BCH 3000PRINCIPLES OF BIOCHEMISTRY

(Semester 2 -2014/15)

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Carbohydrates as the main component of life

Classification

Structure

Chemical reactions and biochemical functions of carbohydrate

CARBOHYDRATE

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CARBOHYDRATE

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● 'hydrate of carbon', with structural formula (CH2O)n

● Carbohydrate = saccharides = biological molecules

● The basic building blocks of carbohydrates are the monosaccharides –i.e. simplest = monosaccharides, eg. glucose

● These are linked together in longer chains to form oligosaccharides (2 - 20 residues - eg. Maltose) and polysaccharides (> 20 residues eg. starch, cellulose , glicogen )

CARBOHYDRATES

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● The root sacchar- comes from the Latin saccharum,"sugar".

● Why saccharide called carbohydrate ?

Most have general formula (CH2O)n

● Many saccharides

a. Modified

b. Contains amino groups, sulphates, phosphates, etc.

CARBOHYDRATES

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● important source of energy for the body - 1g of carbohydrate provides 4.2 kcal of energy

o Almost all of the cells use glucose to distribute energy -brain cells ; erythrocytes (red blood cells) are completely dependant on glucose as an energy source.

● Act as energy storage - glycogen stores act as a readily available energy reserve. A person weighing 70kg has a glycogen reserve of about 350 - 400g, which is about 1.500 kcal.

Functions of carbohydrates

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● Carbohydrates find many uses as structural elements – eg. Cellulose – cell walls in plants, bacteria, exoskeleton of insects,

● Carbohydrates are utilized as raw materials for several industries. For e.g., paper, plastics, textiles etc.

● Marker molecules for cell recognition – Blood types – A,B,O

● Found in biological molecules e g coenzymes and nucleic acids

Functions of carbohydrates

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1. Monosaccharide- one sugar residue. Most well known is glucose, C6H12O6

2. Oligosaccharide- a few (2-9) sugar residues . Most well known is cane sugar or sucrose, C12H22O11.

3. Polysaccharide - many sugar residues. Most common are glycogen, starch and cellulose, from animals, plants and plants.

Classification of Carbohydrates

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1. Two classes- aldoses (aldehyde) and ketoses (keto)

2. In our formula, (CH2O)n, n is 3 or more

3. Simplest are dihydroxycetone, a ketose where n=3, and glyceraldehyde, an aldose where n=3 - Triose

4. Simple sugars – monomeric

5. can have different number of carbon atoms

6. can be combined to form disaccharides and polysaccharides

7. some can have a linear or ring structure

Monosaccharides

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● Aldose = an aldehyde with two or more hydroxylgroups.

● Ketose = a ketone with two or more hydroxyl groups

● Both are trioses = simplest monosaccharides; three-carbon sugars

Monosaccharides

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Aldehyde Ketone

back

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Aldoses and Ketoses

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● Both have the same compositions = TAUTOMERS =

(isomer)

● Tautomers are organic compounds that are

interconvertible by a chemical reaction called

tautomerization.

● Can change from one form to the other but takes a

very long time

● Catalysts can speed up the change

Why tautomer ??

Change from one form (aldehyde) to another (ketone)

Monosaccharides

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● with the same chemical formula

● and often with the same kinds of bonds between atoms,

● but in which the atoms are arranged differently.

● Many isomers share similar if not identical properties in most chemical contexts

Isomers = are molecules

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ENANTIOMER (Optical isomers)

● In glycerldehyde – chiral carbon = 4 different groups

● get 2 isomers = enantiomer mirror images which are not superimposable - D and L isomer

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Enantiomers -if they are –

● mirror images of each other- one is the mirror image of the other,

● two stereoisomers are enantiomers if they are different but each can be superimposed on the mirror image of the other.

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● means that the two molecules differ in their three-dimensional shapes only but that they have the same structural formulas.

● This means they have the same exact groupsattached in the same way.

● Only the three-dimensional orientation of these groups are different.

● enantiomers are stereoisomers

Stereoisomers

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● are stereoisomers that are not enantiomers(nonsuperimposable mirror images of each other).

