carbohydrate chemistry lectures of 1st week

8/2/2019 Carbohydrate Chemistry Lectures of 1st Week

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Carbohydrate Chemistry 1

CARBOHYDRATE CHEMISTRY

DEFINITION:Carbohydrates are organic substances composed of carbon, hydrogen and

oxygen.

CLASSIFICATION:

Carbohydrates are classified into three main groups:

1. Monosaccharides (simple sugars): They consist of a single

polyhydroxyaldehyde (glucose) or ketone units (fructose). They can not be

hydrolysed into simpler units. The sugar unit is sometimes referred to as glycan.

2. Oligosaccharides (oligo = few): contain from two to ten

monosaccharide units joined in glycosidic bonds. e.g. disaccharides (2 units) e.g.

maltose and sucrose, trisaccharides (3 units).....etc.

3. Polysaccharides (poly = many): Also known as glycans. They are

composed of more than ten mon-osaccharide units e.g. starch, glycogen,

cellulose.....etc.

Monosaccharides

They have the common formula (CH2O)n, where n = 3 or some larger number.

CLASSIFICATION OF MONOSACCHARIDES:

Can be carried out by one of two methods:

1. According to the number of carbon atoms: Trioses, tetroses, pentoses,

hexoses, heptoses, octoses.

2. According to the characteristic carbonyl group (aldehyde or ketone

group):

- Aldo sugars: aldoses: monosaccharides containing aldehyde group

e.g. glucose, ribose, erythrose and glyceraldehyde.

- Keto sugars: ketoses: monosaccharides containing ketone group e.g.

fructose, ribulose and dihydroxy acetone.


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N. B.: Monosaccharides can be classified according to both methods e.g. fructose is a keto-hexose i.e. containing ketone group and six carbon atoms.

Forms of Monosaccharides:

Trioses:

D- glyceraldehyde Dihydroxyacetone

Tetroses:

D - erythrose

Pentoses:

Ketose

CH2OH

C = O

H- C OH

CH2 OH

D - erythrulose


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Hexoses:

Heptoses:

D - sedoheptulose

N. B.: Derivatives of trioses, tetroses and heptoses are intermediates in carbohydratemetabolism.

Aldopentoses enter in structure of nucleic acids, coenzymes and certain vitamins.

All simple monosaccharides are white, crystalline, solids, freely soluble in waterand have a sweet taste.

Ring structure of Monosaccharides and Haworth Configuration of Cyclic Sugars:

Monosaccharides of five or more carbon atoms are present in ring forms.

Ring formation involves condensation of the carbonyl group (C-1 in aldoses, and C-2

in ketoses) of the sugar with one of the alcohol groups of the same sugar forming a closed ring

structure.


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The carbonyl carbon atom in the cyclic form will be called the anomericcarbon and

it can exist in two different forms called

(with the hydroxyl group to the right) or

(with the hydroxyl group to the left).

Haworth suggested that monosaccharides are existing in biological systems, not as a

rectangle, but in a ring structure either

pyran (a ring having 5 carbons and an oxygen) or

furan (a ring having 4 carbons and an oxygen).

Aldohexoses exist mainly in the pyran form (link between C-1 and C-5)

while fructose, if phosphorylated or linked to other sugars, will assume the furan

form (link between C-2 and C-5).

In Haworth configuration, all groups to the right of carbons are oriented down

while all groups to the left of carbons are oriented up except those around carbon

5, the reverse orientation occurs.

Aldohexoses can exist either in the pyran form as glucopyranose( between the

aldehyde group at C1 and the alcohol group (C5) or the furan form as

glucofuranose ( between C1 and C4).

They exist mainly in the six membered pyranose ring .

Fructose can potentially form either a 5 membered furan ring (linkage between C2

and OH group of C5) or a 6-membered pyran ring (between C2 & OH group of

C6).


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O

CH2OHCH

2OH

OHH

HOH

CH2OH OH

C

HO - C - H

H - C - OH

H - C

CH20H

O

Fructofuranose


When fructose is linked to other sugars or when it is phosphorylated it assumes

the furanose form.

D- Glucose Ring form


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ASYMMETRIC CARBON ATOM:

It is a carbon atom attached to four different atoms or groups of atom e.g. the

middle carbon of glyceraldehyde. Any compound having asymmetric carbon(s)possess the following properties:

1. Formation of isomers.

2. Shows optical activity.

Isomers are compounds which have the same molecular weight, same

percentage composition, and differ in their physical and chemical properties.

IMPORTANT TYPES OF ISOMERISM:

1. D and L isomers :

They are pairs of compounds that have the same structural formulas but differ in

spatial configuration; one of them is the mirror image of the other and they rotate the

plane of polarised light equally but in opposite directions.

LOOK AT THE PRE-LAST CARBON:

If the OH is on the right: D-sugarIf the OH is on the left: L-sugar

Most of the monosaccharides occurring in mammals are of the D configuration.

