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CARBOHYDRATES STRUCTURE

AND FUNCTIONS

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Carbohydrates (CHO) is widely distributed in

plants and animals. They have important

structural and metabolic roles.

In plants:Glucose synthesised from CO2 and H2O by photosynthesis.

In animal:Animal synthesise CHO from lipid, glycerol and amino acid.

Most animal CHO derived from plants.

Milk is the only animal-derived food that contains significant

carbohydrate

Glucose: The most important CHO

Most dietary CHO absorbed into the blood stream.

Other sugars are converted into glucose in the liver.

Major metabolic fuel of mammals except ruminant.

Universal fuel for fetus.

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Carbohydrate functions:

Carbohydrates provide the majority of energy in

most organisms (simple CHO are sugars, complex

CHO can be broken down into simple sugars)CHO are structural components of cell walls and

cell membranes.

CHO serves as metabolic intermediates (eg.

Glucosephosphate)

CHO (eg, ribose, deoxyribose) are components ofthe nucleotides that form DNA and RNA.

CHO play roles in lubrication, cellular

intercommunication, and immunity.

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Sugar Where found Biochemical importance

D- ribose

D- ribulose

D-xylose

D-arabinose

D-lyxose

D-xylulose

Nucleic acid

Formed in metabolic

processes

Wood gums,proteoglycan,

glycosaminoglycan

Gum arabic, plum

and cherry gums

Heart muscle

Intermediate in

uronic acid pathway

Structural element of nucleic

acids

Ribulose phosphate is an

intermediate in pentose

phosphate pathway.

Constituents of glycoproteins

Constituents of glycoprotein

A constituent of a lyxoflavin

isolated from heart muscle.

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- Carbohydrates are essential to all living organisms and are

the most abundant class of biological molecules.

- They are the end products of photosynthesis (100 billion

tons of CO2 + H2O/yearp cellulose and other plant

products.

- The metabolic breakdown of monosaccharides providesmost of the energy used to power biological processes.

- In structural material (cell walls, connective tissue)

- Important for cell signalling, cell-cell interactions

- Chemically, they are polyhydroxyl aldehydes or ketones.

- Contain three elements - C, H, O, many according to theformula (CH2O)n; where n 3.

- Greek saccharon means sugar

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CHO Classification and nomenclature

1. Monosaccharides (eg. glucose) are simple sugars. They maybe connected by glycosidic linkages to form the following

glycosides.

a) disaccharides (eg. Sucrose are composed of two

monosaccharides.

b) Oligosaccharides (eg, blood group antigens) are composedof two to ten monosaccharides.

c) Polysaccharides (eg, glycogen) are composed of more than

10 monosaccharides.

2. Aldoses and ketoses:

The reactive group (ie, aldehyde or ketone) on a CHOdetermines whether it is and aldose or a ketose.

a) Aldoses, (eg, glucose) possess a reactive group aldehyde

(CHO) group.

b) ketoses (eg, fructose) possess a reactive ketone (C O)

group.

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Nomenclature

Carbon numbering system. In the general

formula for monosaccharides

(CH2O)n (n=number of carbons). Carbons are

numbered sequentially with the carbon

possessing the aldehyde or ketone group

assigned the lowest possible number.

The total number of carbons determines whether

a monosaccharides is a triose (3C), a tetrose

(4C), a pentose (5C) or hexose (6C). Monosaccharide and reactive- group names can

be combined to designated compounds. Eg,

glucose is an aldohexose (ie, a hexose

possessing an aldehyde group.

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aldotriose ketotriose

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Glyceraldehyde

Glyceraldehyde

Is an aldotriose C

C

O H

CH2OHOHH

Glyceraldehyde exists in two

stereoisomeric forms because the

starred carbon is a stereocenter (chiral carbon):

it has four different groups attached.

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Monosaccharide stereoisomers

When the number ofchiral carbon atoms increases in

optically active compound the number of possible

optical isomers also increases.

A compound with n chiral carbon atoms has a

maximum of 2n possible stereoisomers.