● can have different physical properties and different reactivity.

● pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other

● cis-trans isomerism is a form of diastereomerism

Diastereomers (or diastereoisomers)

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Diastereomers

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QUESTION

Which is L isomer?

Which is D isomer?

Fischer Projections

● Glyceraldehyde is actually the basis for the L and D nomenclature

● Solution of D-glyceraldehyde rotates polarized light to the right (dextrorotatory) and L-glyceraldehyde rotates light to the left(levorotatory)

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Plane Polarized Light 26

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● Monosaccharides can have multiple chiral centers

● need some conventions for drawing their structures.

● For linear chains, the stereochemistry is often represented as a Fischer Projection:

Fischer Projections

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● In a Fischer projection the carbon chain is oriented in the vertical direction, with a conformation that projects the carbon bonds onto a flat plane, and with all horizontal bonds projecting out, in front of the plane

● When the molecule is oriented with the C1 aldehyde at the top, pointing away from the viewer, this defines a convention where the C2 hydroxyl group will be on the left for L-glyceraldehyde, and on the right for D-glyceraldehyde

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i. For longer monosaccharides, the assignment of the L and D configuration is determined by the configuration of the chiral carbon farthest away from the C1 carbonyl (ie. Highest chiral number)

ii. Eg. - glucose, a 6-carbon sugar, the C5 carbon is used. If the C5 hydroxyl group is on the left, the molecule is L-glucose. If the hydroxyl group is on the right, it is D-glucose

iii. In a carbon chain with 2 possible configurations for each chiral center, there are a total of 2n stereoisomers for a compound with n chiral carbons

Stereochemistry of Longer Monosaccharides

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Some D-

Aldose

Isomers

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Some D-Ketose

Isomers

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Under natural conditions, only one

enantiomer predominates – the D-isomer

cf. amino acid – L-isomer

e.g. monosaccharide – monosaccharide - = D-

monosaccharide

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Diastereoisomer :

Diastereomers are stereoisomers that are not enantiomers or mirror images of each other. Diastereomers can have different physical properties and different reactivity.

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Tetrose

Chiral carbons

Are they enantiomers?

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Contains 5 carbon atoms 3 chiral carbons = 23

stereoisomers - 4 pairs of enantiomers - untuk

aldose = aldopentose

BUT ketopentose – only 2 chiral carbons 4 isomers only

PENTOSE

Stereochemistry of Longer

Monosaccharides

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Some D-

Aldose

Isomers

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Aldopentose Ketopentose43

HEXOSE

● Monasaccharide with 6 carbon atoms

● Number of isomers – high

● Common hexosee = glucose & fructose,

● mannose & galactose also abundant – all

play important biological roles

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Cyclic structures

• are the common form of monosaccharides with 5 or 6 carbon atoms.

• form when the hydroxyl group on C-5 reacts with the

– aldehyde group or

– ketone group.

O O

Cyclic Structures

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STEP 1 : Number the carbon chain and turn clockwise to form a linear open chain.

Drawing the Cyclic Structure for Glucose

HHO

H

CH2OH

OHC

H

H

OH

OH

C

C

C

OH

C1

2

3

4

5

6

H

OHH

OH

C

H H

OH OH

C C CH

O

CHOCH2123456

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HHO

H

CH2OH

OHC

H

H

OH

OH

C

C

C

OH

C1

2

3

4

5

6

H

OHH

OH

C

H H

OH OH

C C CH

O

CHOCH2123456

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OH

OH

OHOH

CH2OH

O

STEP 2: Fold into a hexagon.

• Bond the C5 –O– to C1.

• Place the C6 group above the ring.

• Write the –OH groups on C2 and C4 below the ring.

• Write the –OH group on C3 above the ring.

• Write a new –OH on C1.

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5

4

3 2

1

Drawing the Cyclic Structure for Glucose

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OH

OH

OHOH

CH2OH

O

-D-Glucose -D-Glucose

OH

OH

OHOH

CH2OH

O

STEP 3 : Write the new –OH on C1

down for the form.

up for the form.

