Carbohydrates Bioc . 201 - PowerPoint Presentation

Slide1

Carbohydrates

Bioc. 201

بسم الله الرحمن الرحيم

1Slide2

Carbohydrates or

Saccharides

Carbohydrates are the most abundant organic molecules in nature.Essential components of all living organisms.The word carbohydrate means "hydrate of carbon“. The suffix –

ose

indicates that a molecule is a carbohydrate, and the prefixes

tri-, tetr-, and so forth indicate the number of carbon atoms in the chain.

2Slide3

Carbohydrates or

Saccharides

Carbohydrates are compounds containing C, H, and O. The general formula for a carbohydrate is (CH2

O)

n

, where n≥3.All carbohydrates contain C==O and ―OH functional groups.

3

D- Glucose

D- FructoseSlide4

Functions of Carbohydrates

The major source of energy.Acting as a storage form of energy in the body.

Serving as cell membrane component that mediate some forms of intercellular communication (e.g. cell adhesion in inflammation).Serving as a structural component of many organisms, including cell wall of bacteria, exoskeleton of insects, and fibrous cellulose of plants.

Essential components of DNA and RNA (Ribose and

Deoxyribose

)4Slide5

Classification of Carbohydrates

5Slide6

Classification of Carbohydrates

There are four classes of Carbohydrates, based on the number of sugar units:

Monosaccharide (simple sugars): cannot be broken down into simpler sugars under mild conditions. Disaccharides: contain 2 monosaccharide units covalently linked.

Oligosaccharides:

contain from 3 - 12 monosaccharide units covalently linked.

Polysaccharides: contain more than 12 monosaccharid

units and can be hundreds of sugar units in length covalently linked.

6Slide7

Monosaccharides

or Simple Sugars

Monosaccaharides are aldehyde or ketone derivatives of straight - chain polyhydroxy alcohols containing at least three carbon atoms.Such substances, for example, D-glucose and D-ribulose

, cannot be hydrolyzed to form simpler

saccharides

.

D- Glucose

D-

Ribulose

7Slide8

Classification of Monosaccharides

Monosaccharides

can be classified according to:1. The number of carbon atoms they contain.

Examples of

monosaccharides

found in humans.

2. The location of the carbonyl (CO) functional group:

Aldose

:

monosaccharide has a terminal carbonyl group (O==CH

-

) called an

aldehyde

group.

Ketose

:

monosaccharide has a carbonyl group (O==C) in the middle linked to two other carbon atoms, called a

ketone

group.

Examples of an

aldose

(A) and a

ketose

(B) sugar.

8Slide9

Classification of Monosaccharides

These terms may be combined so that, for example, glucose is an aldohexoses

where ribulose is a ketopentose. aldo

trioses

and

keto trioses

aldo

tetroses

and

keto

tetroses

aldo pentoses and keto

pentoses

aldo

hexoses

and

keto

hexoses

The simplest monosaccharide are the two 3- carbon

trioses

:

Glyceraldehydes, an

aldose

Dihydroxyacetone

, a

ketose

.

D- Glucose

D-

Ribulose

9Slide10

Classification of Monosaccharides

The most abundant

monosaccharides in nature are the hexoses, which include the aldohexose D-glucose and the ketohexose D-fructose.

D- Glucose

D- Fructose

10Slide11

Classification of Monosaccharides

The

stereochemical

relationships, shown in

Fisher projection

, among the D-

aldoses

with 3 to 6 carbon atoms. The configuration about C2 (

red

) distinguishes the members of each pair.

11Slide12

Classification of Monosaccharides

The

stereochemical

relationships among the D-ketoses with 3 to 6 carbon atoms. The configuration about C3 (

red

) distinguishes the members of each pair.

12Slide13

Structure of Carbohydrates

13Slide14

Chirality

Chiral objects:cannot

be superimposed on their mirror images — e.g., hands, gloves, and shoes. Achiral objects can be superimposed on the mirror images — e.g., drinking glasses, spheres, and cubes.

14Slide15

Chirality

Any carbon atom which is connected to four different groups will be chiral

, and will have two nonsuperimposable mirror images; it is a chiral carbon or a center of chirality.

