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Proteins
Structure and Role
Proteins are the most functionally diverse of all the
biological molecules. Their name is derived from the
Greek word proteios which means first.
Proteome is all the proteins in a living organism, while
proteomics is the study of these proteins.
Proteins
 Virtually
everything a cell is or does depends upon
the proteins it contains.
 Protein molecules carry out essential cellular
functions and form the basis of many cell
structures.
 Proteins show enormous functional diversity – most
proteins have one specific function.
 Easiest way to recall the different functions is to
remember your:
TEACHERS
TEACHERS
T
is for transport proteins which carry other molecules e.g.
haemoglobin
E is for enzymes which catalzye reactions e.g. ATP synthase
A
is for antibodies which are involved in defence against disease
C is for contractile proteins which are involved in movement e.g. actin
and myosin
H
is for hormones which regulate body activity e.g. insulin
E is for exported proteins
R is for receptors which respond to stimuli e.g. insulin receptors
S is for structural proteins e.g. collagen and keratin
What are proteins?
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Are macromolecules composed of linear polymers called
polypeptides.
Polypeptides are formed by condensation polymerisation of
monomers called amino acids.
Amino acids are essential biomolecules, not only because
they are the building blocks of all proteins.
All proteins in life forms on Earth are formed from a set of 20
amino acids.
Most micro-organisms can synthesise the complete set of 20
amino acids, whereas humans can only make 11.
The remaining amino acids must be supplied from the diet
and are called essential amino acids.
Protein construction
Sometimes 100’s or even 1000’s of amino acids join in an
unbranched chain to form a protein. But only 20 different
amino acids are used.
All amino acids have the same basic structure.
 An amino group, an acidic carboxylic group and a side
group known as R. (R varies between amino acids.)
Amino acids
are joined by
Peptide bonds
Polypeptide chain
 When
a number of amino acids join via
peptide bonds, a polypeptide chain is
formed.
 When
amino acids to form peptide
bonds, a water molecule is released.
 This
reaction is called….
Condensation reactions
A
condensation reaction is a chemical reaction in
which two molecules or functional groups
combine to form a larger molecule, together with
the loss of a small molecule usually water.
Proteins are made of…
 Carbon
 Hydrogen
 Oxygen
 Nitrogen
 CHON
 Sometimes
other elements.
Some amino acids may seem
familiar
 Aspartate
 Lysine
 Phenylalanine
 Glycine
 Alanine
 Asparagine
 Glutamate
LEVEL OF
ORGANISATION
Primary
Secondary
Tertiary
Quaternary
STRUCTURAL
FEATURES
TYPES OF BONDS AND
INTERMOLECULAR
INTERACTIONS
Linear sequence of amino Covalent peptide bond linking
acids.
amino acid residues.
Coiling into helices and
Hydrogen bonding.
folding into sheets in parts
of the polypeptide chain.
Non-folded regions are
called random coils.
Folding of the polypeptide
resulting in a 3D shape
that is stabilised by
various intermolecular
interactions.
Hydrogen bonds.
Ionic bonds.
Van der Waals interactions.
Hydrophobic interactions.
Disulfide bonds.
Association of two or
more folded polypeptides
to form a protein.
Ionic bonds.
Van der Waals interactions.
Hydrophobic interactions.
Disulfide bonds.
Primary structure
The sequence of amino acids for a protein
is unique. All molecules of that protein will
be exactly the same. The sequence of
amino acids in a protein defines its primary
structure. This is all held together by strong
covalent bonds.
Secondary structure
Polypeptide chains become folded in various ways. Two
common types are the alpha helix and the beta-pleated
sheet. The base form a backbone for the protein, with the R
groups projecting out from the structure. These structures are
maintained by the hydrogen bonds between neighbouring
NH and CO groups. (H bonds are weak, but when there are
lots of them they are strong.)
Tertiary structure
This refers to the final folded 3D shape of a protein. They may be
globular or compact (Eg, enzymes, antibodies, hormones,
transport proteins) or extended rods (fibrous) proteins that have
mechanical or structural roles.
The folds are created because various points on the secondary
structure are attracted to each other. The strongest bonds are
made between cytosine amino acids. The sulphur atoms form a
disulphide bridge at the amino acid cytosine. Bonds are also
created by weak ionic and H bonds as well as hydrophobic
interactions.
Tertiary (cont)
Amino acids that were distant in the
primary structure may now become very
close to each other after the folding has
taken place
The subunit of a more complex protein
has now been formed. It may be globular
or fibrous. It now has its functional shape
or conformation.
Quaternary structure
Only some proteins need a fourth level of structure.
The quaternary structure refers to the association of
several globular protein units to form a functional
protein.
Denaturing
The weaker bonds in secondary, tertiary and
quaternary structures can be disrupted by
heat, pH changes and the presence of other
chemicals.
This results in a loss of structure and function.
This is also known as denaturing.
Fibrous proteins
Are structural proteins, they can be rod-like or sheet-like.
Examples are:
 Collagen found in connective tissue, cartilage, bones, skin,
tendons and blood vessel walls.
 Elastin is another connective tissue in arteries, skin, ligaments
and lungs. It can stretch in all directions.
 Silks or fibroins are secreted by moths and spiders. They have
high strength and flexibility, but low extensibility.
 They are also contractile, such as myosin (found in
contractile vacuoles).
 Keratin as found in hair, nails, horn, feathers and skin.
Fibrous proteins are:
Water insoluble
Very tough physically, supple and stretchy
Globular proteins
These are easily water soluble and their tertiary
structure is critical to their function.
Their main functions are:
Catalytic: enzymes
Regulatory: hormones such as insulin.
Transport: haemoglobin, protein channels,
Protective: antibodies
Proteins in the Cell Cytoskeleton

Eukaryote cells have a cytoskeleton made up of
straight hollow cylinders called microtubules
(bottom right).

They help cells maintain their shape, they act like
conveyer belts moving organelles around in the
cytoplasm, and they participate in forming
spindle fibres in cell division.

Microtubules are composed of filaments of the
protein, tubulin . These filaments are compressed
like springs allowing microtubules to ‘stretch and
contract’.

13 of these filaments attach side to side, a little
like the slats in a barrel, to form a microtubule.
This barrel shaped structure gives strength to the
microtubule.
Proteins speed up reactions = Enzymes
Catalase speeds up
the breakdown of
hydrogen peroxide,
(H2O2) a toxic by
product of metabolic
reactions, to the
harmless substances,
water and oxygen.
The reaction is
extremely rapid as the
enzyme lowers the
energy needed to
kick-start the reaction
(activation energy)
No catalyst =
Input of 71kJ energy required
Energy
Activation
Energy
With catalase
= Input of 8 kJ energy
required
Substrate
Progress of reaction
Product
Proteins can regulate metabolism =
hormones
When your body detects an increase in the sugar
content of blood after a meal, the hormone insulin is
released from cells in the pancreas.
Insulin binds to cell membranes and this triggers the cells
to absorb glucose for use or for storage as glycogen in
the liver.
Proteins span membranes –protein channels
The CFTR membrane protein is an ion channel that
regulates the flow of chloride ions.
Not enough of this protein gets inserted into the
membranes of people suffering Cystic fibrosis. This
causes secretions to become thick as they are not
hydrated. The lungs and secretory ducts become
blocked as a consequence.
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