The monosaccharides with asymmetric carbon atoms are also optically active,

i.e when a beam of polarized light (light vibrating in one plane) is allowed to pass

through a solution of them, it will be rotated either to the:

1. right (dextrorotatory), or to the

2. left (levorotatory)

CHO CHO

H- C OH HO-C- H

CH2OH CH2OH

D-Glyceraldehyde L-Glyceraldehyde


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Naturally occurring L sugars are not so abundant e.g. L-fucose and L-

rhamnose. In humans the vast majority of the sugars are D. Two exceptions are L-

fucose found in glycoproteins and L- iduronic acid found in GAG.

2. Epimers:

Two monosaccharides differ only in the configuration around one specific carbon

atom (not the anomeric or subterminal ones).

D-glucose and D-mannose are epimers with respect to carbon atom 2,

D-glucose and D-galactose are epimers with respect to carbon atom 4.

3. Aldose-Ketose isomerism:

Two monosaccharides have the same molecular formulae and similar R groups but

differ in theirfunctional groups,

One has an aldehyde group (aldose e.g. glucose)

and the other has a ketone group (Ketose e.g. fructose).

Other examples of aldose-ketose isomers include:

D-glyceraldehyde and dihydroxyacetone,

D-ribose and D-ribulose,

D-mannose and D-fructose. They have similar R group.

H C = O CH2 OH

H - C OH C = O

R R


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4. and anomers:

The asymmetric carbon atom obtained from the carbonyl carbon atom in the ring

forms of monosaccharides is called the anomeric carbon.

In cyclic forms the new asymmetric carbon (anomeric carbon) can exist as two

different isomers called:

- anomer (with the hydroxyl group to the right) and

- anomer (with the hydroxyl group to the left).

- and - anomers are isomers of monosaccharides that differ from each other

only in configuration about the anomeric carbon.

- anomer has the OH to the right (below the plane in the Haworth

configuration).

anomer has the OH group to the left (above the plane in Haworth

configuration).


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Monosaccharides of physiologic importance

1-Pentoses:

-D-ribose is a structural element of ribonucleic acid (RNA)and

coenzymes e.g. ATP, NAD, NADP and others.

D-ribose-phosphate and D-ribulose-5-phosphate are formed from

glucose in the body (HMS).

2-deoxy D-ribose enters in the structure of DNA.

2-Hexoses:

a) D-glucose (grape sugar, Dextrose as D-glucose is dextrorotatory ). It

is the sugar carried by the blood (normal plasma level 70-100 mg/dL) and the

principal one used by the tissues. It is found in fruit juices and obtained by

hydrolysis of starch, cane sugar, maltose and lactose.

b) D-Fructose (honey sugar = levulose as D-fructose is levorotatory). It is

found in fruit juices (fruit sugar ) and obtained from sucrose and inulin by

hydrolysis. Free fructose in solution is in pyranose form, while in the

combined state in sucrose or inulin, it is in furanose form. It is present in thesemen in pyranose form

c) D-galactose:

It is obtained from hydrolysis of lactose. It is a constituent of galactolipids,

glycoprotein and GAGs. It is epimer of glucose

DERIVATIVES OF MONOSACCHARIDES

1. Sugar acids:

oxidation products of monosaccharides

Uronic acids

oxidation of the last carbon By specific enzymes only the primary alcohol group

is oxidized to a carboxyl group e.g. D-glucose yields D- glucuronic acid

D- glucuronic acidis important as a component of manypolysaccharides

and also in detoxication processes).

L-ascorbic acid (vitamin C) is another important naturally occurring sugar acid.


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2. Sugar alcohols:

reduction products of mono-saccharides.

e.g. ribose gives ribitol,

glucose gives sorbitol

mannose gives mannitol

galactose gives galactitol

ribose gives ribitol

fructose gives sorbitol or mannitol

The aldehyde and ketone groups of monosaccharides can be reduced, by addition

of H2 non enzymatically (in the presence of metal catalyst) or by enzymes to yield thecorresponding alcohol (pentitols and hexitols and So on ).

D-Sorbitol D-Mannitol D-Galactitol D-Ribitol

CH2 OH

H- C- OH

HO-C- H

HO-C- H

H- C-OH

CH2OH


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OOH

OH

H

OH

H

OH

OHH

H

HH


myoinositol is a cyclic sugar alcohol occurs in nature and is

important in formation ofphosphatidyl inositol.

CH2 OH CH2 -OH

CH OH H-C OH

CH2 OH H-C-OH

Glycerol H-C- OH

CH2 -OH

D.Ribitol Myoinositol

3. Deoxysugars:

In deoxysugars a hydroxyl group has been replaced by a hydrogen atom. The most

abundant naturally occurring deoxysugars are: 2-deoxy-D ribose in DNA.