Drawing the Cyclic Structure for Glucose

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Summary of the Formation of Cyclic Glucose

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-D-Glucose and β-D-Glucose in

SolutionWhen placed in solution,

• cyclic structures open and close.

• -D-glucose converts to β-D-glucose and vice versa.

• at any time, only a small amount of open chain forms.

-D-glucose D-glucose (open) β-D-glucose

(36%) (trace) (64%)

OH

CH2OH

OH

O

CH

OH

OH

OH

OH

OHOH

CH2OH

O OH

OH

OHOH

CH2OH

O

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Cyclic Structure of Fructose

Fructose

• is a ketohexose.

• forms a cyclic structure.

• reacts the —OH on C-5 with the C=O on C-2.

D-fructose -D-fructoseα-D-fructose

O CH2OH

OH

OH

OH

CH2OHO OH

CH2OH

OH

OH

CH2OH

H OH

H OH

HHO

O

CH2OH

C

C

C

C

CH2OH

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CHO with 5 & 6 carbon atoms normally exist as ring structures

Cyclization = interactions between functional groups at

● C-1 & C-5 hemiacetal (in aldohexose)

● Or between C-2 dan C-5 hemiketal (in ketohexose)

carbonyl carbon becomes new chiral centre = anomeric carbon

Cyclic sugars – 2 different forms - dan = Anomers

CYCLICAL STRUCTURE

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Hemiacetal &

Hemiketal

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New Chiral centre

Glucose

Drawing the Cyclic Structure for Glucose

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1

2

3

4

5

6

6

5

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2

1

New Chiral centre

Fructose

Drawing the Cyclic Structure for Fructose

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● CHO 5 carbon = furanose – furan

● CHO 6 carbon = pyranose – Pyran

● Normally CHO > 5 carbon – in cyclic form

o Free carbonyl group can form @ anomer

o Can change from one form to another

CYCLICAL STRUCTURES

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Furanose

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pyranose

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Which isomer used for reaction?

● Certain reactions – any isomer

● Others – only one anomer

eg. RNA & DNA – requires -D-ribose & -D-deoksiribose

• Fischer Projection – usefull to explain `stereochemsitry’ sugars,

But does not give true picture of overall shape accurately.

use HAWORTH PROJECTION FORMULA

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Fischer Projection

Formula

Haworth Projection Formula

or ?

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Important Simple Monosaccharides

1. Glucose

2. Mannose

3. Galactose

4. Fructose

5. Ribose

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RNA - only ribofuranose

Dinding sel (polysaccharide) - pyranose

KETOPENTOSE – almost all in cyclic form, but

only furanose eg. a -D-ribulose

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1. Mutarotation.

2. Oxidation to CO2 + H2O

Reactions due to aldehyde group

3. Reducing sugars.

4. Reduction to polyols.

Reactions due to alcohol group

5. Esterification.

6. Formation of acetals, also called glycosides

REACTIONS OF MONOSACCHARIDES

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1. Oxidation & Reduction

● Important in biochemistry – provides energy when CHO completely oxidised

● Photosynthesis – reversible process – when CO2 & H2O reduced

● Oxidation reaction can be used to detect the presence of carbohydrates

eg. Aldehyde [O] carboxyl – basis for test for aldose

When aldehyde is oxidised, the oxidising agent is reduced

REACTIONS OF MONOSACCHARIDES

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Because of his property, they are called reducing agents.

Ketose is also a reducing agent – Y?

Ketoses can also be reducing sugars because they can isomerise (a tautomerisation) to aldoses via an enediol:

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● Sugars that contain aldehyde groups that are oxidised to carboxylic acids are classified as reducing sugars.

● They are classified as reducing sugars since they reduce the Cu2+ to Cu+ which forms as a red precipitate, copper (I) oxide.

REDUCING SUGARS

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● Common test reagents are :

Benedicts reagent (CuSO4 / citrate)

Fehlings reagent (CuSO4 / tartrate)

● Remember that aldehydes (and hence aldoses) are readily oxidised

● In order for oxidation to occur, the cyclic form must first ring-open to give the reactive aldehyde (?)