If any of the two groups on the carbon are the same, the carbon atom cannot be

chiral

.

Many organic compounds (e.g. carbohydrates, amino acids) contain more than one

chiral

carbon.

15Slide16

Stereoisomers

The central carbons of a carbohydrate are asymmetric (chiral

) – four different groups are attached to the carbon atoms. This allows for various spatial arrangements around each asymmetric carbon (also called stereogenic centers) forming molecules called stereoisomers.

2

4

=16 steroisomers

Chiral Centers

Chiral

Centers

2

3

=8

steroisomers

16Slide17

Stereoisomers

Stereoisomers have the same order and types of bonds but different spatial arrangements and different properties. For each asymmetric carbon, there are 2

n possible isomers (e.g. an aldohexose contains four asymmetric carbons, there are 24, or 16, possible isomers).

2

4

=16 steroisomers

Chiral Centers

Chiral

Centers

2

3

=8

steroisomers

17Slide18

Stereoisomers

What is the maximum number of possible stereo-isomers of the following compounds?

18Slide19

D- and L- Monosaccharides

(Stereoisomers)

D-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the -

OH

on its asymmetric carbon

on the right.L-monosaccharide: a monosaccharide that, when written as a Fischer projection, has the

-OH on its asymmetric carbon

on the left.

According to the conventions proposed by Fischer:

19Slide20

Isomers and Epimers

Isomers are compounds that have the same chemical formula but have different structures.

Fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H12O6.

20Slide21

Isomers and Epimers

If two monosachharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon), they are defined as

epimers of each other (of course, they are also isomers!).

C4

epimers

C2

epimers

21Slide22

Isomers and Epimers

Glucose and galactose

are C-4 epimers - their structures differ only in the position of the ―OH group at carbon 4. [Note: the carbons in sugars are numbered beginning at the end that contain the carbonyl carbon - that is, the aldehyde or keto group].

C4

epimers

C2

epimers

22Slide23

Isomers and Epimers

Glucose and mannose are C-2 epimers

- their structures differ only in the position of the ―OH group at carbon 2.Galactose and mannose are NOT epimers – they differ in the position of ―OH groups at two carbons (2 and 4) and are, therefore, defined only as isomers.

C4

epimers

C2

epimers

23Slide24

Enantiomers and Diastereomers

A

special type of isomers is found in the pairs of structures that are mirror images of each other. These mirror images are called

enantiomers

.

The two members of the pair are designated as a D- and L- sugar .

The vast majority of the sugars in humans are D-sugars.

24Slide25

Enantiomers and Diastereomers

Diastereomers

: stereoisomers that are not mirror images (e.g. D-erythrose and D-threose). Enantiomers: stereoisomers

that are mirror images (e.g. D-

erythrose

and L-erythrose).

25Slide26

26Slide27

Representation of Carbohydrates Structure

27Slide28

Representation of Carbohydrates

Several models are used to represent carbohydrates:

Fisher Projection.Haworth Projection. Chair Conformation.28Slide29

Fischer Projections

Fischer projections are a convenient way to represent mirror images in two dimensions. Place the carbonyl group at or near the top and the last achiral CH

2OH at the bottom.29Slide30

Fischer Projections

30

Naming Stereoisomers: When there is more than one chiral center in a carbohydrate, look at the chiral carbon farthest from the carbonyl group:

if the hydroxyl group points to right when the carbonyl is “up” it is the

D-isomer

.If the hydroxyl group points to the left, it is the L-isomer. Slide31

Fischer Projections

Identify the following compounds as D or L isomers, and draw their mirror images:

31

Xylose

Fructose

ArabinoseSlide32

Configurations and Conformations

Alcohols react with the carbonyl groups of

aldehyde and ketones to form hemiacetals and hemiketals, respectively.

32Slide33

Configurations and Conformations

A sugar with a 5-membred ring is known as a

furanose in analogy with furan.A sugar with a 6-membred ring is known as a pyranose in analogy with pyran.

33Slide34

Cyclization of

Monosaccharids

The hydroxyl and either the aldehyde or the ketone functions of monosaccharides can likewise react intramolecularly to form cyclic hemiacetals and

hemiketals

.

