Transcript AS 2.1.1 Protein Structure

Proteins from Amino Acids
• Two amino acids joined together make a
dipeptide, as more and more join together by
peptide bonds, a polypeptide is formed
The amino acids in a polypeptide chain are
sometimes referred to as residues because
part of the molecule is lost in the
condensation reaction that produces the
peptide bond
Protein Synthesis
• Polypeptides are made (synthesised)
inside cells on the ribosomes.
• Messenger RNA (mRNA) puts the amino
acids in the right order to make a specific
polypeptide chain
Protein Synthesis
• As the mRNA passes
through the ribosome,
the amino acids are
joined one at a time by a
condensation reaction
forming a peptide bond
• Eventually a longer and
longer chain of amino
acids is formed
• The mRNA determines
the sequence of amino
acids and therefore the
type of protein
Forming Different Proteins
We will look at a peptide chain that is 3
amino acids long.
The R group of an amino acid may be any
one of the 20 available.
To calculate the total number of different
possibilities, we need to multiply the total
number of possibilities at each point.
In this case: 20 x 20 x 20 = 8000
This means that 8000 different sequences
of four amino acids are possible- 8000
different proteins can be made.
Given that most proteins are 100 amino
acids long, in theory the number of different
possible proteins is extremely large
Protein Structure Task
• We can look at proteins
in various levels of
detail. The first is their
primary structure, this is
followed by the
secondary, tertiary and
quaternary structures
• Task: Using Cambridge
Biology p110- 113, and
a two page spread in
your book, summarise
the 4 levels of protein
structure (including the
bonding involved)
Primary Structure
• The sequence of amino acids that forms
the protein
Secondary Structure
• The chain of amino
acids coils or folds to
form an alpha helix or
a beta pleated sheet
• Hydrogen bonds hold
the coils in place
weakly, but as so
many are formed
they give stability to
the protein molecule
Tertiary Structure
• When the coils and pleats themselves coil or fold
• Held together by:
• Disulphide bonds: a covalent bond between 2 sulphur
atoms
• Ionic bonds: the R groups are sometimes charged (+ve
or –ve) so they attract each other
• Hydrogen bonds: +ve hydrogen atoms and –ve oxygen
atoms attract
• Hydrophobic and hydrophilic interactions: hydrophobic
amino acids will be drawn into the centre of the
molecule, and hydrophilic amino acids will be found on
the outside.
Heating Proteins
• Heating a protein increases the kinetic
energy in the molecule
• This causes the molecule to vibrate,
breaking the bonds