302 Carbohydrates Intro

8/3/2019 302 Carbohydrates Intro

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Carbohydrates-structure and functionGeneral Biochemistry-II

(BCH 302)

Dr . Saba Abdi

Asst . Prof. Dept. Of Biochemistry

College Of Science

King Saud University. Riyadh.KSA


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2

Importance of the topicWhy is this topic important?

All organisms utilize carbohydrates important biomolecules

Nutrition: carbos are more than just starch and sugar

Application of previous concepts:

functional groups

stereochemistry

other structural features

} control biological properties

Dr.Saba


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3

Origin of CarbohydrateBefore 1900

}Monosaccharide: cannot be

hydrolyzed into simpler sugars

Glucose C6H12O6

Fructose C6H12O6 no changeH2O, H3O+

no changeH2O, H3O

+

Hydrolysis: waterbreaking; reaction with water,

often in the presence of acid or base

Sucrose C12H22O11H2O, H3O

+

glucose + fructose

Cellulose CnH2nOnH2O, H3O

+

many glucose

Disaccharide: saccharide composed of two simpler sugars

Polysaccharide: composed

of many monosaccharides}Starch CnH2nOn

H2O, H3O+

many glucose

Dr.Saba


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4

QuickTime and aDV/DVCPRO - NTSC decompressor

are needed to s ee this picture.

Origin of Carbohydrate

Sugar general formula = CnH2nOn

Confirmation

sucrose + H2SO4 C + H2O (steam)

dehydrating agent

= Cn(H2O)n

= carbohydrate

= carbon hydrate

sucrose

H2SO4

steam

carbon

Dr.Saba


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Carbohydrates

Widely distributed in nature

Function

Structural

Source of energy

Storage of energy

Chemical structure

Polyhydroxyaldehydes - aldoses

Polyhydroxyketones - ketoses

Classification

monosaccharides (1 unit)

oligosaccharides (2-10 units)

polysaccharides (> 10 units)

5Dr.Saba


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Monosaccharides-molecular structure

Chemical structure

Polyhydroxyaldehydes - aldoses

Polyhydroxyketones - ketoses

Number of carbon atoms

trioses (C-3)

tetroses (C-4)

pentoses (C-5)

hexoses (C-6) heptoses (C-7)

Contain asymmetric carbon atoms C* =>

optically active 6Dr.Saba


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Aldoses (e.g., glucose) have an aldehyde group at one end.

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

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C HHO

C OHH

C OHH

CH2OH

CH2OH

C O

D-fructose

C

C OHH

C HHO

C OHH

C OHH

CH2OH

D-glucose

OH

Dr.Saba


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Enantiomers Mirror image isomers are called enantiomers.

There are two series: D- and L-.

In the D isomeric form, the OH group on the

asymmetric carbon (a carbon linked to four

different atoms or groups) farthest from thecarbonyl carbon is on the right.

The number of stereoisomers is 2n, where n is the

number of asymmetric centers.

The 6-C aldoses have 4 asymmetric centers. Thus

there are 16 stereoisomers (8 D-sugars and 8 L-

sugars). 8Dr.Saba


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Levorotation and dextrorotation

Optical isomers rotate the beam of plane-polarized light for the same angle, but in

opposite direction.

Equimolar mixture of optical isomers hasno optical activity - racemic mixture

Dextrorotation and levorotation refer,

respectively, to the properties of rotatingplane polarized light clockwise (for

dextrorotation) or counterclockwise (for

levorotation).9Dr.Saba


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. A compound with dextrorotation is called

dextrorotatory ordextrorotary,while a

compound with levorotation is calledlevorotatory orlevorotary

Both D- and L- isomers can be dextrotatory

or leavorotatory. A dextrorotary compound

is often prefixed "(+)-" or "d-". Likewise, a

levorotary compound is often prefixed "()-

" or "l-".

