Does Cooperative Binding cause the sigmoid shape in the Dissociation curve?

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Angusburgerman
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I have trouble getting my head around the dissociation curve and what it shows. I understand what cooperative binding of Hb and O2 is and how it works.

Is it the fact that with each O2 binding to Hb, it makes the next binding of O2 easier, up until the 4th binding where it is actually more difficult to bind, that causes this dissociation curve?
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RM_Asclepius
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The dissociation curve becomes steeper after the binding of the first O2 molecule due to positive cooperativity however, the curve becomes steep once again after the binding of the third O2 molecule as the probability of an O2 molecule binding to the single unoccupied haem group on the Hb molecule is low. It is important to know that positive cooperativity only occurs after the first O2 molecule binds to the Hb and its effect is not amplified subsequently.
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macpatgh-Sheldon
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Angusburgerman

I think these details are interesting, and co-operativity is not the full story as I will delve into later.

Unless you are doing a degree in biological sciences or preparing for the FRCPath exams some years after obtaining your medical degree, the really relevant and important part, rather than the molecular biochemistry, is the physiological significance of the sigmoid shape of the oxygen dissociation curve AND the Bohr shift in the tissues where pCO2 is high.

In the alveoli, where pO2 is high, drastically more oxygen is taken up by Hb to be carried by the pulmonary veins to the left atrium, thence to the left ventricle and the systemic circulation which carries it to where it is needed.
In the tissues, where oxygen needs to be released from Hb [obviously], the dissociation curve shifts to the right to enable exactly this.

Now for the molecular biochemistry/genetics:-
A comparison with myoglobin {which has a single haem chain] makes things a little clearer. Myoglobin [present in muscle - Greek myos = muscle as in myocyte] has a dissociation curve that is a hyperbola, so it is not a good molecule for transfer of oxygen from the lungs to the tissues. You can also look up the features of the haemoglobins of creatures that live at high altitudes and those dwelling at other hypoxic niches.

The Hill coefficient [n] is an index of the co-operativity of a molecule that binds oxygen and represents the haem-haem interaction - n is 1 for myoglobin and 3 for Hb.

The other comparison to make is with HbF [foetal Hb]. This has a higher affinity for oxygen than HbA [A for adult] because of the need to "steal" away O2 from the pregnant mother's Hb.

Apart from cooperativity described by the other member above, there is a crucial other mechanism that explains the shape of the dissociation curve. In the erythrocyte, there is also 2,3-BPG (biphosphosphoglyvcerate, an intermediary in respiration). 2,3-BPG has a high affinity for deoxygenated Hb, but this affinity reduces with O2 binding. In conjunction with the fact that binding of BPG to Hb reduces the affinity of Hb for O2, this explains the reduction of oxygen binding to Hb as it becomes more and more saturated with oxygen. (this is a bit difficult to follow, so let us take an example). In the tissues, Hb is deoxygenated; ----> this results in more BPG binding to it ----> affinity of Hb for O2 decreases ----> O2 is released [which is what is needed in the tissues].

The opposite occurs in the lungs - work it out step by step in this case - then it will stick in your mind.

If you are looking for a first class degree, read the below, too:

A] Specific amino acids are involved in the binding of BPG to Hb, including the N-terminal valine residues.

B] HbF owes its high affinity for O2 to the substitution of a serine in place of histidine at position 21 in the beta-globin chain (memory aid: imagine baby [so HbF] is crying inside uterus [the more absurd, OR extreme [very large or tiny or vividly coloured], or sexy your imagination of an item to remember, the more easily you will remember it - well-known fact!] and mum bends her head down to her belly and goes Shush - actually do this in your mind after closing your eyes!] Serine ----> histidine, then imagine the mum' belly expanding massively to accommodate a 21-yr-old "baby" (lol) = position 21 [I bet you will remember this and me in 2030].

Best of luck and be safe!

M
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