Aldose have 2n-2

Aldose has two achiral carbons (C1 and C6).

Ketose have 2n-3

Ketose has three achiral carbons (C1, C2, and C6)

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Glucose with 6CD- glucose chiral carbon n=4

D-glucose with 24=16 stereoisomers.

(comprise all possible aldohexoses)

# almost all naturally occuring sugars have

the D-configuration.

In all D-aldose the hydroxyl group is to the right

on the chiral carbon atom farthest from the most

oxidised carbon in the molecule (C no 5 in a six

carbon sugars)

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Stereoisomers that are not enantiomers (mirror image

isomers) are called diastereomers.Aldopentoses;

D-ribose and L-ribose are enantiomers, as are D-

arabinose are diastereomers because they are isomers but

not mirror image.

Diastereomers that differ in the configuration at a single

asymmetric carbon atom are called epimers.

D-glucose and D-galactose are epimers because their

structures differ only in the configuration of the OH group

at carbon 4.D-mannose and D-galactose are not epimers because

their configuration differ at more than one carbon.

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CHO

C

C

OH

OH

C HOH

C

H

H OHCH

2OH

H

D- l

l

D- l t

l

CHO

C

C

OH

OH

C OHH

C

H

H OHCH

2OH

H

T i t r m r r l pim r , t y

iff r i fi r ti t ly t r -

t r ( l r t).

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Chiral carbon- carbon bonded to four different atoms or groups of

atoms).

Isomers are compound with the same chemical formula. Optical

isomers are alike with respect to what atoms are bonded to each

other but different in how the atom are oriented in space.

Epimers- are isomers with conformation that differ only at one

carbon atom.

Enantiomers are isomers that are mirror images.

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Fischer Projections

In a Fischer projection, the sugarmolecule is oriented so that themost oxidized carbon is to the top.The stereocenter carbons arearranged so that the groups not part

of the main chain projecthorizontally toward the viewer.

The molecule is in the all eclipsedform.

CO H

CH OH

C

CH2OH

H OH

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Monosaccharides are drawn in Fischer

projections with the most oxidized carbonclosest to the top. The carbons are numbered

from the top. If the the stereocenter with the

highest number has the OH to the right, the

sugar is D. If the OH is to the left, the sugar isL.

Most common sugars are in the D form.

Note: Fisher projections represent an alleclipsed conformation.

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1

2

3

4

1

2

3

4

5

65

CC

O H

C

OHH

C

H OH

H

CH2OH

OH

CH2OH

CC

OOH

C OHH C

H

H OH

CH2OH

D-ril t

D-fr tt

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Cyclic forms for sugars

Most simple sugars of four or more

carbons exist in the cyclic (hemiacetal orhemiketal) form.

A hydroxy group in the sugar reacts with

the carbonyl group. The new OH bearing carbon is now a

stereo center and is called an anomeric

carbon. If the OH on the ring is up the carbon is

F, if the OH is down it is E.

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Formation of hemiacetals and hemiketals a) form an aldehyde b) form a ketone

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Cyclic forms for sugars Fischer projections forE D glucose

C

CC OHOHC OHHC

H

H

CH2OH

H

OH

OH

C

CC OHOHC OHHC

H

H

CH2OH

H

OHH C

CC OHOHC OHHC

H

H

CH2OH

H

OH H

D-glucose E D-glucosecyclic form

O

F D-glucosecyclic form

O

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Sugars with 5 C mostly exist in their cyclized form (intramolecular hemiacetal

formation):

y Since C1 becomes a chiral center,there are two new possible

stereoisomers that are named E and F.

yAnomers: Isomeric forms that differ

only in their configuration about

the hemiacetal or hemiketalcarbon atom. E,F anomers

y anomeric carbon the hemiacetal

carbon atom.