● So any sugar that contains a hemi-acetal will be a reducing sugar.

● But glycosides which are acetals are not reducing sugars.

hemi-acetal

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Another example of reagent for reducing sugars – Tollen’s Reagent

Use silver ammonium complex Ag (NH3)2+ as oxidising agent mirror precipitate on the walls of test tube

RCHO + 2Ag(NH3)2+ + OH-

RCOO- + 2 Ag+ + 3NH3 + NH4+ + H2O

Other methods – use enzyme – glucose oxidase –to detect glucose

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● Hydroxyl group (OH) in CHO reacts with acids ester

● E.g. Phosphate ester – intermediate in the breakdown of CHO energy

● Phosphate ester is normally formed when the phosphate from ATP reacts with sugars phosphorylated sugar – important in the metabolism of CHO

ESTERIFICATION REACTION

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Monosaccharides have several OH groups – can bind/exchange with other groups modify the original structures

A. Phosphate esters

Phosphate esters – found in many metabolsic pathways - ATP, ADP, etc

B. Acid & lactones

Oxidation of monosaccharides produces

1. Aldonic acids – e.g. aldose reacting with alkaline solution of Cu+

2. Lactone & uronic acids - [O] with enzim

MONASACCHARIDE DERIVATIVES

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Mannose

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Reduction of C=O polyhydroxy compounds = eg D-mannitol & D-glucitol

Ribitol or Adonitol is a crystalline pentose alcohol (C5H12O5) formed by the reduction of ribose. It occurs naturally in the plant Adonis vernalis

C. Alditols

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● Mannitol or 1,2,3,4,5,6-hexanehexol (C6H8(OH)6)

is a vasodilator which is used mainly to reduce

pressure in the cranium,

● Chemically, mannitol is an alcohol and a sugar,

or a polyol; it is similar to xylitol or sorbitol.

● Mannitol is also used as a sweetener for people

with diabetes.

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● 2 derivatives of amino sugars – in polysaccharides – glucosamine & galactosamine

● Exchange of OH with NH2

D.Amino sugars

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● glycosides are certain molecules in which a sugar part is bound to some other part

● Bond = glycosidic bond

● Formed when H2O is removed from OH – of saccharide & other compounds containing OH-

● Found in plants/animals

● e.g. - Salicin, a glycoside related to Aspirin

NB: OH- must be attached to anomeric carbon

E. Glycoside

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Salicin, a glycoside related to Aspirin

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● OH- from sugar with another OH- ether

bond

● OH- - must be anomeric

● bond= glycosidic bond

● product = glycoside

furanose = furanoside, pyranose = pyranoside.

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Glycosidic bonds between monosaccharides = basis for the formation of oligosaccharides and polysaccharides

Bonds = between a anomer or b anomer and another OH- of another sugar

lots of combinations - OH- must be numbered to differentiate

notation for glycosidic bond – which anomeric atom involved

e.g.. (1 4), (1 6), (1 1)

OLIGOSACCHARIDES

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Formed when two monosaccharides are joined together by glycosidic bond

Plays important roles in living organisms

Simplest and most important oligosaccharide = disaccharide

Examples of disaccharide = sucrose, lactose, maltose

OLIGOSACCHARIDES

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4 important characteristics to differentiate

disaccharides

1. monomer found and their configuration

2. which carbon involved in the bond

3. the arrangement of the monomers

4. the anomeric configuration of the

hydroxyl group

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Because of the variation in the glycosidic bond,

can get various types of polymers – branched

or linear

Chemical characteristics of poly- &

oligosaccharides formed will depend on

1. Chemical characteristics of

monosaccahrides

2. The type of glycosidic bonds formed (i.e.

which anonmer; which carbon atom etc )

e.g. The differences between celluloses

and starch is because of the difference

in the glycosidic bond formed between

glucose

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Bond: (1 4)

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Example.

