Haemoglobin

• Haemoglobin


  • Water soluble globular protein
  • Consists of two beta polypeptide chains and two alpha helices
  • Each molecule forms a complex containing a haem group
  • Carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group
  • Each molecule can carry four oxygen molecules
• Partial pressure of oxygen increases


  Affinity of haemoglobin for oxygen increases
• Loading


  Oxygen binds to haemoglobin in the lungs
• Partial pressure of oxygen decreases


  Affinity of haemoglobin for oxygen decreases
• Unloading


  Oxygen is released from haemoglobin in respiring tissues
• After unloading, haemoglobin returns to the lungs where it binds to oxygen again
• Dissociation curves


  Illustrate the change in haemoglobin saturation as partial pressure changes
• Partial pressure is high


  Haemoglobin has high affinity for oxygen and is highly saturated
• Partial pressure is low


  Haemoglobin has low affinity for oxygen and is less saturated
• Binding the first oxygen molecule

  Increases the affinity of haemoglobin for oxygen
• Oxygen dissociation curve


  • Initially shallow as it is hard for the first oxygen molecule to bind
  • Steep increase as binding the first oxygen makes it easier for the other oxygen molecules to bind (positive cooperativity)
  • Gradient flattens out as the likelihood of the fourth oxygen finding a binding site is low
• Fetal haemoglobin


  Has a higher affinity for oxygen compared to adult haemoglobin to better absorb oxygen at low partial pressure in the placenta
• Carbon dioxide is released by respiring cells


  Affinity of haemoglobin for oxygen decreases, causing oxygen to be released (Bohr effect)
• Carbon dioxide creates acidic conditions


  Changes the shape of haemoglobin, making it easier for oxygen to be released