Proteins from Amino Acids

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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
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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

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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

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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
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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)

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Primary Structure

• The sequence of amino acids that forms the protein

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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

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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.

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Heating Proteins

• Heating a protein increases the kinetic energy in
  the molecule
• This causes the molecule to vibrate, breaking the
  bonds