These "d-" and "l-" prefixes should not be confused with the "D-"

and "L-" prefixes which is based on the actual configuration of

each enantiomer, with the version synthesized from naturally

occurring (+)-glyceraldehyde being considered the D- form

10Dr.Saba


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The (D)-Aldose FamilyThe (D)-Aldotrioses

C

C

CH2OH

HO H

H O

C

C

CH2OH

H OH

H O

One stereocenter two enantiomers

(L)-(-)-glyceraldehyde (D)-(+)-glyceraldehyde

C

C

CH2OH

H OH

H O

Dr.Saba


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Alternate representation

C

C

CH2OH

H OH

H O

The (D)-Aldose FamilyFischer Projections

Emil Fischer

Determined relative structure of(D)-aldoses

Nobel Prize in Chemistry 1902

Most natulral saccharides are (D)-form

(D)-(+)-glyceraldehyde

C

C

CH2OH

H OH

H O

Vertical lines

= broken wedges

Horizontal lines

= solid wedges

Fischer projection

CHO

H OH

CH2OH

Dr.Saba


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The (D)-Aldose FamilyThe (D)-Aldotetroses

Two stereocenters four stereoisomers

Two (D) and two (L)

(D)-(-)-erythrose (D)-(-)-threose

Not found in nature

C

C

CH2OH

H OH

C

OH

H OH C

C

CH2OH

H OH

C

OH

HO H

Dr.Saba


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The (D)-Aldose FamilyThe (D)-Aldopentoses

Three stereocenters eight stereoisomersFour(D) and four(L)

(D)-(-)-ribose

RNA (ribonucleic acid)

DNA (deoxyribonucleic acid)

(D)-(-)arabinose (D)-(-)-lyxose

Not found in nature

(D)-(+)-xyloseC

C

CH2OH

H OH

H OH

CH

C

OH

OH

C

C

CH2OH

H OH

H OH

CHO

C

H

OH

C

C

CH2OH

H OH

HO H

CH

C

OH

OH

C

C

CH2OH

H OH

HO H

CHO

C

H

OH

Dr.Saba


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The (D)-Aldose FamilyThe (D)-Aldohexoses

Four stereocenters 16 stereoisomers eight(D) and eight(L)

(D)-(+)-allosenot found in nature

C

C

CH2OH

H OH

H OH

CH OH

CH OH

C

OH

(D)-(+)-altrose

C

C

CH2OH

H OH

H OH

CH OH

CHO H

C

OH

(D)-(+)-glucosemost abundant

monosaccharide

C

C

CH2OH

H OH

H OH

CHO H

CH OH

C

OH

(D)-(+)-mannose

C

C

CH2OH

H OH

H OH

CHO H

CHO H

C

OH

Dr.Saba


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16

The (D)-Aldose FamilyThe (D)-Aldohexoses

Four stereocenters 16 stereoisomers eight(D) and eight(L)

(D)-(-)-gulose

not found in nature

C

C

CH2OH

H OH

HO H

CH OH

CH OH

C

OH

(D)-(-)-idose

C

C

CH2OH

H OH

HO H

CH OH

CHO H

C

OH

(D)-(+)-galactose

fairly common

C

C

CH2OH

H OH

HO H

CHO H

CH OH

C

OH

(D)-(+)-talose

C

C

CH2OH

H OH

HO H

CHO H

CHO H

C

OH

Most important aldoses: glucose, ribose, galactose

Dr.Saba


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Epimers

Pairs of monosaccharides different only in

configuration around only one specific C-

atom. For example, glucose and galactose are C-4

epimerstheir structures differ only in the

position of the -OH group at carbon 4.

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Cyclization of monosaccharides

monosaccharides with five or more carbonsare predominantly found in a ring (cyclic)

form, in which the aldehyde (or ketone)

group has reacted with an alcohol group onthe same sugar

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O C

H

R

OH

O C

R

R'

OHC

R

R'

O

aldehyde alcohol hemiacetal

ketone alcohol hemiketal

C

H

R

O R'R' OH

"R OH "R

+

+

Dr.Saba


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Cyclic Structures

most stable are rings with 5 and 6 members

most common in nature

in accordance to oxygen containing

heterocycles monosaccharides are called

with 5 atoms in cyclefuran

with6 atoms in cyclepyranoses

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O

O

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Cyclic Structures

Glucose forms an

intra-molecular

hemiacetal, as the C1aldehyde & C5 OH

react, to form a 6-

member pyranose

ring, named afterpyran

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H O

OH

H

OHH

OH

CH2OH

H

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose -D-glucose

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4

5

6

1 1

6

5

4

3 2

H

CHO

C OH

C HHO

C OHH

C OHH

CH2OH

1

5

2

3

4

6

D-glucose(linear form)

Dr.Saba


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Cyclic Structures

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CH2OH

C O

C HHO

C OHH

C OHH

CH2OH

HOH2C

OH

CH2OH

H

OH H

H HO

O

1

6

5

4

3

2

6

5

4 3

2

1

D-fructose (linear) -D-fructofuranose

Fructose forms either

a 6-member pyranose ring,

by reaction of the C2 keto

group with the OH on C6,

or a 5-member furanose ring,

by reaction of the C2 keto

group with the OH on C5.

