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 Affinity of Hb for oxygen  

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Hi everyone. I was reading about the affinities of the 4 binding sites of hemoglobin for oxygen and was confused about one thing

If the 4th binding site has the highest affinity for oxygen then why is it empty when plasma pO2 is 40mmHg while at this pressure the other sites are occupied??

I mean, if something has great affinity for an object then it should be very difficult to remove that object from it. So if the 4th site has the greatest affinity, then it should still have been occupied by oxygen at pO2 40mmHg.

And since it is written in Kaplan Physiology that the 1st binding site binds oxygen at the lowest pO2 (remains attached under physiologic conditions), then the 1st site should be the one with highest affinity since it is still attached to oxygen when the other sites are empty...


The amount of oxygen bound to the hemoglobin at any time is related, in large part, to the partial pressure of oxygen to which the hemoglobin is exposed. In the lungs, at the alveolar–capillary interface, the partial pressure of oxygen is typically high, and therefore the oxygen binds readily to hemoglobin that is present. As the blood circulates to other body tissue in which the partial pressure of oxygen is less, the hemoglobin releases the oxygen into the tissue because the hemoglobin cannot maintain its full bound capacity of oxygen in the presence of lower oxygen partial pressures ( Wikipedia,2014).

Also see this link : nod

I hope that is helpfully,,

Best regards,,,

Future Doctor: M. Alrubaie


Thanx for the reply. What you said is understood but my question was not this.
My question is, that if it is said that the 4th site on Hb has the highest affinity for Oxygen then why is it that at low partial pressures of oxygen around tissues the 4th site is empty while the 1st is occupied??...shouldn't we give the 'highest affinity' title to the 1st site rather than the 4th??...



You need to understand cooperative kinetics.

Cooperative kinetics state that the affinity for one molecule increases as more molecules are bound to it.

The 1st site binds oxygen but affinity is lower than when 2 molecules combine which have more affinity then single binding site.

When 3 out of 4 combine the affinty for oxygen increases further.

When finally the 4th site is bound the affinity for oxygen is said to reach maximum and that is why 4th site has maximum affinity because 04 molecules are bound to oxygen ( by virtue of cooperative kinetics, this is the state in which Hb has maximum oxygen affinity).

The reason for this cooperative binding is why the Hb-Oxygen curve is sigmoidal in shape.

instead of a hyperbolic curve which would be expected with myoglobin where all the molecules bind or disassociate to reach a saturation level and no cooperativity is exhibited.

I hope that clears it up.

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