E

F

* *

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Cyclic forms for sugars- Haworth

1. Draw a five- or six-membered ring

O O

pyranose form furanose form

2. Anomeric C to right ofO. Place OHup or down.

3. In D- sugars, the last C is always up.

furan (5 membered ring) Pyran (6-membered ring)

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CH2C O

OH

COH H

C OHH

CH

CH2OH

OH

O

CH2OH

OH

OH

OH

CH2

OH

D-fructoseF-D-fructofuranose

+ E isomer

Anomeric CF-OH

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CH2C O

OH

COH H

C OHH

CH

CH2OH

OH

O

CH2OH

OH

OH

OH

CH2

OH

D-fructoseF-D-fructofuranose

+ E isomer

Anomeric CF-OH

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Cyclic forms for sugars- Glucose

OCH

2OH

HH

OHH

OH

OH

H O

H

HOCH2OH

HH

OH H

OH

OH

H O

HH

OCH

2

OH

HH

OH

H

OH

OH

HH

O H

E form

(alpha)

F form(beta)

arrows showelectron movement

Pyranose ring

form

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The pyranose ring structure ofF-D-glucose

may be written as F-D-glucopyranose. The carbonyl carbon (C-1 in this case) is call

the anomeric carbon atom and so the E- and

F-forms rapidly interconvert via the open

chain structure, to give an equilibrium

mixture.

This proces is called mutarotation.

In the case of ketose, fructose, the anomeric

carbon atom (that carried the carbonyl

moiety) is C-2 and hence two isomers(anomers) exist which differ in their

configuration about the carbon atom, ie, the

EFdesignation refers to the configuration

about C-2 not C-1.

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Cyclic forms for sugars- Ribose also exists mainly in the cyclic

form.

O

CH2OH

H

H

H

OH

H

OH

O

HCH

2OH

HH

OH

H

OH

O

H

O H

F D-ribose

F D-ribofuranose(furanose ring form)

arrows showelectron movement

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Fisher configuration

The pyranose ring of six carbon sugar can

exist in either a boat and a chair configuration.

In the boat form, there is considerable steric

hindrance between the various groups

attached to the carbon atoms of the ring andhence this form is less favourable

energetically.

The chair form predominates, where all the

axial positions are occupied by hydrogen

atoms.

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E D-glucose: the chair conformer

OH

H H

H

HOH

H

O

OH

OH

CH2OH

1

2

3

4

5

6

Four of the five bulky groups (OH and CH2OH on C2,3,4,5)

on the ring are in the more stable equitorial positions!

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Fischer configuration

ofE and F-glucose

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Equilibrium mixture

of D-glucose

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Oxidation-reduction reactions

In the present of oxidizing agents, metal ions such as Cu2+, and

certain enzymes, monosaccharides readily undergo several

oxidation reactions.

Oxidation of

a) Aldehyde group yields and aldonic acid

b)Terminal CH2OH group (but not the aldehyde group)

gives a uronic acid.c) Aldehyde and CH2OH gives and aldaric acid.

The carbonyl groups in both aldonic and uronic acids can react

with an OH group in the same molecule to form a cyclic ester

known as a lactone.

Lactone are commonly found in nature.

L-ascorbic acid (vitamin E) is a lactone derivative of D-

glucuronic acid.

Sugars that can be oxidised by weak oxidizing agents such as

Benedicts reagent are called reducing sugars. The reaction

occurs only with sugars that can be revert to the open chain

form, all monosaccharides are reducing sugars.

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Aldehyde oxidn aldonic acid

Oxidation of Monosaccharides

COOH

CC

OHOH

C OHH

C

H

H OHCH

2OH

H

D-gluconic acid

Term CH2OH oxidn

uronic acid

CHOCC

OHOH

C OHHC

H

H OHCOOH

H

D-glucuronic acid

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Aldehyde + term CH2OH oxidn

aldaric acid

COOH

CC OHOHC OHHC

H

H OH

COOH

H

D-glucaric acid

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O

CH2OH

HH

OHH

OH

OH

H OO

CH2OH

HH

OHH

OH

OH

H

OH

H

Aldoses react with Tollens reagent (Ag(NH3)2

+) to give a

lactone (cyclic ester). The silver ion plates out as a mirror.

