Adult and Foetal Haemoglobin

• Oxygen dissociation curve


  Shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2)
• Partial pressure of oxygen

  The pressure exerted by oxygen within a mixture of gases; a measure of oxygen concentration
• Haemoglobin saturation


  When all of its oxygen binding sites are taken up with oxygen; when it contains four oxygen molecules
• Haemoglobin affinity for oxygen


  The ease with which haemoglobin binds and dissociates with oxygen
• High affinity haemoglobin


  • Binds easily and dissociates slowly
• Low affinity haemoglobin


  • Binds slowly and dissociates easily
• In liquids like water, oxygen associates at a constant rate, providing a straight line on a graph, but with haemoglobin oxygen binds at different rates as the pO2 changes
• Haemoglobin's affinity for oxygen changes at different partial pressures of oxygen
• Explaining the shape of the oxygen dissociation curve


  1. Difficult for first oxygen molecule to bind, making binding slow at low pO2

  2. Haemoglobin changes shape, making it easier for remaining oxygen molecules to bind, speeding up binding at medium pO2

  3. As haemoglobin approaches saturation, it takes longer for fourth oxygen molecule to bind due to shortage of binding sites, causing curve to level off at high pO2
• Cooperative binding


  The shape change of haemoglobin leading to easier oxygen binding
• Foetal haemoglobin


  Has a higher affinity for oxygen than adult haemoglobin, allowing a foetus to obtain oxygen from its mother's blood at the placenta
• Foetal haemoglobin vs adult haemoglobin

  Foetal haemoglobin curve shifts to the left, meaning at any given pO2 it has a higher percentage saturation
• After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin
• Haemoglobin


  A quaternary protein, made up of four globin polypeptides and four haem groups
• The structure of haem is identical in all types of haemoglobin, but the globin chains can differ substantially between species
• The globin polypeptides determine the precise properties of haemoglobin
• There are a wide range of haemoglobin types that exist, varying in their oxygen-binding properties and binding/release conditions
• Environmental factors can have a major impact on the evolution of haemoglobin within a species
• Effects of altitude


  The partial pressure of oxygen is lower at higher altitudes, so species living there have haemoglobin adapted to bind very readily to oxygen to obtain sufficient saturation