Hemoglobin as a Buffer: How It Becomes Weak & Strong Acid

[Asmaa Mohammad]

Hemoglobin works as a buffer. It has 6 times more buffering power than plasma proteins.

My book says that Hemoglobin carrying CO2 (deoxyHb) is a stronger buffer than Hemoglobin carrying O2, because deoxyHb dissociates less (i.e. it forms a weaker acid = a stronger buffer).

I don't understand the sentence in bold. How does deoxyHb dissociate? Google searches is very complex? So I hope some one will explain that point for me.

Also, and that's the most important part, what makes deoxyHb forms a weak acid and oxyHb forms a stronger acid? And how this will affect the strength of buffering?

And what's the relation of dissociation of hemoglobin with the formation of strong or weak acids?

 

[epenguin]

Not happy about the phrase "Hemoglobin carrying CO2 (deoxyHb)" - is it a mistyping? Also it would be helpful if you could reproduce the entire relevant passage verbatim here.

These are words and you can describe what they are talking about in various different words! I'm not sure that the quoted ones are the most helpful! Hopefully there are also diagrams - and again the thing can be displayed in different ways in different diagrams. You will need to get used to how different diagrams are displaying the same thing. The most complete one will be a 3-D one of pH on one axis, [O₂] or p_O2 or log p_O2 etc, on the second, and amount of O₂ bound or saturation on a vertical axis. But what is often seen instead is a slice, or a series of slices through that surface. When you see that I recommend trying to visualise also the surface.

The bit about the weak acid is OK. A crude model for Hb can be as follows. There is partial competition for binding to Hb between H⁺ and O₂. It is as if the pK_a or some group in Hb is about 8, and in HbO₂ is about 6.7. So Hb is a weaker acid than HbO₂. It is actually saying the same thing in other words if you say that when starting with deoxy-Hb, each oxygen molecule that then binds to it in the pH range 6.5 to 9 displaces on average a fraction of a proton. (I hope talking of pK's is something familiar to you, if not ask again.)

It was easier for me to give that explanation than to explain somebody else's explanations that I don't like. . One weak acid is no stronger a buffer than another weak acid! The difference between them is the pH range in which they buffer. I can't make any sense of this phrase about stronger buffering unless pH range is stated. Try to understand the phenomenon, not just one person's description of it.

Your question (if I understand it) of what makes this proton release and uptake happen is that oxygen binding and proton binding cause the protein to fold in different ways, expose ionising amino acid sidechains to water in different ways, and the different conformations will have different affinities for oxygen and for protons. But that's several pages of a textbook, or a whole book, not for here. Your first have to get clear what the phenomenon is that these mechanisms that come out of decades of studies by crystallography, nmr, and every technique in the book, explain.

Re Google searches, I expect you will very soon find something helpful if you search for 'alkaline Bohr effect' or just 'Bohr effect'.
(The main effect is called the alkaline Bohr effect because it takes place in the pH range 6.5 - 9. Affinity increases (p_½ decreases with increasing pH. Below pH 6.5 the direction of the effect is opposite.)

 

[Asmaa Mohammad]

Not happy about the phrase "Hemoglobin carrying CO2 (deoxyHb)" - is it a mistyping? Also it would be helpful if you could reproduce the entire relevant passage verbatim here.

The original text is this:
"Hemoglobin is an important buffer. It has 6 times more buffering power than plasma proteins. Therefore, it can buffer H+ inside RBCs (They are formed during CO2 Transport) and can carry CO2 with minimal change in pH. Hemoglobin carrying CO2 (deoxyHb) is a stronger buffer than hemoglobin carrying O2, since deoxyHb dissociates less (i.e. it forms a weaker acid = a stronger buffer)."

(I hope talking of pK's is something familiar to you, if not ask again.)

I apologize, but it's not familiar to me. So, I can't say I really got what you said :(

 

[epenguin]

OK as the question was about buffers, I thought you would know something about them. You need to if you are following physiology like this. So you might try to read that up, you will not go very far without meeting pK_a. Actually studying something like that when you need it because you have an application to deal with is more motivating and fixes the learning better. Although the ideas are quite limited, they cause students quite a lot of problems somehow and we get probably more questions on them than any other subject in the homework help biology and chemistry section, so that is available to you.

