Download Proteins * Structure and Function

Proteins – Structure and
Function
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.
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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.
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The 20 naturally-occurring amino acids
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Proteins – Structure and
Function
Peptide bonds and dipeptides
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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.
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Portion of polypeptide chain
Amino
acids
Peptide
bond
Hydrogen
bonds form
 - helix
Primary structure
Secondary
structure
- the
folding
of the of
polypeptide
chain
- the
sequence
amino acids
in a
intopolypeptide
an α-helix or
a β-pleated sheet.
chain
Hydrogen
bonds
Portion of polypeptide chain
Hydrogen
bonds form
Hydrogen
bonds
Secondary structure
- the folding of the polypeptide chain
into an α-helix or a β-pleated sheet.
 - pleated sheet
 - pleated
sheet
 - helices
Amorphous
regions
Tertiary structure
- the secondary structures fold up to
form a very precise threedimensional structure
Bonds responsible for the tertiary structure…
Hydrogen bonds
Shared electrons spend longer at
these atoms, forming a slight
negative charge
bonds to
molecule
C
O-
+H
N
bonds to
molecule
hydrogen
bond
Hydrogen
bonds form
between these
polar groups
High temperatures and altered pH can
split these bonds
Bonds responsible for the tertiary structure…
Ionic bonds
bond to
molecule
O
Basic group
H
C
Acidic group
O-
+H
Ionic
bond
H
N
bond to
molecule
Ionic bonds can be split by changing
the pH
Disulphide bonds
CH2
CH2
Bonds responsible for the tertiary structure…
cysteine
R group
SH
S
HS
CH2
S
CH2
disulphide
bond
(covalent)
Disulphide bonds can be split by
reducing agents
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.
van der Waal’s forcesBonds responsible for the tertiary structure…
These are weak forces of attraction between non-polar
groups
Water excluded from these hydrophobic side chains
helps keep the side chains together
Valine
R group
Phenylalanine
R group
CH(CH3)2
CH2
Weak van der Waals’
force of attraction
These forces can be split by a rise in
temperature
H20
NOTE: the cell is an aqueous environment
1
2
H20
3
4
H20
5
6
7
8
H20
H20
9
H20
2
13
3
1
12
4
6
H20
13
H20
H20
H20
12
H20
H20
H20
H20
Globular proteins
form a
spherical mass with a specific 3-D
H20
shape (tertiary and quaternary
H20
structure)
They fold up so that hydrophilic
H20
groups are on the outside and
H20
hydrophobic groups
are inside the
molecule
11
H20
H20
H20
Hydrophilic
Hydrophobic
R groups H20 R groups
10
5
11
7
8
H20
10
9
H20
H20
Quaternary structure
This is where two or more polypeptide chains
are joined to form a protein,
e.g. haemoglobin
collagen
Haemoglobin is an example of a globular protein with
quaternary structure
-chain
subunit
-chain
subunit
4 polypeptide
chains
- 2  -subunits
- 2  -subunits
Haem groups
4 haem prosthetic
groups
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
Polypeptide chains
Hydrogen bonds
Fibres form a triple-helix of polypeptide
chains
These chains are held together by
hydrogen bonds
The structure of proteins
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Protein structure
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Denaturing proteins
If the bonds that maintain a protein’s shape are broken, the
protein will stop working properly and is denatured.
denaturation:
bonds broken
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.
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Biuret test for proteins
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PROTEINS
dingbats – say
what you see
S
2-