Proteins

1
Proteins

• Higher Level Material
• Section of syllabus 7.5

2
Protein structure

• There are 4 levels of protein structure

QUARTERNARY
TERTIARY
SECONDARY
PRIMARY
3
Primary Structure

• A protein molecule may contain several hundred
  amino acids linked into a long chain.
• The TYPES of amino acids and the SEQUENCE in
  which they are joined is the primary structure.
• A change in a single amino acid can completely
  change the properties of the protein.

4
Secondary Structure

• Regular, repeating structures
• Amino acids in a chain interact with each other,
  coiling into shapes eg. a-helix or ß-pleated
  sheet
• Hydrogen bonds form between the O of the carboxyl
  group of one amino acid and an H of the amine
  group of another amino acid.
• Eg of a-helix keratin in hair, wool, feathers
• Eg of ß-pleated sheet silk

5
Tertiary Structure

• The way in which a protein molecule coils up to
  form a precise 3D shape.
• Structure maintained by bonds between different
  amino acids.
• Eg. hydrogen bonds, ionic bonds, disulphide
  bridges.
• This allows for very precise structures, eg
  active sites on enzymes.

6
Quarternary Structure

• Many proteins are made of 2 or more polypeptide
  chains.
• The association of these chains by various types
  of bonding is called the quarternary structure.
• Eg. insulin is made of 2 polypeptide chains,
  haemoglobin is made of 4.
• Sometimes a non-protein component is joined to
  the chain. This is called a PROSTHETIC group.
• Proteins with prosthetic groups are called
  CONJUGATED proteins.
• Eg. glycoproteins in the plasma membrane.

7
Fibrous v Globular Proteins

• Fibrous
• Long, narrow shape
• Insoluble in water
• Structural role. Eg. collagen for strengthening
  bone, tendon, skin.
• In movement. Eg. myosin causes contraction of
  muscle fibres to cause movement in animals.

• Globular
• Round shape, folded into a ball
• Soluble in water
• Transport role. Eg. haemoglobin for binding to
  oxygen in the lungs and delivering to cells.
• Enzymes. Eg. catalase converts toxic hydrogen
  peroxide into water and oxygen.