Haemoglobin

• Haemoglobin

  A large protein with a quaternary structure that carries oxygen around the body
• Structure of haemoglobin

  • Primary structure: amino acid sequence of the four polypeptide chains
  • Secondary structure: polypeptide chains folded into an alpha helix
  • Tertiary structure: each polypeptide chain is folded in a precise shape
  • Quaternary structure: all four polypeptides linked to form an almost spherical form
• Haem group

  Contains Fe2+ ion which can bind to oxygen
• Haemoglobin can bind to four O2 molecules
• The function of haemoglobin is to load oxygen in the lungs and deliver it to the respiring tissues
• Affinity


  The level of attraction haemoglobin has to oxygen
• Loading/associating


  When haemoglobin binds with oxygen
• Unloading/disassociating

  When haemoglobin releases its oxygen
• Role of haemoglobin in oxygen transport

  • High affinity in the lungs due to high partial pressure of oxygen and low partial pressure of carbon dioxide
  • Loads/associates oxygen in the lungs and becomes saturated
  • At respiring tissues, low affinity due to low partial pressure of oxygen and high partial pressure of carbon dioxide
  • Unloads/dissociates/delivers oxygen and becomes unsaturated
• Different haemoglobins have different affinities due to changes in the environment (e.g. pH or temperature)
• How haemoglobin is a protein with a quaternary structure

  • 4 oxygen molecules can bind to it
  • Each polypeptide chain has a haem group which can bind to one O2 molecule
• Oxygen dissociation curve explanation

  1. Low pressure: difficult for first oxygen molecule to bind

  2. Higher oxygen partial pressure: first oxygen molecule changes quaternary structure, easier for other subunits to bind (positive cooperativity)

  3. End of curve: longer for oxygen molecule to bind due to difficulty finding a binding site
• Oxygen dissociation curve

  Left: greater affinity of haemoglobin for oxygen<|>Right: lower affinity of haemoglobin for oxygen
• Bohr effect

  The greater the CO2 concentration, the more readily haemoglobin releases its oxygen
• Low CO2 concentration

  Haemoglobin changes shape into one of high affinity
• High CO2 concentration

  Lower affinity, oxygen disassociates more readily
• CO2 dissolves in the blood forming acid, increasing H+ ions and lowering pH
• Haemoglobin in lungs vs. respiring tissue

  • Higher affinity in lungs due to low CO2 concentration and high oxygen concentration
  • Lower affinity in respiring tissue due to high CO2 concentration and low oxygen concentration
• Low-affinity haemoglobin


  Good for respiring tissues as it means more readily oxygen is unloaded
• Benefit of fetal haemoglobin having high affinity

  • Oxygen dissociates from mother's haemoglobin and associates with fetal haemoglobin at low partial pressures of oxygen
  • Can reach 100% saturation even at low partial pressures of oxygen
• Adults do not keep fetal haemoglobin because high affinity means less oxygen is unloaded at respiring tissues
• Affinity for small organisms

  Low affinity due to large surface area to volume ratio, losing heat, and needing to respire to generate heat
• Small organisms have a low affinity for oxygen so they unload enough oxygen for the cells' demand for more respiration