Haemoglobin

• Haemoglobin is a globular protein with a quaternary structure of 4 polypeptide chains. Each chain has a prosthetic haem group, which contains an Fe2+ ion.
• Each haem group can reversibly combine with an oxygen molecule to form oxyhaemoglobin. So one haemoglobin can carry 4 O2 molecules.
• Haemoglobin + oxygen <-> oxyhaemoglobin

  Hb + 4O2 <-> HbO8
• Partial pressure of oxygen is the measure of oxygen concentration.

  The higher the pO2 the greater the oxygen concentration.
• Haemoglobin’s affinity for oxygen varies depending on pO2:

  • high pO2 (in lungs): high affinity for O2 (combines readily with O2), so high saturation of oxygen
  • low pO2 (in respiring tissues): low affinity for O2 (releases O2 readily), so low saturation of oxygen
• Cooperative binding: the binding of the first oxygen molecule changes the shape of the haemoglobin, making it easier for each successive oxygen molecule to bind.
• Oxygen dissociation curve

  A) Unloading of O2

  B) Cooperative binding

  C) loading of O2

  
• Effects of altitude

  • High altitudes have low oxygen availability
  • Species living at high altitudes have haemoglobin with a higher affinity for oxygen so bind more readily to oxygen
  • Oxygen dissociation curve shifts to the left
• Effects of metabolic rate

  • Organisms with a high metabolic rate need a lot of oxygen for respiration
  • Haemoglobin have lower affinity for oxygen so releases oxygen more readily
  • Oxygen dissociation curve shifts to the right
• Bohr Effect:

  • CO2 lowers the pH of blood
  • High pCO2 gives haemoglobin a low affinity for oxygen, so the rate at which oxyhaemoglobin dissociates to release oxygen increases
  • Oxygen dissociation curve shifts to the right
  • At a given pO2, the percentage saturation of haemoglobin with O2 is lower