Proteins – Structure and Function - PowerPoint Presentation

Slide1

Proteins – Structure and FunctionSlide2

Introducing proteins

Proteins

are a diverse group of large and complex polymer molecules, made up of long chains of

amino acids

.

They have a wide range of biological roles, including:

structural: proteins are the main component of body tissues, such as muscle, skin, ligaments and hair

catalytic: all enzymes are proteins, catalyzing many biochemical reactions

signalling: many hormones and receptors are proteins

immunological: all antibodies are proteins.Slide3

The general structure of amino acids

All amino acids have the same general structure: the only difference between each one is the nature of the

R group

.

The R group therefore defines an amino acid.

amino

group

carboxylic acid group

R group

The R group represents a side chain from the central ‘alpha’ carbon atom, and can be anything from a simple hydrogen atom to a more complex ring structure.Slide4

The 20 naturally-occurring amino acidsSlide5

Proteins – Structure and FunctionSlide6

Peptide bonds and dipeptidesSlide7

Polypeptides

When more amino acids are added to a dipeptide, a

polypeptide

chain is formed.

A protein consists of one or more polypeptide chains folded into a highly specific 3D shape.

There are up to four levels of structure in a protein:

primary, secondary, tertiary and quaternary. Each of these play an important role in the overall structure and function of the protein.Slide8

Portion of polypeptide chain

Amino acids

Peptide bond

 - helix

Hydrogen bonds

Hydrogen bonds form

Primary structure

- the sequence of amino acids in a polypeptide chain

Secondary structure

- the folding of the polypeptide chain into an

α

-helix or a

β

-pleated sheet.Slide9

 - pleated sheet

Portion of polypeptide chain

Hydrogen bonds

Hydrogen bonds form

Secondary structure

- the folding of the polypeptide chain into an

α

-helix or a

β

-pleated sheet.Slide10

 - helices

 - pleated sheet

Amorphous regions

Tertiary structure

- the secondary structures fold up to form a very precise three-dimensional structureSlide11

C

O

-

+

HN

Hydrogen bonds

bonds to moleculebonds to moleculeShared electrons spend longer at these atoms, forming a slight negative charge hydrogen bond

High temperatures and altered pH can split these bonds

Bonds responsible for the tertiary structure…Hydrogen bonds form between these polar groupsSlide12

C

O

-

+

HN

O

HH

Ionic bondsbond to moleculebond to moleculeBasic groupAcidic group

Ionic bondIonic bonds can be split by changing the pH

Bonds responsible for the tertiary structure…Slide13

HS

CH

2

SH

CH2

SCH2

SCH2

Disulphide bondsDisulphide bonds can be split by reducing agentscysteine R group

disulphide bond(covalent)Bonds responsible for the tertiary structure…Slide14

Polar and Non Polar

The arrangement

of

the atoms in some molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative charge. If this is the case, the molecule is called a polar molecule, meaning that it has electrical poles. Otherwise, it is called a non-polar molecule.

Whether molecules are polar or non-polar determines if they will mix to form a solution or that they don't mix well together. Also, polar molecules are water soluble, while non-polar molecules are fat soluble.Slide15

CH(CH

3

)

2

CH2

van der Waal’s forces

These forces can be split by a rise in temperaturePhenylalanine R groupValine R groupWeak van der Waals’ force of attraction

These are weak forces of attraction between non-polar groupsWater excluded from these hydrophobic side chains helps keep the side chains togetherBonds responsible for the tertiary structure…Slide16

CH

3

CH

2

SH

CH2

C-OO

CH

2HS

CH3

CH2

+

HN

NH

CH

2

OH

CH

2

C

NH

2

O

+

NH

3

(CH

2

)

4

CH

2

OH

3

4

1

7

5

2

8

11

14

13

10

12

9

6

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

Lysine

Tyrosine

Asparagine

Serine

Alanine

Cysteine

Aspartate

Histidine

Basic R group

Acidic R group

Polar R group

Polar R group

Polar R group

Polar R group

Non-polar R group

Non-polar R groupSlide17

CH

3

CH

2

S

CH

2

C

O

O-

CH

2

S

CH

3

CH

2

HN

HN

CH

2

HO

CH

2

C

NH

2

O

+

NH

3

(CH

2

)

4

CH

2

HO

3

4

1

7

5

2

8

11

14

13

10

12

9

6

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

Ionic bond

Hydrogen bonds

Disulphide bond

van

der

Waal’s forcesSlide18

H

2

0

H

2

0

H20H20

H20H20H20H

20H20H20H2

0H20H20H2

0H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

H

2

0

1

5

11

2

3

4

6

7

8

9

10

12

13

1

5

11

2

3

4

6

7

8

9

10

12

13

NOTE: the cell is an

aqueous

environment

Hydrophilic R groups

Hydrophobic R groups

Globular

proteins form a spherical mass with a specific 3-D shape (tertiary and quaternary structure)

They fold up so that

hydrophilic

groups are on the outside and

hydrophobic

groups are inside the molecule Slide19

Quaternary structure

This is where two or more polypeptide chains are joined to form a protein,

e.g. haemoglobin

collagenSlide20



-chain subunit



-chain subunit

Haem

groups

Haemoglobin is an example of a

globular

protein with quaternary structure

4 polypeptide chains

2

 -subunits

2  -subunits

4

haem

prosthetic groupsSlide21

Fibrous proteins

Fibrous protein molecules form long chains or fibres

(they have primary, secondary, tertiary and quaternary structure)

Their fibrous nature makes them insoluble in water...

... this makes them useful for structure and support

Collagen

found in skin, teeth, bones, tendons, blood vessel walls

Fibres form a triple-helix of polypeptide chains

These chains are held together by hydrogen bonds

Polypeptide chains

Hydrogen bondsSlide22

The structure of proteinsSlide23

Protein structureSlide24

Denaturing proteins

If the bonds that maintain a protein’s shape are broken, the protein will stop working properly and is

denatured

.

Changes in temperature, pH or salt concentration can all denature a protein, although the specific conditions will vary from protein to protein.

Fibrous proteins lose their structural strength when denatured, whereas globular proteins become insoluble and inactive.

denaturation: bonds brokenSlide25

Biuret test for proteinsSlide26

PROTEINSdingbats – say what you seeSlide27
Slide28

NNNNNNNNNNN

MSlide29

S

2-Slide30

-LINSlide31

- jSlide32

- PASlide33

-k +ySlide34

C

TSlide35

+

arySlide36
Slide37
Slide38
Slide39

+

ary

-d