H6).

two hydrocarbon (HC) groups attached. Again, these canbe either alkyl groups (CnH2n+1) or ones containing benzenerings (C6H6). Notice that ketones never have a hydrogenatom attached to the carbonyl group.

3. In ketones,the carbonylgroup has

For example− glucose is an

aldose;

− fructose, astructural isomerof glucose, is aketose

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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 havingdifferent structures, and

- Stereoisomers have the same molecular formulaand the same structure, but they differ inconfiguration, that is, in the arrangement of atomsin space.

Saccharides with identical functional groups but withdifferent spatial configurations have different chemicaland biological properties.

Compounds that are mirror images of each other butare not identical, comparable to left and right shoes, arecalled enantiomers

Any carbon atom which is connected to four differentgroups is called asymmetric carbon or a chiral carbonand will have two nonsuperimposable mirror images ora center of chirality

Monosaccharides contain one or more asymmetric C-atoms: 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

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D and L designations are based on theconfiguration about the single asymmetriccarbon in glyceraldehyde. Glyceraldehyde is a

chiral molecule — itcannot besuperimposed on itsmirror image.

The two mirror-image forms ofglyceraldehyde areenantiomers of eachother.

Enantiomers are two carbohydrates that are completemirror images of one another.

An example of an enantiomer is the D and Lisomers of glucose

The blue indicates the D-isomer and the redindicates the L-isomer

mirror images

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Enantiomers

Mirror imageconfigurations

These are not mirror images of one another.

Diastereoisomers. Two carbohydratesare said to be diastereoisomers ifthey have the opposite configurationat one or more of the chiral centerspresent in the carbohydrate but thetwo carbohydrates are not mirrorimages of one another.

An example of twocarbohydrates that arediastereoisomers are D-Glucose and D-Altrose

The differingstereogenic centers.

Epimers are a special type ofdiastereoisomers in thatthey only differ at one of thestereogenic centers.

An example of epimersthat differ at onestereogenic center is D-Glucose and D-Mannose,

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For sugars with more than one chiral center, theD or L designation refers to the asymmetriccarbon farthest from the aldehyde or keto group.

The chirality of the carbohydrate

Most naturallyoccurring sugarsare D isomers.

D & L sugars aremirror images ofone another. Theyhave the samename. For example,D-glucose and L-glucose are shownat right.

Number of Stereoisomers: When a molecule hasmore than one chiral carbon, each carbon canpossibly be arranged in either the right-hand orleft-hand form, thus

if there are n chiral carbons, there are 2npossible stereoisomers.

D-Glucose(an aldose)

• D-Glucose have four asymmetric carbon atoms (No. 2,3, 4 & 5)2n = 24 = 16

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4. Pyranose and Furanose1. If the carbon chain is long enough, the alcohol at one

end of a monosaccharide can attack the carbonylgroup 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 glucosecan exist in both a straight-chain and ring form. D-glucose can cyclize in two ways forming eitherpyranose or furanose structures.

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Ribose Deoxyribose

3. In reality, an aqueous sugar solution contains only0.02% of the glucose in the chain form, the majority ofthe 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

Steps of cyclation

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-D-Glucose -D-Glucose

D-Glucose

Cyclation ofGlucose

4. There are two possible structures for the pyranose andfuranose forms of a monosaccharide, which are calledthea-anomers andb-anomers

5. Cyclization of glucose produces a new asymmetriccenter at C1, and the 2 stereoisomers are calledanomers, α & β.

α (OH below the ring) & β (OH above the ring)

a-anomers

b-anomers

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6. The reactions that lead to the formation of a pyranoseor a furanose are reversible. For examplea-D-glucopyranose andb-D-glucopyranose areinterconvertable to give an equilibrium mixture that is63.6% of the β-anomer and 36.4% of the α-anomer.

7. The 2:1 preference for the β-anomer can beunderstood by comparing the structures of thesemolecules. In the β-anomer, all of the bulky -OH or -CH2OH

substituents lie more or less within the plane of the six-membered ring.

In the -anomer, one of the -OH groups is perpendicular tothe plane of the six-membered ring, in a region where itfeels strong repulsive forces from the hydrogen atoms thatlie in similar positions around the ring.

As a result, the β-anomer is slightly more stable than the-anomer.

8. The six-membered pyranose ring is not planar due tothe tetrahedral geometry of its saturated carbonatoms. Instead, pyranose rings adopt two classes ofconformations, termed chair and boat.

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9. Since carbohydrates contain bothalcohol and aldehyde or ketonefunctional groups, the straight-chainform is easily converted into the chairform - hemiacetal ring structure.

10. The chair form is more stable because of less sterichindrance as the axial positions are occupied byhydrogen atoms.

tetrahedral

11. Furanose rings, like pyranose rings, are not planar andcan be puckered so that four atoms are nearly coplanarand the fifth is about 0.5 Å away from this plane. Thisconformation is called an envelope form that resemblesan opened envelope with the back flap raised.

12. In the ribose moiety of most biomolecules, either C-2 orC-3 is out of the plane on the same side as C-5. Theseconformations are called C2-endo and C3-endo,respectively.

The color indicates the fouratoms that lieapproximately in a plane

The C2-endo and C3-endoforms of β-D-ribose

Envelope Conformationsof β-D–ribose,

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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 aninstrument called apolarimeter (Lippichtype)

• Rotation is either(+) dextrorotatory or(-) levorotatory

Polarimeter

The D and L form of monosaccharide rotate plane of apolarized light in the opposite direction by the sameamount.

PrismPolarizeslightvertically

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When the sample tubeis empty, the planes ofpolarization of thepolarizing prism and theanalyzing prisms aresame and αobs is 0°

One now needs to rotate the analyzer prism for its plane ofpolarization to coincide with the plane of the emergent light. Thiscorresponds to the maximum intensity of the transmitted light.

When the sample tubehas a solution of achiral (optically active)substance, the plane ofpolarization of theemergent polarized lightchanges.

Magnitude of rotation depends upon:1. The nature of the compound2. The length of the tube (cell or sample container) usually

expressed in decimeters (dm)3. The wavelength of the light source employed; usually

either sodium D line at 589.3 nm or mercury vapor lampat 546.1 nm

4. Temperature of sample5. Concentration of analyte (g/100 ml)

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1. D-glucose +52.72. D-fructose -92.43. D-galactose +80.24. L-arabinose +104.55. D-mannose +14.26. D-arabinose -105.0

6. D-xylose +18.87. Lactose +55.48. Sucrose +66.59. Maltose +130.410. Invert sugar -19.811. Dextrin +195

Specific rotation of various carbohydrates at 20oC

D = Na D line, T = temperature (oC), obs : observedrotation (0, specify solvent), l = length of tube (dm,decimeter), c = concentration (g/100ml), and [] =specific rotation

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EXERCISE How many isomers are there of

How many chiral carbon atomsare there in

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Carbohydrate synthesis CHO’s are made in the chloroplast and

cytosol Starch = chloroplast Sucrose = cytosol

Chloroplast organelles arefound in plants and algae Enclosed by a double

membrane Have their own small

genome The inner membrane is

impermeable to ionssuch as H+, and to polarand charged molecules

Glyceraldehyde-3 Phosphate (G 3-P) Glyceraldehyde-3 Phosphate (3-phosphoglycerate) is

the first product of photosynthesis through threerounds of the Calvin cycle to fix three CO2 molecules toproduce one molecule of G 3-P

● G 3-P is converted tostarch in the chloroplast,and to sucrose in thecytosol for export

● G 3-P synthesis isbalanced by phosphatelevels & triose phosphatelevels in the compartments