2.5 Proteins

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Transcript 2.5 Proteins

2.5 Proteins
Specification reference: 3.1.2
Learning Objectives
• How are amino acids linked to form
polypeptides – the primary structure of
proteins?
• How are polypeptides arranged to form
the secondary structure and then the
tertiary structure of a protein?
• How is the quaternary structure of a
protein formed?
• How are proteins identified?
Starter Activity: Word-Search
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ALPHA HELIX
AMINO ACID
BIURET
CONDENSATION
DIPEPTIDE
DISULPHIDE BOND
GLOBULAR
HAEMOGLOBIN
HYDROGEN BOND
HYDROLYSIS
IONIC BOND
MONOMER
PEPTIDE BOND
POLYMER
POLYMERISATION
POLYPEPTIDE
PROSTHETIC
TERTIARY
QUATERNARY
Why are Proteins
polymers?
Proteins consist
of long chains of
amino acids.
There are over 20 naturally
occurring amino acids, which
differ in the composition of
the R group.
Two amino acids may be
linked together by a
condensation reaction to
form a ‘dipeptide’.
Since the amino acids may be joined in any
sequence there is an almost infinite variety of
possible proteins.
•The chain of amino acids is referred to as the
protein’s primary structure.
•The chain is folded (often into a helix) to give
the secondary structure.
•The secondary structure is folded on itself to
form the tertiary structure.
•The combination of a number
of polypeptide chains along with
associated non-protein groups
results in the quaternary.
Quaternary Structure Of A Protein
•These shapes are due to the fact
that proteins are amphoteric, i.e.
they have both positive and
negative charges on them.
•The attraction of these opposite
charges forms weak electrostatic
(hydrogen) bonds causing the
chain to form a complex 3D
structure – globular proteins.
•Ionic bonds, disulphide bridges,
hydrogen bonds and hydrophobic
interactions all contribute to the
final shape of a given protein
molecule.
All enzymes and some
hormones are globular
proteins and their functions
depend on the precise shape
of the protein molecule.
•Sometimes the protein consists of
long parallel chains with cross-links –
fibrous proteins.
•These are insoluble and have
structural functions, e.g. collagen in
cartilage; keratin in hooves, feathers
and hair, actin and myosin in muscle.
If a globular protein is heated
or treated with a strong acid or
alkali the hydrogen bonds are
broken and it reverts to a more
fibrous nature – a process
called DENATURATION.
Proteins sometimes occur in
combination with a nonprotein substance (prosthetic
group); these are called
conjugated proteins, e.g.
haemoglobin.
Test for Proteins
The Biuret test detects peptide bonds.
• Place a sample of the solution to be tested in a test
tube and add an equal volume of sodium hydroxide
solution at room temperature.
• Add a few drops of very dilute (0.5%) copper (II)
sulphate solution and mix gently.
• A purple coloration indicates the presence of peptide
bonds and hence a protein. If no protein is present,
the solution remains blue.
• Alternatively use Biuret reagent to test for protein. A
purple colour shows protein is present; a blue colour
indicates that protein is absent.
Protein shape and function
Proteins perform many different roles in
living organisms. Their roles depend on
their molecular shape, which can be of 2
basic types.
• Fibrous proteins, such as collagen, have
structural functions.
• Globular proteins, such as enzymes and
haemoglobin, carry out metabolic functions.
It is the very different structure and shape of
each of these types of proteins that enables
them to carry out their functions.
Fibrous Proteins e.g. Collagen
•
•
•
•
These form long chains which run parallel
to one another. These chains are linked by
cross-bridges and so form very stable
molecules. One example is collagen. Its
molecular structure is as follows:
The primary structure is an unbranched
polypeptide chain.
In the secondary structure the polypeptide
chain is very tightly wound.
In the tertiary structure the chain is
twisted into a second helix.
Its quaternary structure is made up of 3
such polypeptide chains wound together
in the same way as individual fibres are
wound together in a rope.
Collagen is found in tendons.
Tendons join muscles to bones.
When a muscle contracts the bone is
pulled in the direction of the
contraction. The individual collagen
polypeptide chains in the fibres are
held together by cross-linkages
between amino acids of adjacent
chains.
•The points where one collagen
molecule ends and the next begins
are spread throughout the fibre
rather than all being in the same
position along it.
Questions
1. Explain why the quaternary structure of
collagen makes it a suitable molecule for a
tendon.
2. Suggest how the cross-linkages between
the amino acids of polypeptide chains
increase the strength and stability of a
collagen fibre.
3. Explain why the arrangement of collagen
molecules is necessary for the efficient
functioning of a tendon.
Answers
1. It has 3 polypeptide chains wound together to
form a strong, rope-like structure that has
strength in the direction of pull of a tendon.
2. It prevents the individual polypeptide chains
from sliding past one another and so they gain
strength because they act as a single unit.
3. The junctions between adjacent collagen are
points of weakness. If they all occurred at the
same point in a fibre, this would be a major weak
point at which the fibre might break.
Plenary: Use the following key words to write
an essay on proteins. You must include all key
words! Monomer
Amino Acid
Biuret
Condensation
Dipeptide
Disulphide Bond
Globular
Haemoglobin
Hydrogen Bond
Hydrolysis
Ionic Bond
Peptide Bond
Polymer
Polymerisation
Polypeptide
Primary
Prosthetic
Secondary
Tertiary
Quaternary
Alpha Helix
Learning Objectives
• How are amino acids linked to form
polypeptides – the primary structure of
proteins?
• How are polypeptides arranged to form
the secondary structure and then the
tertiary structure of a protein?
• How is the quaternary structure of a
protein formed?
• How are proteins identified?