biological molecules - proteins

8/8/2019 Biological Molecules - Proteins

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Prepared by Raajeswari Rajendran

Unit 3 Structure and

Function of Cells

Proteins


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Proteins are polymers made up of

amino acids

They consist of carbon, hydrogen,oxygen, nitrogen, phosphorus and

sulphur

They perform many functions


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An overview of protein functions


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Polypeptides

PolypeptidesAre polymers of amino acids

A protein

Consists of one or more polypeptidesAll polypeptide chains are polar due to

the presence of the carboxyl group and

amino group at the terminal end


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Amino Acid Monomers

Amino acids

Are organic molecules consisting of anasymmetrical carbon atom attached to a

carboxyl group, an amino group, a hydrogen

atom and a side group, R

Differ in their properties due to differing sidechains, called R groups

There are about 20 types of amino acids in

nature due to 20 different types of R groups

and the specific sequence of amino acids in a

polypeptide chains determines the function of a

protein


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Amino acids

Can form zwitterions in water whenthe amino and carboxyl groups ionize

They are amphoteric in nature acting

as acids and bases at the same time


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Amino acids

20 different amino acids make up

proteins

O

O

H

H3N+ C C

O

O

H

CH3

H3N+ C

H

C

O

O

CH3 CH3

CH3

C C

O

O

H

H3N+

CH

CH3

CH2

C

H

H3N+

CH3 CH3

CH2

CH

C

H

H3N+ C

CH3

CH2

CH2

CH3N+

H

C

O

O

CH2

CH3N+

H

C

O

O

CH2

NH

H

C

O

O

H3N+ C

CH2

H2C

H2N C

CH2

H

C

Nonpolar

Glycine (Gly) Alanine (Ala) Valine (Val) Le cine (Le ) I le cine (Ile)

Methi nine (Met) Phenylalanine (Phe)

C

O

O

Trypt phan (Trp) Pr line (Pr )

H3C

S

O

O


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O

OH

CH2

C C

H

H3N+

O

OH3N

+

OH CH3

CH

C C

HO

O

SH

CH2

C

H

H3N+ C

O

OH3N

+ C C

CH2

OH

H H H

H3N+

NH2

CH2

O

C

C C

O

O

NH2 O

C

CH2

CH2

C CH3N+

O

O

O

Polar

Electrically

charged

O O

C

CH2

C CH3N+

H

O

O

O O

C

CH2

C CH3N+

H

O

O

CH2

CH2

CH2

CH2

NH3+

CH2

C CH3N+

H

O

O

NH2

C NH2+

CH2

CH2

CH2

C CH3N+

H

O

O

CH2

NH+

NH

CH2

C CH3N+

H

O

O

Serine (Ser) Threonine (Thr)Cysteine

(Cys)

Tyrosine(Tyr)

Asparagine(Asn)

Glutamine(Gln)

Acidic Basic

Aspartic acid(Asp)

Glutamic acid(Glu)

ysine ( ys) Arginine (Arg) Histidine (His)


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Amino Acid Polymers

Amino acids - are linked by peptide bonds

produced by condensation reactions to form apolypeptide chainOH

DESMOSOMES

OH

CH2

C

H

C

H O

H OH OH

Peptidebond

OH

OH

OH

H H

HH

H

H

H

H

H

H H

H

SH Side chains

SH

OO

O O O

H2O

CH2 CH2

CH2 CH2 CH2

C C C C C C

C CC C

Peptidebond

Amino end( -terminus)

Backbone

(a)

(b)Carboxyl end(C-terminus)


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Protein Conformation and Function

A proteins specific conformation

Determines how it functions

Proteins can be globular e.g. catalytic

proteins such as enzymes, hormones or

fibrous e.g. structural proteins such as

collagen and keratin

There are 4 levels of protein structure:

1. Primary structure

2. Secondary structure

3. Tertiary structure

4. Quaternary structure


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Primary structure

Is the unique sequence of amino acids in a

polypeptide formed by peptide bonds

Amino acidsubunits

+H3NAmino

end

o

Carboxyl end

oc

GlyProThrGly

Thr

Gly

GluSeu

ysCysPro euMet

al

ys

al

euAsp

Ala

alArgGlySer

ProAla

Gly

lle

SerProPheHisGluHis

Ala

Glu

alalPheThrAla

Asn

Asp

SerGlyPro

ArgArg

TyrThr

lleAla

Ala

eu

euSer

ProTyrSerTyrSerThr

Thr

Ala

al

alThrAsnPro

ysGlu

Thr

ys

SerTyrTrp

ysAla eu

Glu

leAsp


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Secondary structureIs the folding or coiling of the polypeptide into a

repeating configurationIncludes the E helix (e.g. -keratin) and the Fpleated sheet (e.g. silk proteins)

The secondary structures are maintained byhydrogen bonding

O C E helix

F pleated sheet

Amino acidsubunits

NCH

C

O

C N

H

C

O H

R

C N

H

C

O H

C

R

N

HH

RC

O

R

C

H

N

H

C

O H

NC

O

R

C

H

N

H

H

C

R

C

O

C

O

C

N

HH

R

C

C

O

N

HH

C

R

C

O

N

H

R

C

H C

ON

HH

C

R

C

O

N

H

R

C

H C

O

N

HH

C

R

C

O

N H

H C R

N H O

O C N

C

RC

H O

CHR

N H

O C

RC

H

N H

O CH C R

N H

CC

N

R

H

O C

H C R

N H

O C

RC

H

H

C

R

N

H

C

OC

N

H

R

C

H C

O

N

H

C

H H


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Tertiary structureIs the overall three-dimensional shape of a

polypeptide - results from interactions betweenamino acids and R groups

The tertiary structure is maintained by ionic

bonds, disulfide bridges, hydrogen bonds,

hydrophobic and an der Waals interactions

O

O

O

N 3+ -O

O

SS

3

3

3

3

Hydrophobicinteractions and

van der Waals

interactions

Polypeptide

backboneHyrdogen

bond

Ionic bond

CH2

Disulfide bridge


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Quaternary structure

Is the overall protein structure that results from the

aggregation of two or more polypeptide subunitsE.g. collagen and hemoglobin

Polypeptidechain

Coll

F Chains

E ChainsHemoglobin

IronHeme


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The four levels of protein structure

+H3NAmino end

Amino acidsubunits

Ehelix


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What Determines Protein

Conformation?

Protein conformation

Depends on the physical and chemical

conditions of the proteins environment


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DenaturationIs when a protein unravels and loses its native

conformationDenaturation

Renaturation

Denatured proteinNormal protein


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Causes of Protein Denaturation

1. pH2. Temperature

3. Salt concentration

4. Detergents and other chemicals thatdisrupt chemical bonds

biological molecules - proteins

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