Cyclization using C1 to C5, results in hemiacetal formation.Cyclization

using C2 to C5 results in hemiketal formation.

In both cases, the carbonyl carbon is new

chiral

center

and becomes an

anomeric carbon.

34Slide35

Cyclization of Monosaccharids

Has the aldehyde or ketone at the top of the drawing.

The carbons are numbered starting at the aldehyde or ketone end. The compound can be represented as a straight chain or might be linked to show a representation of the cyclic, hemiacetal form.35

Fisher projection of glucose. (Left) Open chain (linear form) Fisher projection. (Right) Cyclic Fisher projection.

Fischer Projection:

D-Glucose

Slide36

Cyclization of Monosaccharids

36

Fischer Projection: D- Fructose Slide37

Cyclization of Monosaccharides

Haworth Projection:

Represents the compound in the cyclic form that is more representative of the actual structure.This structure is formed when the functional (carbonyl) group (ketone or aldehyde) reacts with an alcohol group on the same sugar to form a ring called either a

hemiketal

or hemiacetal ring, respectively.

The reaction of (a) D-glucose in its linear form to yield the cyclic

hemiacetal

α

-D-glucopyranose, and (b) D-fructose in its linear form to yeild the hemiketal

α

-D-fructofuranose. The cyclic sugars are shown as both Hawoth projections and space-filling models.

37Slide38

Haworth Projections

A common way of representing the cyclic structure of monosaccharides

Monosaccharides do not usually exist in solution in their “open-chain” forms: an alcohol group can add into the carbonyl group in the same molecule to form a pyranose ring containing a stable cyclic hemiacetal or hemiketal. All of the atoms on the

right

are pointed

down in the Haworth structure. All of the atoms on the left are pointed up in the Haworth structure.

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Cyclization of Monosaccharids

In the

pyranose form of glucose, carbon-1 is chiral, and thus two stereoisomers are possible: one in which the OH group points down ( -hydroxy group) and one in which the OH group points up ( -hydroxy group). These forms are anomers of each other, and carbon-1 is called the anomeric

carbon.

39Slide40

Cyclization of Monosaccharids

The new carbon

stereocenter created in forming the cyclic structure is called an anomeric carbon.Stereoisomers that differ in configuration only at the anomeric carbon are called

anomers

.

The anomeric carbon of an aldose is carbon 1; that of the most common ketoses is carbon 2.

40Slide41

Cyclization of Monosaccharids

41

Anomeric

carbon

always above ring for D-

saccharidesSlide42

42Slide43

Chair Conformation

The use of Haworth formulas may lead to the erroneous impression that furanose and

pyranose rings are planner.Chair Conformation: The most stable conformation of cyclohexane that resembles a chair.The chair structure consists of a six-membered ring where every C-C bond exists in a staggered conformation.

Molecules will try to adopt the most stable conformation that minimizes strain.

43Slide44

Chair Conformation

When going from the Haworth projection to the chair conformation, the anomeric carbon’s substituent that points

down in the Haworth projection is going to be axial, and the substituent that points up in the Haworth projection is going to be equatorial. An axial –OH on the anomeric carbon makes the sugar an α sugar, while an equatorial –OH on the

anomeric

carbon makes the monosaccharide a β sugar.

Besides the substituents on the anomeric carbon, everything else is drawn relative to the Haworth projection. In other words, all the other

substituents are drawn pointing up if they were pointing up in the Haworth projection, and pointing down if they were pointing down in the Haworth projection.

44Slide45

Chair Conformation

45Slide46

Physical and Chemical Properties of Carbohydrates

46Slide47

Physical Properties of Monosaccharides

Most monosaccharides

have a sweet taste (fructose is sweetest; 73% sweeter than sucrose). They are solids at room temperature. They are extremely soluble in water: – Despite their high molecular weights (MW), the presence of large numbers of OH groups make the monosaccharides much more water soluble than most molecules of similar MW. – Glucose can dissolve in minute amounts of water to make a syrup (1 g / 1 ml H2O).

