What are proteins?
What are proteins?
 Proteins are made up of C, H, O, N and some S and P
 Proteins are the building blocks of life
 There are millions of different proteins
 Proteins are the most abundant molecules in cells
 Proteins make up more than 50% of a cell’s dry mass
What do proteins do?
Proteins perform a wide range of biological functions:
• As enzymes they catalyse reactions.
• Carrier proteins transport molecules across
membranes.
• Antibodies defend against disease.
• Structural proteins support cells and tissues.
• Hormones transmit information.
• Transport proteins such as haemoglobin carry oxygen.
• Contractile proteins enable muscles to contract.
How do proteins do all this?
Proteins can carry out all these different functions
because each different protein has a specific molecular
shape which enables the protein to do its job.
It is structure of a protein that allows it to carry out its
function.
So, what goes into a protein?
So, what goes into a protein?
 Proteins are polymer molecules.
 The monomer molecules making up proteins are
called amino acids.
 There are 20 different naturally occurring amino acids.
 There are over 100,000 combinations of amino acids
forming known proteins.
What is an amino acid?
All amino acids have the same general structure:
• A carboxyl group (-COOH)
• An amino group (-NH2) attached to a C atom
• A variable group called R
It is the R group that differs from one amino acid to
another
Are all amino acids ‘equal’?
No, of the 20 naturally occurring amino acids 8 are
known as ‘essential amino acids’. These 8 cannot be
synthesised by the body and must be obtained from
the diet.
The remaining 12 can be synthesised by the body.
Joining amino acids together
When amino acids join together, they do so by a
condensation reaction.
This means one water molecule is removed, using the
OH group from the carboxyl group of one amino acid,
and one H from the amino group of another.
The resulting bond is called a peptide bond.
Peptides
 Two amino acids joined together form a dipeptide.
 Three amino acids joined together form a tripeptide.
 A polypeptide is made up of many amino acids joined
together.
 When a polypeptide bonds with other polypeptides it
forms a protein containing thousands of amino acids.
Proteins have 4 structural levels.
Proteins are big, complicated, 3-dimensional molecules.
The structure is described in four ‘levels’:
• Primary
• Secondary
• Tertiary
• Quaternary
Primary structure
 The primary structure of a protein is the sequence of
amino acids in the chain.
 The primary structure determines the eventual shape
of the protein, hence its function.
Secondary structure
 The amino acids in the primary structure of a protein
do not lie flat and straight.
 Hydrogen bonds form between the amino acids in the
chain.
 This makes the protein coil into an a helix or fold into
a b pleated sheet.
 This is the secondary structure.
Tertiary structure
 The coiled or folded chains often coil or fold further.
 More bonds form due to interactions between the
R-groups of the polypeptide chain.
 This is called the tertiary structure.
 For proteins formed from a single polypeptide chain
this is the final 3D structure of the protein.
Quaternary structure
 Some proteins are made up of several polypeptide
chains held together by bonds.
 The quaternary structure is how these chains are put
together.
 The best known example is haemoglobin, which is
made of four polypeptide chains bonded together. For
proteins such as haemoglobin, the quaternary
structure determines the final 3D structure.
Haemoglobin
Protein bonds
The four structural levels in proteins are held together
by different bonds:
• Peptide bonds (primary)
• Hydrogen bonds (secondary and tertiary)
• Ionic bonds (tertiary)
• Disulphide bonds (tertiary)
• Hydrophobic and hydrophilic interactions (tertiary)
Quaternary structure depends on the tertiary structure
of the individual polypeptides, and so is influenced by
all these bond types.
Types of protein
There are two different types of protein and they are
different shapes. The shape of a protein molecule is
related to its function.
• Globular proteins – these are round, compact and
easily soluble so they can be transported in fluids.
Examples are haemoglobin and enzymes.
• Fibrous proteins – these are tough and rope-shaped.
They tend to be found in connective tissues such as
tendons. Collagen is an example of a fibrous protein.
Haemoglobin
Haemoglobin is a globular protein.
It’s structure is curled up so that hydrophilic side chains
face outwards and hydrophobic side chains face
inwards.
This makes haemoglobin soluble and therefore good for
transport in the blood.
Collagen
Collagen it made of three polypeptide chains, tightly
coiled in a strong triple helix.
The chains are interlinked by strong covalent bonds.
Minerals can bind to the triple helix to increase its
strength.
Plenary
 Name the two groups found in all amino acid
molecules.
 Name the bond that joins amino acids together in
proteins.
 Name the four types of bond that determine the
structure of a protein.
 Name the four structural levels of a protein.