4. Aminosugars:

They are monosaccharides in which the hydroxyl group at carbon 2 is replaced by an

amino group (NH2).The amino group of an aminosugar may be acetylated. Naturally occurring

aminosugars include, D-glucosamine, D-galactosamine and D-mannosamine.

D-Glucosamine - D- galactosamine - D -mannosamine

5 . Amino sugar acids (e.g. Neuraminic acids):

They are nine-carbon amino sugar derivatives consisting of 6-carbon aminosugar

linked to 3-carbon acid; the amino group is usually acetylaletd.

OH

OH

HOH

OH

H

CH2OH

H NH2

OH

OH

HOH

CH2OH

OH

H

H NH2

H

H

O

CH2OH

H

H

H

OH

H

OH

HOH

NH2


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Neuraminic acid is an important building unit of the structural polysaccharides in the

cell coats of higher animals. Sialic acids are N-acetyl derivatives of neuraminic acid

which is present in glycolipids(Gangliosides).

Neuraminic acid N-acetylneuraminic acid (NANA)

(D-Mannosamine-pyruvic acid)

6. Glycosides There are two types of glycosides :

Glycosides are compounds formed from a condensation between:

O- Glycosides: A sugar and another sugare.g. disaccharides or oligo and

polysaccharides.

N-glycosides: A sugar & an aglycone (non sugar radical), e.g.

Nucleosides = pentose + nitrogenous base. These are components of nucleotides

and nucleic acids.

Adenosine Uridine

COOH

C = O

CH2

H C OH

H2 N C H

HO C - H

H - C OH

H-

C

N

N

N

N

NH2

H

OHOH

H

O

CH2OH

N

N

O

H

OHOH

H

O

CH2OH

19


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DISACCHARIDES

Disaccharides have two monosaccharides, either the same or different, linked by O-

glycosidic linkage.

Depending on their reactions with Fehling, s solution they are classified into

a

group (has free anomeric carbon) , and a non reducing group

(both anomeric carbons are involved in the formation of the glycosidic

bond).

A . Reducing disaccharides :

1.MALTOSE( Malt sugar ):

Chemistry: Contains two D-glucose residues, in pyranose form, linked by -1-4

glucosidic bond. It is a abbreviated as Glc (1 4) Glc.

OH

CH2OH

H

OH

O

OH

H

H

H

H H

OH

OHH

HOH

CH2OH

H

OHO

Maltose

The second glucose residue has a free anomeric carbon atom and is capable of:

Existing in and forms.

Reducing alkaline metal salts.

Forming osazone.

Sources:

Germinating cereals and malt.

Intermediate product of the action of amylases on starch.

Hydrolysis: By acid or by maltase enzyme into two D-glucose units.

Fermentation: It is fermented by yeast enzymes.


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2. LACTOSE (Milk sugar ):

Chemistry: -D-galactopyranose and -D-glucopyranose linked by -1,4-

galactosidic linkage. It is abbreviated as Gal (1 4) Glc

- Lactose

Since it has a free anomeric carbon on the glucose residue, lactose is:

Reducing disaccharide.

Capable of forming osazone.

Able to occur in and forms

Sources: It is only found in milk.

Hydrolysis: By acid or lactase enzyme in the intestine, it yields D-galactose and D-

glucose.

Fermentation: It is non fermentable.

B. Non reducing disaccharides :

SUCROSE ( Cane sugar = Table sugar = Beet sugar )

Chemistry: It is a disaccharide of -D fructofuranose and -D-glucopyranose

linked by ( -2,-1)-fructosidic linkage or (-1, 2)-glucosidic linkage. It is abbreviated

as Glc (1 2) Fru.

O

CH2OH

OH

H

H

OH

H

H

O

CH2OH

OH

H

OH

H

OH

OH

OH

H

H

H

OH

CH2

OH

H

OH

H

O

CH2

OH

H

OH H

OHO

H

OH

HOCH2

HO

H


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The anomeric carbon atoms of the two hexoses are linked to each other, thus, sucrose

contains no free anomeric carbon atom so:

It does not act as a reducing sugar.

Does not form osazone.

Does not occur in and forms.

Sources: Sugar Cane and sugar beet .

Hydrolysis: By acids or by sucrase (invertase enzyme) toD-glucose and D-

fructose.

Inversion: It is hydrolysis of sucrose to D-glucose and D-fructose because it is

accompanied by a net change in optical rotation from dextro (of sucrose), to levo (of

invert sugar).

Invert sugar: e.g. natural honey

It is an equimolar mixture of D- glucose and D- fructose produced by hydrolysis of

sucrose and it is levorotatory since the strongly levorotatory fructose produced by

hydrolysis, changes (inverts) the previous dextrorotatory action of sucrose.Fermentation: It is fermentable sugar.

carbohydrate chemistry lectures of 1st week

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