Cu2+

+ Cu2O (red-orange)

Benedicts reagent (a blue copper ion solution) also gives a

lactone. The blue color fades as reaction occurs.

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Reduction

Reduction of aldehyde and ketone groups of monosaccharide

yields the sugar alcohols (alditols).

D- Glucose D-glucitol or D-sorbitol

Sorbitol is used to prevent moisture loss in candy making,

pharmaceutical product.

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Modern methods for blood glucose determination

(Glucose oxidase method)

H2O2 thus formed can be measured by several methods.

y H2O2 reacts with Fe(CN) in the presence of luminol to produce

luminescence proportional to the initial glucose concentration.

y H2O2 is first converted to water and oxygen by the enzyme peroxidase

(POD). Then, 4-aminophenazone, an oxygen acceptor, takes up the

oxygen and together with phenol forms a pink coloured chromogen which

can be measured at 515mm.

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Reduction of Monosaccharides

The most important reduced sugar is deoxyribose. (In

DNA)

When the carbonyl of a sugar is reduced to analcohol, alditols are produced. The two shownabove are used to sweeten nonsugar gum.

D-sorbitol D-xylitol

CH2OH

C

C

OH

OH

C HOH

C

H

H OH

CH2OH

H

CH OH

CH2OH

CH2OH

C

C

OH

OH H

HC

C

O H

C

HH

C

H OH

OHH

CH2OH

D-deoxyribose

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Isomerization

Monosaccharides undergo several types of

isomerization.

After several hours an alkaline solution of D-glucose

also contain D-mannose and D-fructose.

Isomerization involved intramolecular shift of a H

atom and relocation of a double bond. The

intermediate formed is called an enediol.

Reversible transformation of glucose to fructose is

an aldose ketose interconversion. Conversion of glucose to mannose is referred to as

an epimerization.

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Isomerisation of D-glucose to form D-mannose and D-fructose

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Esterification

Like all free OH group, those of carbohydrates can be

converted to esters by reactions with acids.

Phosphate and sulfate esters of carbohydrate molecules are

among the most common ones found in nature.

Sulfate esters of carbohydrates molecules are foundpredominantly in proteoglycan component of connective

tissue.

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Esters of Monosaccharides The OH groups of sugars can react with

phosphoric acid to give phosphate esters.

O

H O O-

HH

OH

H

OH

OH

H H

OH

F -glucose- -phosphate

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Gl id

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Glycosides The anomeric OH can react with another OH on

an alcohol or sugar.W

ater is lost to form anacetal/ketal

O

H OH

H

H

OH

H

OH

OH

H

H

OH

H O H

O

H OH

H

H

OH

H

OH

OH

H

H

O H

H O

Acetal

carbon

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Important monosaccharides

Glucose

Primary fuel for living cells.

Fructose

Fruit sugar.

Important member of ketose family.On a pergram basis, fructose is twice as sweet as sucrose-used in smaller

amount.

Always used as a sweetening agent in processed food product.

Galactose

Important as a precursor for variety of biomolecules ie, lactose in lactatingmammary glands, glycolipids, certain phospholipids, proteoglycan and

glycoprotein.

Synthesis of the above substances is not diminished by diet lack of galactose

or disaccharide lactose (galactose+glucose).Why? Galactose is epimer to

glucose, epimerase enzyme will interconvert the glucose to galactose.

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A i S

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Amino Sugars

OCH2

HH

OH

H

OH

NH3

+

H OH

H

OH

E-D- l i

OCH2

HH

OH

H

OH

NH

H OH

HOH

C OCH3N- tyl-E-D- l i

A i S 2

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Amino Sugars-2

NHC

O

CH3

OR

OH

OH

OH

COO-

CH OHCH OH

CH2

OH

N- t l r i i ii li i

R=

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c) Deoxysugars Monosaccharides in which an -H has replaced an OH group.

Eg, L-fucose and 2-deoxyribose. .

Fucose always found in glycoprotein and 2-deoxyribose in

DNA.

cho for pkk 5203

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