This ring is more stablefor all ketones.

Dr.Saba


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Cyclization of glucose produces a new asymmetric centerat C1 (in

frucose at C2) . The 2 stereoisomers are called anomers, & .

Haworth projections represent the cyclic sugars as having essentially

planar rings, with the OH at the anomeric C1:

(OHbelow the ring) (OH above the ring). The and anomers of D-glucose interconvert in aqueous solution by

a process called mutarotation.

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H O

OH

H

OHH

OH

CH2OH

H

-D-glucose

OH

H H O

OH

H

OHH

OH

CH2OH

H

H

OH

-D-glucose

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4

5

6

1 1

6

5

4

3 2

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Because of the tetrahedral nature of carbonbonds, pyranose sugars actually assume a

"chair" or "boat" configuration, depending

on the sugar.

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O

H

HO

H

HO

H

OHOH

HH

OH

O

H

HO

H

HO

H

HOH

HOH

OH

-D-glucopyranose -D-glucopyranose

1

6

5

4

3

2

Dr.Saba


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Reducing sugars

If the oxygen on the anomeric carbon of asugar is not attached to any other structure,

that sugar can act as a reducing agent and is

termed a reducing sugar. Such sugars canreact with chromogenic agents (for

example, Benedict's reagent or Fehling's

solution) causing the reagent to be reduced

and colored, with the anomeric carbon of

the sugar becoming oxidized to a carboxyl

group.

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Reactions of monosaccharides*

1. Esterification

2.Oxidation (only aldose sugar)

3. Reduction 4. Cyanohydrin

5.Osazone (test for identification of sugar)

6. Furfurals 7. Enolization

(* Refer to hand out of reactions)25Dr.Saba


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Sugar derivative

sugar alcoholFormed by reduction of an aldehyde

or ketone group of monosaccharide; e.g., ribitol.

sugar acid - the aldehyde at C1, or OH at C6, is

oxidized to a carboxylic acid; e.g., gluconic acid,

glucuronic acid, ascorbic acid (vitamin C).

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COOH

C

C

C

C

H OH

HO H

H OH

D-gluconic acid

D-glucuronic acid

CH2OH

OHH

CHO

C

C

C

C

H OH

HO H

H OH

COOH

OHH

CH2OH

C

C

C

CH2OH

H OH

H OH

H OH

D-ribitol

Dr.Saba

i


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amino sugaran amino group substitutes for a hydroxyl. An example isglucosamine (component of chitin).The amino group may be acetylated, as in N-

acetylglucosamine.

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H O

OH

H

OH

H

NH2H

OH

CH2OH

H

-D-glucosamine

H O

OH

H

OH

H

NH

OH

CH2OH

H

-D-N-acetylglucosamine

C CH3

O

H

Dr.Saba


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N-acetylneuraminate (N-acetylneuraminic acid, also

called sialic acid) is often found as a terminal residue

of oligosaccharide chains of glycoproteins.

Sialic acid imparts negative charge to glycoproteins,because its carboxyl group tends to dissociate a

proton at physiological pH, as shown here.

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NH O

H

COO

OH

H

HOH

H

H

R

CH3C

O

HC

HC

CH2OH

OH

OH

N-acetylneuraminate (sialic acid)

R =

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Glycosidic BondsThe anomeric hydroxyl and a hydroxyl of another sugar orsome other compound can join together, splitting out waterto form a glycosidic bond:

R-OH + HO-R' R-O-R' + H2OE.g., methanol reacts with the anomeric OH on glucose toform methyl glucoside (methyl-glucopyranose).

Glycosidic bonds are readily hydrolyzed by acid but resistcleavage by base

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O

H

HO

H

HO

H

OH

OHHH

OH

-D-glucopyranose

O

H

HO

H

HO

H

OCH3

OHHH

OH

methyl--D-glucopyranose

CH3-OH+

methanol

H2O

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O- and N-glycosides

If the group on the non-carbohydrate

molecule to which sugar is attached is -OH

group the structure is an O-glycoside. If the group is an-NH2, the structure is N-

glycoside

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.