47Slide48

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions: REDUCTION

The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, such as NaBH4. Reduction of the C=O group of a monosaccharide gives sugar alcohols called alditols.The products named by replacing the -

ose

ending with

-itol.

Reduction of D-glucose:

48Slide49

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions: REDUCTION

D- Fructose

D-

Sorbitol

(sugar alcohol)

D-

Mannitol

Reduction of D-fructose:

49Slide50

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions: OXIDATION

Monosaccharides are reducing sugars if their carbonyl groups oxidize to give carboxylic acids.Oxidation of an aldose

yielding an

aldonic

acid (suffix –onic acid).

50

Oxidation of D-glucose:Slide51

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions: OXIDATION

51

Oxidation of D-glucose:Slide52

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions: OXIDATION

In a basic solution, ketoses are converted into aldoses.Fructose, a ketohexose, is also a reducing sugar. In a basic solution such as Benedict's, the carbonyl group moves from carbon 2 to carbon 1, so it can be oxidized as glucose. 52

Oxidation of D-fructose:Slide53

Chemical Properties of Monosaccharides

Oxidation-Reduction Reactions:

Oxidation

Loss of electrons

Gain of O

Loss of H (H

+

& e)

Reduction

Gain of electrons

Loss of O

Gain of H (H

+

& e)

53Slide54

Chemical Properties of Monosaccharides

54

Oxidation-Reduction Reactions:

The products of oxidation and reduction of D-Mannose are:Slide55

Chemical Properties of Monosaccharides

55

Oxidation-Reduction Reactions:Slide56

Chemical Properties of Monosaccharides

Amino Sugars:

Replace a hydroxyl group with an amino (-NH2) group.Only three amino sugars are common in nature: D-glucosamine, D-mannosamine and

galactosamine

.

56Slide57

Chemical Properties of Monosaccharides

Formation of Phosphate Esters:

Phosphate esters can form at the 6-carbon of aldohexoses and aldoketoses. Phosphate esters of monosaccharides are found in the sugar-phosphate backbone of DNA and RNA, in ATP, and as intermediates in the metabolism of carbohydrates in the body.

57Slide58

Chemical Properties of Monosaccharides

Glycosidic

Bond Formation:Carbohydrates can form glycosidic bonds with other carbohydrates and with noncarbohydrates.Glycoside

:

a carbohydrate in which the -OH of the

anomeric carbon is replaced by –OR (-H2O)Glycosidic bond

: the bond from the anomeric carbon to the -OR group.

58Slide59

Disaccharides

59Slide60

Disaccharides

Disaccharides are formed when two monosaccharide units are joint by a glycosidic linkage.

On hydrolysis, disaccharides will split into two monosaccharides by disaccharide enzymes (e.g. lactase).The most common disaccharides are:Maltose (Glucose + Glucose).Lactose (Glucose + Galactose).

Sucrose (Glucose + Fructose).

60Slide61

Disaccharides

Maltose:

Contains two D-glucose residues joined by a glycosidic linkage between carbon atom 1 (the

anomeric

carbon) of the first glucose residue and carbon atom 4 of the second glucose.The

free anomeric carbon of its glucose residues makes maltose a

reducing sugar

since it can be oxidized

α

-D-glucopyranosyl-(1 4)-D-glucopyranose

61Slide62

Disaccharides

Lactose (milk sugar):

Occurs naturally only in milk.Made up of D-galactose and one unit of D-glucose joined by a b-1,4-glycosidic bond.

The

free

anomeric carbon of its glucose residues makes lactose a reducing sugar.

β

-D-galactopyranosyl-(1 4)-D-glucopyranose

62Slide63

Disaccharides

Sucrose:

The most abundant disaccharide, occur throughout the plant kingdom and is familiar to us as common table sugar. Sucrose is formed by one unit of D-glucose and one unit of D-fructose joined by an α-1,2-glycosidic bond.

α

-D-glucopyranosyl-(1 2)-

β

-D-fructofuranoside

63Slide64

Disaccharides

Sucrose:

Sucrose is a non-reducing sugar (C1 of glucose and C2 of fructose are the anomeric carbon atoms).

