Haemoglobin
Alpha chain
Beta chain
Haem group
Alpha chain Beta chain
> Haemoglobin (Hb) is a conjugated protein with a quaternary structure.
> It is found in red blood cells.
> It is a conjugated protein because it has proteins attached to non-proteins.
> A quaternary structure is more than one polypeptide chain joined together.
> Haemoglobin has four polypeptide chains joined together. These chains are of two types,
Beta and Alpha.
> The haemoglobin also has a prosthetic group called haem. Haem consists of an iron
atom enclosed in a ring.
> Each haem group can combine with one oxygen molecule.
> This means that four oxygen molecules can bind to a single haemoglobin molecule.
> In the lungs, oxygen binds to haemoglobin to create oxyhaemoglobin.
> This reaction is reversible. When the oxygen leaves oxyhaemoglobin near the body cells
it turns back to haemoglobin again.
Partial pressure of oxygen;
> The partial pressure of oxygen (pO2) is a measure of the concentration of oxygen.
> Haemoglobins affinity for oxygen depends on the partial pressure of oxygen;
- Oxygen loads onto haemoglobin to form oxyhaemoglobin easily where there is a
high partial pressure of oxygen.
- Oxygen unloads from the oxyhaemoglobin when there is a low partial pressure of
oxygen.
> Oxygen enters the blood capillaries in the alveoli in the lungs.
> The partial pressure of oxygen here is high; therefore oxygen loads onto the
haemoglobin.
> When cells respire, they use up oxygen.
> This lowers the partial pressure of oxygen.
> This means that oxygen is unloaded from the oxyhaemoglobin to the respiring cells.
The dissociation curve;
> The dissociation curve shows how saturated haemoglobin is at various partial pressures
of oxygen.
Where the partial pressure of oxygen is high, like
in the lungs, the haemoglobin has a high affinity
for oxygen. This means that oxygen can easily
bind to the haemoglobin so it has a high
saturation of oxygen.
Where the partial pressure of oxygen is low, like
in respiring tissues, the haemoglobin has a low
affinity for oxygen. This means that the oxygen
cannot bind as easily, so it is unloaded. This
lowers the saturation of oxygen of the
haemoglobin.
Why is the graph ‘S’ shaped?
> When there is a partial pressure of oxygen of 0, there is no oxygen bound to the
haemoglobin.
> At low partial pressures of oxygen, the polypeptide chains are tightly bound together, so it
makes it difficult for the oxygen molecules to reach the haem groups to bind, therefore the
curve rises only gently.
> As one molecule of oxygen binds to one of the haem groups the polypeptide chains open
up a bit revealing the other haem groups. This makes it a lot easier for the oxygen
molecules to bind to the haemoglobin. This makes the curve rise steeply.
> At very high partial pressures of oxygen, the haemoglobin becomes saturated so the
curve levels off.
Loading – this is the partial pressure of oxygen at which haemoglobin becomes 95%
saturated.
Unloading – this is half the saturation level.
Carbon dioxide levels affect oxygen unloading;
> Haemoglobin gives up oxygen more readily at higher partial pressures of carbon dioxide.
> It is a way to get more oxygen to cells that are respiring (that are using oxygen and need
more).
> When cells respire they produce carbon dioxide.
> This raises the partial pressure of carbon dioxide.
> This makes the oxygen unload more easily from the haemoglobin.
> This will make the dissociation curve shift to the right.
> This is called the Bohr Effect.
> The saturation of oxygen will be lower for a given partial pressure of oxygen.
> This means more oxygen is being released.
The reaction;
> Carbon dioxide diffuses into the red blood cells and is converted into carbonic acid.
> Carbonic acid dissociates to form hydrogencarbonate ions and hydrogen ions.
> The hydrogencarbonate ions diffuse into the plasma.
> The hydrogen ions remain inside the red blood cell and are mopped up by the
haemoglobin to form haemoglobinic acid.
> It’s formation forces haemoglobin to unload oxygen.
> Increased carbon dioxide causes a decrease in pH.
Haemoglobin varies between different organisms;
High altitude environments;
> At high altitudes, there is less oxygen available. The partial pressure of oxygen lowers.
> Organisms that live here have a dissociation curve to the left of the human dissociation
curve,
> This means that the haemoglobin of these organisms has a higher affinity of oxygen.
> This means that it will take lower partial pressures of oxygen for haemoglobin to bind to
the oxygen. The oxygen will bind more easily at lower partial pressures of oxygen (low
loading pO2).
> The haemoglobin will also have a low unloading partial pressure of oxygen.
Mammalian foetus;
> The human foetus has a dissociation curve to the left of the human dissociation curve.
> This means the foetal haemoglobin has a higher affinity for oxygen that the human
haemoglobin.
> This is so that the foetus can retrieve a sufficient amount of oxygen to supply the cells.
Lugworms;
> Lugworms live in conditions where the oxygen levels can get very low.
> At these times the lugworms need to make sure they get enough oxygen to their cells.
> They have a dissociation curve to the left of the human dissociation curve.
> This means their haemoglobin has a higher affinity of oxygen, a low unloading and low
loading partial pressure of oxygen.
Ducks and mackerel;
> These organisms are fast moving and live in oxygen rich conditions.
> This means that their haemoglobin doesn’t need such a high affinity for oxygen since the
environment they live in is oxygen rich.
> They have a dissociation curve to the right of the human dissociation curve.
> This means that they have a lower affinity of haemoglobin for oxygen.
> This means that the oxygen is less able to bind to the haemoglobin.
> They have a high unloading pO2 and a high loading pO2.
> They need a higher partial pressure of oxygen to be able to bind oxygen with
haemoglobin.
> They also need a higher partial pressure of oxygen to be able to unload oxygen.
