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Proteins
• Learning objective
• To be able to describe the primary,
secondary, tertiary and quarternary
structure of proteins
• In a protein the polypeptide chain may
be hundreds of amino acids long.
• Amino acid polymerisation to form
polypeptides is part of protein
synthesis.
• It takes place in ribosomes, and is
special because it requires an RNA
template.
• The sequence of amino acids in a
polypeptide chain is determined by the
sequence of the genetic code in DNA.
Protein Structure
• Polypeptides are just a string of amino
acids,
• They fold up to form the complex and
well-defined three-dimensional
structure of working proteins.
• To help to understand protein structure,
it is broken down into four levels:
Primary Structure
• This is just the sequence of amino acids in
the polypeptide chain, so is not really a
structure at all.
• However, the primary structure does
determine the rest of the protein structure.
• Finding the primary structure of a protein is
called protein sequencing, and the first
protein to be sequenced was the protein
hormone insulin, by the Cambridge
biochemist Fredrick Sanger, for which work
he got the Nobel prize in 1958.
Secondary Structure
• Interactions between the R groups of
the amino acids in the chain cause the
chain to twist and fold into a three
dimensional shape
• Lengths of the chain may first coil into
α-helices or β-pleated sheets.
• These are known as the secondary
structures.
α-helices
• Within the helix, hydrogen bonds form
between the C=O of the carboxylic acid
group and the NH of the amine group of
the different amino acids.
β-sheets
• Several chains may link together with
hydrogen bonds holding the parallel
chains together.
H
N
H
C C
O
N
H
C C
O
N
H
C C
O
N
H
C C
O
N
C C
N H
O
C
H
C
O
C N
H
C
O
C N
H
C
O
C N
H
C
O
C N
H
C
O
C N
C O
Tertiary structure
• The polypeptide chain often folds and bends to
produce a three dimensional shape
• The tertiary structure is held together by bonds
between the R groups of the amino acids in the
protein, and so depends on what the sequence of
amino acids is. There are three kinds of bonds
involved:
– hydrogen bonds, which are weak.
– ionic bonds between R-groups with positive or
negative charges, which are quite strong.
– sulphur bridges - covalent S-S bonds between
two cysteine amino acids, which are strong.
• The secondary structure is due to
backbone interactions and is thus
largely independent of primary
sequence.
• The tertiary structure is due to side
chain interactions and thus depends on
the amino acid sequence
Quaternary Structure
• This structure is found in proteins containing
more than one polypeptide chain, and simply
means how the different polypeptide chains
are arranged together.
• The individual polypeptide chains are usually
globular, but can arrange themselves into a
variety of quaternary shapes.
Globular and fibrous proteins
• Proteins can be divided into two
distinct groups
Globular proteins
• The polypeptide chain is folded into a
compact spherical shape
• These proteins are soluble due to
hydrophilic side chains that project
from the outside of the molecules
• Globular proteins are therefore
important in metabolic reactions
Globular proteins
• Enzymes – 3D shape gives the ability to
form enzyme – substrate complexes
and catalyse reactions
• Transport proteins – 3D shape allows
them to bind with other molecules eg:
proteins in cell membranes or
haemoglobin in red blood cells.
• Antibodies – precise shape binds to
microorganisms
Fibrous proteins
• These do not fold into a ball shape
• They remain in long chains, often with
several polypeptide chains cross linked
together for additional strength
• They are insoluble
Fibrous proteins
• Keratin – in hair and skin
• Collagen – in the skin, tendons,
cartilage and blood vessel walls
Types of proteins
• Fibrous proteins
– e.g. collagen
– Insoluble
– structural
• Globular proteins
– e.g.enzymes
– Soluble
– 3D shape
Tertiary structure
• Bonding between R-groups
gives rise to a 3D shape
• H-bonds =O HNaffected by temp & pH
• Ionic bonds –NH3-COOaffected by pH
• Disulphide bridge
--CH2S-SCH2affected by reducing agents
Examples of Quaternary
Structures
• Haemoglobin, the
oxygen-carrying
protein in red blood
cells, consists of four
globular subunits
arranged in a
tetrahedral (pyramid)
structure.
• Each subunit
contains one iron
atom and can bind
one molecule of
oxygen.
Examples of Quaternary
Structures
• Immunoglobulins, the
proteins that make
antibodies, comprise
four polypeptide
chains arranged in a
Y-shape.
• The chains are held
together by sulphur
bridges. This shape
allows antibodies to
link antigens
together, causing
them to clump.
Examples of Quaternary
Structures
• Actin, one of the proteins found in
muscles, consists of many globular
subunits arranged in a double helix to
form long filaments.
Just to be awkward…
• A few proteins have both structures:
the muscle protein myosin has a long
fibrous tail and a globular head, which
acts as an enzyme
•
This diagram
shows a molecule
of the enzyme
dihydrofolate
reductase, which
comprises a single
polypeptide chain.
It has been drawn
to highlight the
different secondary
structures.
• Can you identify
which is which?
1. β sheet
2. α – helix
3. α – helix
3
2
1
Questions
1.
2.
3.
Name the elements found in proteins.
What are the monomers of proteins?
Draw the structure of an amino acid (label the
groups).
4. What is the name for the bond between two amino
acids?
5. What bonds are found in the secondary structure
of proteins?
6. What is the test for protein?
7. List 6 functions of proteins.
8. What is hydrolysis?
9. What is denaturation?
10. What kind of protein is an enzyme?
11. What is the function of a fibrous protein?
Answers
1.
Name the elements found in proteins.
• CHONS
2. What are the monomers of proteins?
• Amino acids
3. Draw the structure of an amino acid (label the
groups).
H
O
O
C
The group that
determines what
amino acid is
R
C
H
N
H
carboxylic acid
group
amine group
H
4.
What is the name for the bond between two amino
acids?
•
5.
Peptide
What bonds are found in the secondary structure
of proteins?
•
6.
Hydrogen bonds
What is the test for protein?
•
7.
Biuret test gives purple with protein
List 6 functions of proteins.
•
8.
Enzymes, structural, carriers, pumps, hormones,
antibodies
What is hydrolysis?
•
9.
Breakdown of a substance by adding water
What is denaturation?
•
Altering the shape of a protein
10. What kind of protein is an enzyme?
•
Globular
11. What is the function of a fibrous protein?
•
Structural