● Arrangement – starts with non-reducing end –left –anomeric & enantiomeric – with prefixes

● Cyclic configuration with suffix

● Atoms between glycosidic bonds – the number inside bracket between residues

Not all oligosaccharides are dimeric; also possible to have trimers, tetramer and bigger

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● 2 Monosaccharides = -D-Glucose & -D- fructose

● Glucose = aldohexose = pyranose ; Fructosee =ketohexose = furanose

● -C-1glucose attached to fructose

● Not reducing sugar - why? – because both anomeric groups are involved in glycosidic bond

● But free glucose and fructose are reducing sugars.

When sucrose is digested, hydrolysed glucose & frctoseenergy

Sucrose -

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● A disaccharide formed from -D- galactose & -

D-glucose

● Galctose = epimer of glucose - ie. Reverse position at C-4

● Glycosidic bond = (14) between anomeric C-1 ( form) of galactose and C-4 carbon of glucose

Lactose

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Because anomeric carbon of glucose is NOT involved in the bond formation, can be in either &

Lactose = reducing sugar because the groups at the anomeric carbon atom (glucose) is not involved in glycosidic bond formation; hence can react with oxidising agents

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Lactose= milk sugar

Human can be allergic to milk or milk products – why???

lack of lactase (breaks lactose to galactose and glucose) lactose will accumulate

Lactose will be acted on by lactase bacteria produce Hydrogen gas, CO2 & organic acids – problems with digestion – bloating and diarrhea

Although lactose can be degraded to galactose, galactose has to be isomerised to glucose before being absorbed

can accumulate GALACTOSEMIA – mental retardation

LACTOSE INTOLERANCE

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a disaccharide – hydrolytic product of starch

2 molecules of D-glucose --D-glucose & -D-glucose joined by (1 4) bond

Different from celobiose

o hydrolysis of cellulose

o different glycosidic bond - D-glucose attached through (1 4)bond

o maltose can be digested by humans, cellobiose cannot

Maltose

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Epimers are diastereomers that differ in configuration of only one stereogenic center

Diastereomers are a class of stereoisomers that are non-superposable, non-mirror images of one another

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Epimers121

various functions

sequence of monmer determines the primary structure – normally simple monomers

● 1 type of monomer = homopolysaccharide

● 2 @ more = heteropolysaccharide

● normally not complex – not more than 2 residues

● Cf. Protein & nucleic acids - well defined length’ –polysaccharide chain - random length -

POLYSACCAHARIDE

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1. Important examples - amylose & amylopectin -starch in plants and & glycogen in mammals and bacterial cells

2. amylose, amylopectin & glycogen = homopolysaccharide (glucan) - deposited in the liver, otot

= polymer -D-glucopyranose-Difference – bond between residues

Storage Polysaccharide

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● amylose - linear, α(1 4)

● Amylopectin & glycogen α(1 4) + α(1 6), branched polymer

● Glycogen - more branched; if not they are very similar

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Regular and simple structure regular secondary structure

α(1 4) bond , each residue leans slightly compared to the previous resisue helical conformation

However, the helix is not stable- e.g. amylose form random coils

Iodine – can stabilise helix – because it can fit the core of the helix

Complex – blue in colour

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Glycogen & amylopectin – cannot get blue colour???

● Branches – inhibit formation of helix

● To form a helix – need 12 residue for ever turn

● amylopectin - 10 -12 residues, glycogen - 8 residue – not enough

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Plants – do not use/synthesise structural protein

use specialIsed polysaccharide

animal – use both

main polymer in plants - woody/fibrous

Polymer linear - D-glucose ( glucan)

Joined through (1 4)bond

Cellulose

STRUCTURAL POLYSACCHARIDE

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Animals- can digest starch – can cleave bond

Cellulose - cannot – requires symbiotic bacteria -produce enzyme cellulase

Ruminant - OK- cellulase is present

White ants – protozoa – can digest cellulse

Fungi - eg. mushroom – live on rottting wood, etc

Other polysaccharides also found.e.g.

xylans = polymer (1→ 4) - linked D-xylopyranose

Glucomanan = hemicellulose

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● Cellulose – Not confined to plants only

