Haemoglobin

• Haemoglobin molecules


  A group of chemically similar molecules found in a wide variety of organisms
• Structure of haemoglobin


  • Primary structure: sequence of amino acids in the four polypeptide chains
  • Secondary structure: each polypeptide chain is coiled into a helix
  • Tertiary structure: each polypeptide chain is folded into a precise shape
  • Quaternary structure: all four polypeptides are linked together to form an almost spherical molecule, each associated with a haem group containing a ferrous (Fe) ion
• Loading oxygen


  The process by which haemoglobin binds with oxygen, takes place in the lungs
• Unloading oxygen


  The process by which haemoglobin releases its oxygen, takes place in the tissues
• Haemoglobins with high affinity for oxygen

  Take up oxygen more easily, but release it less easily
• Haemoglobins with low affinity for oxygen


  Take up oxygen less easily, but release it more easily
• Role of haemoglobin


  • Readily associate with oxygen at the surface where gas exchange takes place
  • Readily dissociate from oxygen at those tissues requiring it
• Haemoglobin


  Binds with oxygen and releases it in the environment of the body
• Haemoglobin changes its tertiary structure


  Therefore affects its oxygen binding properties
• Haemoglobin has two requirements that may appear to contradict each other
• Haemoglobin's remarkable property


  It changes its affinity (chemical attraction) for oxygen under different conditions
• Presence of carbon dioxide


  Causes a change in the shape of the haemoglobin molecule
• New shape of the haemoglobin molecule


  Binds more loosely to oxygen
• Haemoglobin binding more loosely to oxygen


  Releases its oxygen
• Affinity of haemoglobin for oxygen under different conditions

  • High oxygen, low carbon dioxide concentration - high affinity
  • Low oxygen, high carbon dioxide concentration - low affinity
• The shape of the haemoglobin molecule changes when it binds the first oxygen molecule</b>
• Change in haemoglobin shape


  Makes it easier for the next oxygen molecule to bind
• Binding of more oxygen molecules


  Further changes the shape of haemoglobin
• Changing haemoglobin shape


  Reduces the probability of the next oxygen molecule binding
• Different haemoglobins have different affinities for oxygen
• Reason for different haemoglobin affinities


  Each species produces a haemoglobin with a slightly different amino acid sequence, leading to different tertiary and quaternary structures