7 Haemoglobin

  • Created by: lee8444
  • Created on: 09-03-20 17:30

Haemoglobin molecules

  • Haemoglobin proteins have a quaternary structure
    • almost spherical
    • 4 polypeptides
    • each polypeptide has a haem group
    • each haem group has a ferrous (Fe2+) ion
    • each Fe2+ can combine with a single O2
    • total of 4 oxygen molecules per haemoglobin
  • Efficient at loading oxygen in certain conditions and unloading it under other conditions
  • Loading/unloading
    • can be called loading and associating or unloading and dissociating
    • haemoglobin with a high affinity for oxygen take oxygen up more easily but release it less easily
    • haemoglobin with a low affinity for oxygen takes up oxygen less easily but releases it more easily
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Role of haemoglobin

  • To be efficient, haemoglobin must:
    • easily associate with oxygen at the gas-exchange surface (lungs)
    • easily dissociate from oxygen at the tissues requiring it
  • The affinity changes depending on the conditions
  • This happens as the partial pressures of oxygen and carbon dioxide cause the tertiary structure of haemoglobin to change causing the affinity to change

Different haemoglobins

  • Many organisms possess haemoglobin
  • The affinity of haemoglobin changes depending on the organism
  • This was due to the shape of the molecule which changes due to mutations during evolution
  • This is because the mutation may cause a new amino acid which changes the primary, secondary and eventually the tertiary and quaternary structure of the haemoglobin produced
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Oxygen dissociation curve

  • Under different partial pressures of oxygen, the affinity for oxygen changes
  • The shape of the haemoglobin makes it difficult for the first oxygen molecule to bind to a haem group as the sub-units are closely bound causing the graph to have a shallow gradient at the start as little oxygen is taken up
  • After the first oxygen molecule has bound to a haem group, the quaternary structure of the haemoglobin changes making it easier for a second oxygen molecule to bind to a haem group. Therefore, a small increase in the partial pressure of oxygen causes the gradient to rapidly increase (positive cooperativity)
  • After the third oxygen molecule has bound to the haemoglobin, due to probability, it is less likely that an oxygen molecule is going to bind to the fourth site on haemoglobin. This causes the graph to level off under high partial pressures of oxygen
  • The further left the curve, the greater the affinity for oxygen
  • The further right the curve, the lower the affinity for oxygen
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Effect of carbon dioxide concentration

  • Haemoglobin's affinity for oxygen in high concentrations of carbon dioxide decreases - The Bohr effect
  • At the gas exchange surface, the concentration of carbon dioxide is low because it diffuses out of the organism. This causes the affinity for oxygen to increase so oxygen is readily taken up at the gas-exchange surface. This is aided by the relatively high concentration of oxygen at the gas-exchange surface - at this point, the oxygen dissociation curve is to the left
  • At the respiring tissues, the concentration of carbon dioxide is high so the affinity for oxygen is reduced. Helped by the relatively low concentrations of oxygen at the respiring tissues, oxygen is readily unloaded. The oxygen dissociation curve has shifted to the right
  • All of this is because the dissolved carbon dioxide lowers the pH of the blood
  • This causes the haemoglobins quaternary structure to change as certain bonds change
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Loading, transport & unloading

  • Carbon dioxide is constantly being removed at the gas-exchange surface
  • pH is slightly raised due to less carbon dioxide so less acidic
  • Higher pH changes the shape of haemoglobin into one that enables oxygen to load easily
  • The changed shape also has a higher affinity for oxygen so oxygen isn't lost whilst being transported to the tissues
  • In respiring tissues, carbon dioxide is being produced
  • This lowers the pH of the blood around the tissue
  • The lower pH changes the shape of the haemoglobin which makes it easier for oxygen to unload along with a lower affinity for oxygen
  • This causes oxygen to be released into respiring tissues
  • The more a tissue is respiring, the more oxygen is unloaded for respiration
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Human/animal haemoglobin

  • Human haemoglobin becomes saturated with oxygen in the lungs but in reality not all haemoglobin has all for haem groups associated with an oxygen molecule
  • At atmospheric pressure, human haemoglobin has a saturation of around 97%
  • Normally, only one oxygen molecule per haemoglobin is released at a respiring tissue
  • However, at an exercising tissue, 3 oxygen molecules will be unloaded for more respiration
  • Different species have different haemoglobin with different dissociation curves that are adapted to their environment
  • Animals in environments with low partial pressures of oxygen have evolved to have haemoglobin to have a higher affinity for oxygen so that more oxygen is taken in
  • e.g. the lugworm lives on the seashore - as it is not very active it is covered by seawater which covers the lugworm's burrow and oxygen is taken in from the water and circulated. When the tide goes out, the lugworm can extract less oxygen from its surrounding water and so to extract as much as possible, the lugworm has adapted to have a high affinity for oxygen so that as much oxygen as possible is taken up - the dissociation curve is further to the left than humans
  • e.g. the llama lives at high altitudes where there is less oxygen - shifted to the left
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