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DisaccharidesComposed of two monosaccharide molecules

Useful vocabulary:

Linked by glycoside (an ether), part ofacetal functional group

Other anomeric carbon = hemiacetal functional group

glycoside linkage

C-O-C

OO

O

CH2OH

HO

HO

HO

HOHO

CH2OH

OH

Dr.Saba


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DisaccharidesCarbohydrate Ring Numbering

Anomeric carbon receives lowest number

Carbon 1 in aldoses

Carbon 2 (rarely 3) in ketoses

All other carbons numbered in order

OO

CH2OH

HO

HO

12

3

45

6

O

Numbering for an aldohexose

Dr.Saba


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Naming disaccharides By convention the name describes the compound with its

nonreducing end to the left, and we can build up the

name in the following order.

(1) Give the configuration ( or) at the anomeric

carbon joining the first monosaccharide unit (on the left) to

the second.

(2) Name the nonreducing residue; to distinguish five- and

six-membered ring structures, insert furano or pyrano

into the name.

(3) Indicate in parentheses the two carbon atoms joined bythe glycosidic bond, with an arrow connecting the two

numbers

(4) Name the second residue

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DisaccharidesLactose

1,4--D-galactopyranosyl-D-glucopyranose

Present in mammalian milk (up to 8 % by weight; varies with species)

Readily digested by infant mammals; requires enzyme lactase

Adults often less tolerant due to low levels of lactase

It is a reducing sugar

Lactose

O

OO

CH2OHHO

HO

OH

HO

CH2OH

OHHOH3O

+/H2O

hydrolysis

+O

OH

CH2OH

HOHO

HO

Glucopyranose

(glucose)

Galactopyranose

(galactose)

O

CH2OHHO

HO

HO OH

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DisaccharidesSucrose

1,2--D-fructofuranosyl--D-glucopyranose

Unusual structure: 1,2--glycoside

Most common disaccharide in nature

Produced only by plants such as sugar cane, sugar beats

An -glycoside: readily digested by mammals

Sucrose contains no free anomeric carbon atom; the anomeric carbons of

both monosaccharide units are involved in the glycosidic bond . Sucrose

is therefore a nonreducing sugar.

Sucrose

O

OO

CH2OH

HOHO

HO

OH

OH

CH2OH

CH2OH

H3O+/H2O

hydrolysis

O

OH

CH2OH

HOHO

HO

Glucopyranose(glucose)

Fructofuranose(fructose)

+ O

OH

OH

CH2OHHO

CH2OH

Dr.Saba


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.

Maltose, a cleavage product of starch (e.g.,

amylose), is a disaccharide with an (14)glycosidiclink between C1 - C4 OH of 2 glucoses.

Because the disaccharide retains a free anomeric

carbon (C-1 of the glucose residue on the right) ,maltose is a reducing sugar

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H O

OH

H

OHH

OH

CH2OH

H

O H

OH

H

OHH

OH

CH2OH

H

O

HH

1

23

5

4

6

1

23

4

5

6

maltose

Dr.Saba


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.

Cellobiose, a product of cellulose breakdown,The (14) glycosidic linkage between twoglucose molecules.

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H O

OH

H

OHH

OH

CH2OH

H

O OH

H

H

OHH

OH

CH2OH

H

H

H

O1

23

4

5

6

1

23

4

5

6

cellobiose

Dr.Saba


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.

Trehalose is found in plants and insects

formed by an ,-1,1-glucoside bond

between two -glucose units

It is non reducing sugar

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Polysaccharides

Generally called glycans

Contains a number of monosaccharide units linked

by glycosidic bonds

Divided into two broad groups:(i) Homopolysaccharides-contains only one type of

monomer. Examples:Starch,cellulose,glycogen

(ii) Heteropolysaccharides- Contain two or more

types of monomers. Examples: hyaluronic acid,

chondriotin sulphate, heparin, and mureins.

40Dr.Saba


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PolysaccharidesCellulose

Linear 1,4--D-glucopyranose polymer

~5,000 - 10,000 glucopyranose molecules per cellulose molecule

repeating subunit:glucopyranose

OO

OO

OOO

HO

CH2OH

OH

HO

CH2OH

OH

CH2OH

HO

OH

Most abundant organic substance in nature

Function: support structure in plants

Wood is ~50% cellulose by weight

Strength due to intermolecular hydrogen bonding

H3O+/H2O

hydrolysis

O

OH

CH2OH

HOHO

HO

Many glucopyranose

Not easily digested by mammalsDr.Saba


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CelluloseCellulose, a major constituent ofplant cell walls, consists of long linear

chains of glucose with (14) linkages.

Every other glucose is flipped over, due to linkages.