α

-D-glucopyranosyl-(1 2)-

β

-D-fructofuranoside

64Slide65

Oligosaccharides

65Slide66

Oligosaccharides

A linear or branched carbohydrate usually from 3 to 12 monosaccharide units joined by glycosidic bonds.

Often found covalently attached to proteins or membrane lipids to form glycoproteins and glycolipids.

66Slide67

Polysaccharides

67Slide68

Polysaccharides

Polysaccharides are formed by the linkage of many monosaccharide units by glycosidic bonds.On hydrolysis, polysaccharides will yield more than 10

monosaccharides.Polysaccharides are not reducing sugars because the anomeric carbons are connected through glycosidic linkages.

68Slide69

Polysaccharides

Polysaccharides are classified as:

Homopolymers: consist of one type monosaccharide, e.g.:

Glucans

:

are homopolymers of glucose.

Galactans:

are

homopolymers

of

galactose

.

Hetropolymers

:

consist of more than one type monosaccharide.

The most common polysaccharides are:

Starch and glycogen (

energy-storage polymers)

.

Cellulose and chitin (

structural polymers).

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Storage Polysaccharides

Starch (plants)The storage form of carbohydrate in plants ingested by humans (

Amylase enzyme hydrolyzes starch to disaccharides).Plants store starch mostly in the roots and seeds.When a plant seed is in an energy poor state, the starch is

broken down

and used for energy and/or precursors.

70Slide71

Storage Polysaccharides

There are two forms of starch:

1. α-amylose: a linear polymer of glucose units liked with α (1- 4)

glycosidic

bond.

71Slide72

Storage Polysaccharides

There are two forms of starch:

2. Amylopectin: a highly branched polymer of all α,D-glucose subunits. The linkages in the backbone are all α(1-4).

Every 24 to 30 residues along a backbone there is a branch point.

The linkages at the branch points are

α(1-6). 72Slide73

Storage Polysaccharides

Glycogen

(animals)The main storage polysaccharide in animal cells. It is abundant in the liver and muscle.A branched

homopolymer

of glucose.

On hydrolysis, it forms glucose, which maintains normal blood sugar level and provides energy. It is the energy-reserve carbohydrate for animals.

73Slide74

Storage Polysaccharides

Glycogen (animals)

Similar to amylopectin: It has

backbone

of glucose residues linked in an

α (1- 4) configuration and Branches

that are linked α (1- 6).

Different from

amylopectin

:

In that it is

more highly branched

.

It contains α (1- 6) branch every 8-13 residues along a backbone.

74Slide75

Structural Polysaccharides

Cellulose (

plants

)

The most abundant polysaccharide in the world.

The main structural component of plant cells.

It is part of the cell wall and is a major component of wood.

The rigid nature of this polymer makes it an excellent structural element.

An

unbranched

linear polymer of D-glucose linked by

β

(1-4) glycosidic

bonds.The only difference between cellulose and

starch is the β- linkage.75Slide76

Structural Polysaccharides

Cellulose (

plants)

The glucose

β

(1- 4) glucose bond are very stable

and difficult to hydrolyzed by chemicals.

A very few species of bacteria contain an enzyme that can hydrolyzed the

β

(1- 4) bond.

Animals, such as cow that derive most of their nutrients from plant material, contain bacteria in their gut that can hydrolyze the glucose

β

(1- 4) glucose bond of cellulose.

76Slide77

Structural Polysaccharides

Chitin (invertebrates)The

major structural component of the hard, shell-like exoskeleton of invertebrates, such as insects, lobsters, crabs, shrimp, and other shellfish; also occurs in cell walls of algae, fungi, and yeasts.composed of units of N-acetyl-β-D-glucosamine joined by

β

-1,4-glycosidic bonds.

77Slide78

References

Biochemistry (Lippincott´s Illustrated Reviews series) by Champe P, Harvey R, and Ferrier D.

Clinical Chemistry by Bishop M, Fody E and Schoeff L. Biochemistry, by Voet D and Voet J.

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http://www.wiley.com/college/voet/047119350X/image_gallery/ch11/index.html

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https://www.inkling.com/read/lippincotts-illustrated-biochemistry-ferrier-6th/chapter-7/ii--classification-and-structure

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