● Marine Invetebrata - eg. Tunicates - cellulose in the mantle

● in connective tissue - human

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Tunicates 135

Tunicates136

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Similar to cellulose

smalll difference - homopolymer N-asetil-D-glucosamine - minor constituent in fungi and algae- replace cellulose

Role in invertebrate - exoskeleton of arthropod & mollusks

CHITIN

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In vertebrata - previously known as mucopolysacharide

● chondroitin sulphate

● keratan sulphate connective tissue

● dermatan sulphate – skin

● Hyaluronic acid

1. All are polymers = repeating units of disaccharides

2. Sugar (CHO) = N-asetylgalactosamine @ N-asetylglucosemine

GLYCOSAMINOGLYCAN

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Main functions of glycosaminoglycan

● formationn of matrix that bind protein components with connective tissue eg. Proteoglycan – inside cartilage - filemental structure synthesised from hyaluronic acid with protein core

● Protein - keratin sulphate chain and chondroitinsulphate chains are attached

Structure - collagen fibers becomes compact and strong

● Bonds – electrostatic between sulphate and basic collagen side chains

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Non Structural function of glycosaminoglycan

Hyaluronic acid

● very soluble in water- found in-synovial fluid- lubricating agent for joints

● vitreous humor – eyes – agent for increasing fluidity

Heparin - anticoagulant – in body tissues – bound strongly to blood proteins (prothrombin III) –prevets enziymes in the coagulation of blood

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Polysaccharides in bacterial cell walls

● Gram + & Gram – based on cell wall

● Gram + = peptidoglycan = polysaccharide –peptide complex which are multilayered and crosslinked

● Gram - = single layer of peptidoglycan covered with membrane layer

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Gram + bacteria149

Gram - bacteria150

Importance of other peptidoglycas

● A few antibiotics inhibit bacterial growth by preventing peptidoglycan layers

● Lysozymes can dissolve cell walls cell lysis bacterial death

● Also found in bacteriophage, white of egg, eyedrops of humans

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● Many proteins are bound to saccharide = glycoprotein

● Different functions

● Saccharide chain (= glycan) bound to protein -2 ways

◊ Bound to N of the amino group of asparagine - (N-Iinked Glycans)

◊ Through

N-acetylglycosmine @

N-acetylgalactosamine

Differet structures – complex branched structures

Different functions – protein indicators – old proteins that need to be destroyed

GLYCOPROTEIN

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ABO blood group system, the classification of human blood based on the inherited properties of

red blood cells (erythrocytes) as determined by the presence or absence of the antigens A and B, which are carried on the surface of the red cells.

These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group system

Persons may thus have type A, type B, type O, or type AB blood

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eg. immunoglobin – sialic acid residues will be cleaved slowly, then receptor will recognise and bind to the protein, engulf the protein

• O-linked Glycans

Different functions – e.g..

● Antartic fish – have glycoprotein which acts as an “anti freez’ – fluids do not freeze although temperature below freezing point

● Mucins - glycoprotein – in salive – increase viscosity of fluid

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Humans can produce antibodies against the A & B, but NOT O; i.e. O is antigenic antigenic

Normally, antibodies react against other antigens. eg. Type A carries antibodies against B - therefore when receiving blood type B - will clot & precipitate

Blood type O carries antibodies against A & B cannot receive blood type A and B; but CAN donate to both

Bllod Type AB – carries A & B antigens; therefore no antibodies against A @ B ; only donatieto AB only

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Blood group antigens – a cell recognition phenomenon

Cells need to be marked –(on the surface) so that they can interact with other cells – can recognize own cells from other external cells

In animals – there a layer of saccharides bound to a protein

or lupid in the membranes – e.g. glycocalyx - can interactwith bacteria in the intestine; collagen

To act as signal, need to have a specific protein boundspecifically - imunoglobulin

Other examples – lectin – interact between cells andproteins in the intercellulr matrix – to maintain the structureof tissues and organs

Oligosaccharides as CELL MARKERS

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