Question from YenLuM (5th January 2010):

sir,may u explain to me da significance fact that:-
a)with an increase in partial pressure of CO2,the O2 dissosiation curve for the shift to the right?.
b)O2 dissociation curve for myoglobin is displaced to the left of that for haemoglobin

Answer:

a) This question concentrates on the effect of partial pressure of CO2 on haemoglobin. First, we should consider what a right shift means.

The graph on the left shows a right shift in the dissociation curve of haemoglobin. Actually a dissociation curve of haemoglobin also measures how fast it saturates with oxygen. (Dissociation or breakdown of oxyhaemoglobin to haemoglobin and oxygen, and saturation- combination of haemoglobin and oxygen, are 2 sides of the same coin.) A right shift means the curve is "gentler" or has a slower rising gradient - ie. it saturates slower with oxygen or it dissociates faster. (More oxygen is released to the blood or tissues by haemoglobin.) Therefore increasing partial pressure of CO2 in blood will cause the haemoglobin in the red blood cells to release more oxygen into the blood.

Next, we should explain why does this happen. CO2 in plasma diffuses into blood to form carbonic acid. The enzyme carbonic anhydrase acts on this bicarbonate to rapidly convert it into bicarbonate and hydrogen ions. An increase in CO2 means an increase in H ions in blood. This can cause a change in pH of blood towards the acidic side. As this is dangerous, haemoglobin steps in to remove H ions by combining with it to form a product called haemoglobinic acid (HHb). HHb is a weak acid and thus diminishes the effect of H ion in changing the pH of the red blood cell. ( In this way, haemoglobin acts as a buffer to maintain pH in the red blood cell.) The reaction of haemoglobinic acid is tied up with oxyhaemoglobin in this reaction:

The more haemoglobin acid is formed, the more oxyhaemoglobin has to dissociate to provide free haemoglobin to react with the extra H ions. This causes the effect of release of more oxygen. At the same time, the bicarbonate (negative ion) diffuses out of the red blood cell. This causes a decrease in negative ion concentration, and will enable chloride ions to diffuse in to replace this loss. It was recently found that chloride ions can act as an allosteric inhibitor to oxyhaemoglobin formation. (The increase in chloride ions causes the reaction between haemoglobin and oxygen to become inhibited or slower. So less oxygen can combine with haemoglobin, leaving oxygen molecules free to diffuse out of the red blood cell.

Such situations where the dissociation curve shifts are called Bohr effect. This particular situation (where the dissociation curve shifts to the right) occurs in the tissues where cell respiraton creates an increase in partial pressure of CO2- resulting in the red blood cell releasing more oxygen to the tissues. The reverse situation occurs in the lungs, where increased partial pressures of CO2 diffuses out of the blood into the lungs, and chloride ions diffuse out.

b) Shift of dissociation curve to the right:

In this case, myoglobin curve for saturation/dissociation with oxygen is more towards the left compared with haemoglobin. The myoglobin curve is considered hyperbolic, whereas the haemoglobin curve is considered sigmoid. This is not due to CO2 but to the fact that myoglobin is structurally different from haemoglobin. Myoglobin is a monomer with a haeme group that saturates with oxygen faster than haemoglobin. Haemoglobin is a polymer of 4 monomers that saturates with oxygen at an increasing state ( the more molecules of oxygen present, the faster haemoglobin saturates. This is called the cooperative binding of oxygen. Myoglobin does not exhibit this property. It just binds with oxygen at a fast rate.) Therefore the curve is to the right as faster saturation means slower dissociation.  

** The faster binding of myoglobin makes it an ideal component for storing oxygen in muscle, especially in animals that require oxygen for long-term muscular activity (e.g. migrating birds). In the event that all oxygen is used up in haemoglobin, then only the myoglobin steps in to release its stock of oxygen - like a second storage. That is one of the reasons why migrating birds can fly for hours without stopping, something humans can't do ( we don't have as much myoglobin in our muscles compared to the birds.)

Hope this answers your question.