This promotes intra-chain and inter-chain H-bonds and van der Waals

interactions, that cause cellulose chains to be straight & rigid, and pack

with a crystalline arrangement in thick bundles - microfibrils

Multisubunit Cellulose Synthase complexes in the plasma membrane spin

out from the cell surface microfibrils consisting of 36 parallel, interacting

cellulose chains.

These microfibrils are very strong.

The role of cellulose is to impart strength and rigidity to plant cell walls,

which can withstand high hydrostatic pressure gradients. Osmoticswelling is prevented.

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Chitin

It is a structural homopolysaccharide

It is a linear molecule composed ofN-

acetylglucosamine monomers linked by (14) glycosidic bond

It forms extended fibers similar to that of

cellulose It is component of exoskeleton of insects

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Polysaccharides

OO

HO

CH2OH

HOO

OHO

CH2OH

HO O

O

OHO

CH2OH

HO

Amylose

Starch

Two forms: amylose, amylopectin

Amylose

Linear coiled polymer of glucopyranose linked by

-1,4 glycosidic bonds

20-25% of starch OO

HO

CH2OH

HO OO

HO

HO O

O

OHO

CH2OH

HO

O

O

OHO

CH2OH

HO

Amylopectin

Amylopectin

Branched polymer containing glucopyranose linked by

-1,4 glycosidic bonds. Branch points has

-1,6glycosidic bonds, 12 glucose in a branch

75-80% of starch

1,4--D-glucopyranose polymer

Function: plant glucose/energy storage

Hydrolysis glucopyranose

Easily digested by mammals

Glucose storage in polymeric form minimizes

osmotic effects

Dr.Saba

CH2OH CH2OH gl cogen


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.

Glycogen, the glucose storage polymer in animals, is similarin structure to amylopectin.

But glycogen has more(16) branches.

The highly branched structure permits rapid glucose releasefrom glycogen stores, e.g., in muscle during exercise.

The ability to rapidly mobilize glucose is more essential toanimals than to plants.

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H O

OH

H

OHH

OH

CH2OH

H

O H

H

OHH

OH

CH2OH

H

O

HH H O

OH

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

2

HH H O

H

OHH

OH

2

HH

O

1

OH

3

4

5

2

glycogen

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Glycosaminoglycans

Glycosaminoglycans (mucopolysaccharides) are

linear polymers ofrepeating disaccharides.

The constituent monosaccharides tend to be

modified, with acidic groups, amino groups, sulfated

hydroxyl and amino groups, etc.

Glycosaminoglycans tend to be negatively charged,

because of the prevalence of acidic groups.

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Hyaluronate(hyaluronan) is a glycosaminoglycan with a

repeating disaccharide consisting of 2 glucose derivatives,

glucuronate (glucuronic acid) &N-acetyl-glucosamine. Theglycosidic linkages are (13) & (14). Form ground

substance of connective tissue.

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H O

H

H

OHH

OH

COO

H

H O

OH HH

NHCOCH3H

CH2OH

H

OO

D-glucuronate

O

1

23

4

5

61

23

4

5

6

N-acetyl-D-glucosamine

hyaluronate

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.

Inulin :Plant polysaccharide of fructose residues

connected by 2-1 linkage

Chitin: Polymer of N-acetylglucosamine linkedthrough -1,4 glycosidic bond. Found in fungal

cell wall and arthropod cuticle

Mannan: Polymer of mannose linked by -1,4

and -1,3 glycosidic bonds. Found in cell wall ofbacteria yeast and some plants.

Heparin: Consists of glucosamine N-sulphate and

sulphate esters of glucuronic acid. Found in

granules of mast cells has anticoagulant

properties.

hDr.Saba


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Physical propreties of

carbohydrates Monosaccharides are colorless crystalline

solids, very soluble in water, but only

slightly soluble in ethanol, low molecularweight, sweet tasting

Disaccharides are low molecular weight,

sweet,crystalline, less soluble in water thanmonosaccharides

Polysaccharides are high molecular weight,

not sweet, not soluble and not crystalline.49Dr.Saba


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Complex Carbohydrates

Carbohydrates can be attached by

glycosidic bonds to non-carbohydrate

structures :(a) Nitrogen bases (in nucleic acids)

(b)Aromatic ring (in steroid and bilirubin)

(c) Proteins (in glycoproteins andglucosaminoglycans)

(d) Lipids (in glycolipids)50Dr Saba

302 Carbohydrates Intro

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