Oxygen dissociation curves

• Affinity - the degree to which two molecules are attracted to each other
• What are the benefits of having an oxygen dissociation curve further to the right?

  The move to the right lowers the affinity of haemoglobin for oxygen. Oxygen more readily dissociates at the same partial pressure of oxygen (ppO2) for aerobic respiration.
• How does a curve to the left for a llama show that its haemoglobin is well adapted for its environment?

  Llama lives at high altitudes where oxygen is scarce.

  The curve shows haemoglobin has high affinity for oxygen so can become more saturated with oxygen more easily.

  A small change in partial pressure results in a large change in percentage saturation.
• The sigmoid (S-shaped) curve is more efficient for a respiratory pigment than a linear relationship because a small change in partial pressure results in a large change in percentage saturation. It is more efficient in unloading oxygen to respiratory tissues with a low partial pressure.
• Associate - the loading of oxygen onto the haemoglobin
• Dissociate - the unloading of oxygen from the oxyhaemoglobin
• Partial pressure of oxygen (ppO2) - the pressure oxygen exerts on a system
• Hb - haemoglobin
• Affinity - ability for 2 molecules to bind
• Structure of haemoglobin:

  • Quaternary protein - 3D globular shape
  • 4 haem groups (prosthetic added at the Golgi body)
  • 4 iron ions - 1 per haem group
  • 4 O2 molecules - 1 per haem group
  • 2 alpha helixes, 2 beta pleated sheets - 4 polypeptide chains
  • 2 genes code for haemoglobin
  • Fully saturated when 4 O2 molecules are attached
  • O2 associates with haemoglobin to form oxyhaemoglobin
• Contracting muscle:

  • high O2 demand
  • high metabolic rate
  • needs more O2 for aerobic respiration
  • needs more ATP for muscle contraction
  • 80% of O2 has been dissociated to the muscle tissue from oxyhaemoglobin
• Relaxed muscle:

  • Low O2 demand
  • low metabolic rate
  • needs less O2 for aerobic respiration
  • 20% of O2 has been dissociated to the tissue from haemoglobin
• Curve is S-shaped due to cooperative binding of O2 to Hb
• Number of O2 molecules dissociated into the tissue = maximum % saturation - % saturation at a given point
• It is difficult for the last O2 molecule to associate with the fourth haem group so it doesn't reach 100% saturation
• At high ppO2 e.g. in the lungs, oxygen can associate with the haemoglobin to form oxyhaemoglobin
• Gas exchange at the lungs - association - at high ppO2 and supply of oxygen to the body cells at a capillary bed - dissociation - at lower ppO2
• Myoglobin:

  • respiratory pigment found inside cells - e.g. muscle cells
  • curve is extremely to the left
  • Only one polypeptide chain and one haem group
  • only combines with one molecule of O2
  • used as an oxygen store in muscle cells - supplementary source of O2
  • The pigment has a very high affinity for oxygen
  • associate O2 at much lower ppO2 - over a large range
  • Only dissociates O2 to the cell to maintain aerobic respiration for longer

  In some species Hb acts very similarly to myoglobin. Storing oxygen until very low ppO2 as it has a high affinity for oxygen
• Curve to the left:

  • Hb has a higher affinity to O2

  Adv

  • associate O2 onto the Hb at lower ppO2
  • foetal Hb + placenta
  • llama + high altitude

  Disadv

  • O2 is harder to dissociate at the body tissues
  • need very low ppO2 to dissociate O2
  • less O2 for aerobic respiration in adult humans
• Curve to the right:

  • increases in CO2
  • High rate of aerobic respiration
  • = produces more CO2
  • = produces more carbonic acid (CO2 + H2O by carbonic anhydrase)
  • = more hydrogen carbonate ions that diffuse into the plasma. Chloride ions diffuse in to maintain electrochemical charge - chloride shift
  • Also = more hydrogen ions that have a higher affinity for Hb than O2
  • More O2 dissociated from Hb. Hb acts as a buffer. Hb + H+ = haemoglobinic acid.
  • More O2 delivered to cells to maintain high rate of aerobic respiration
• Curve to the right:

  Adv

  • more O2 is supplied to body tissues for aerobic respiration (at any given ppO2 for these tissues) - more dissociation = Bohr Shift
  • Hb has a lower affinity for O2

  Disadv

  • need higher ppO2 to associate O2 at gas exchange surface