Anyway, useful to know that the pK_a of an acid is the pH at which it is half protonated and half unprotonated. I'm going to represent an acidic sidechain of haemoglobin as -AH⁺. When the pH equals the pK_a half these group will be -AH⁺ and half -A . (These two forms are in a very rapid equilibrium, is protons transfer from -AH⁺ to water, with the opposite transfer taking place equally fast.) I hope you know what pH is - this is a must!

The pH inside the red blood cell is about 7.4, and the system tries to keep this as constant as possible. Metabolic oxidation produces acid - each molecule of O₂ produces one of CO₂ which becomes carbonic acid and finally (nearly) one proton H⁺ and one bicarbonate ion HCO₃^-.

Without mentioning pK's again, protein sidechains contain a lot of weak acid and basic groups.Now more than one acid group of an Hb subunit may participate in the Bohr effect, but it works like there was just one. Other side chain groups can also buffer at this pH but one of these groups is, we call it, oxygenation-linked or oxygen-linked. In oxyhaemoglobin at pH 7.4 this group is less than 20% protonated or more than 80% unprotonated. When the haemoglobin is deoxygenated it undergoes a change of 3-D confirmation that makes it become weaker acid, and that oxygen linked acid group becomes about 80% protonated. So the oxygen molecule that has oxidised C has created one proton, but about 0.6 of a proton (meaning 60% of the produced protons) you could say is taken up by the haemoglobin without involving any change of pH.

(Which I suppose physiologists could call buffering - it is not what I think of as buffering in a physical chemistry sense.)

Another function of the Bohr effect (which we can summarise those protons and oxygen being antagonistic and pushing each other off the haemoglobin) is that as the blood returns through capillaries, more acid as you go towards the veins, the acid pushing the oxygen off means that the blood is desaturated more completely, the haemoglobin carrying capacity is more completely used.

There are a lot of studies of the modulations of the Bohr effect in different organisms which have various slightly different Hb's, according to their physiology and lifestyle, these differences can be reasonably explained by the different structures. Just one is an extreme Bohr effect call the Root effect which acidifying the blood dries oxygen into swimbladders of fish ensuring their buoyancy and enabling them to adjust it.

 

[SciencewithDrJ]

Hemoglobin works as a buffer. It has 6 times more buffering power than plasma proteins.

My book says that Hemoglobin carrying CO2 (deoxyHb) is a stronger buffer than Hemoglobin carrying O2, because deoxyHb dissociates less (i.e. it forms a weaker acid = a stronger buffer).

I don't understand the sentence in bold. How does deoxyHb dissociate? Google searches is very complex? So I hope some one will explain that point for me.

Also, and that's the most important part, what makes deoxyHb forms a weak acid and oxyHb forms a stronger acid? And how this will affect the strength of buffering?

And what's the relation of dissociation of hemoglobin with the formation of strong or weak acids?

Just think along the lines of where you would expect to find more protons (H+). In tissues (where CO2 is more abundant) or in the lungs (where O2 is more abundant). Wouldn't you therefore expect to find a higher affinity for H+ once deoxyHB is in tissues? A stronger base accepts protons more readily.

 

[Asmaa Mohammad]

Just think along the lines of where you would expect to find more protons (H+). In tissues (where CO2 is more abundant) or in the lungs (where O2 is more abundant). Wouldn't you therefore expect to find a higher affinity for H+ once deoxyHB is in tissues?

OK, I think I would find more protons (H+) in tissues than in lungs, due to the existence of larger amount of CO2, hence the deoxyHB has to have higher affinity for H+, right?

A stronger base accepts protons more readily.

I didn't get this line. Why we are talking about strong bases here?! The book said that deoxyHB would form a weak acid, and I don't understand why and how.

And still I don't understand what is meant by deoxyHB dessociation? And how that will affect the buffering power of deoxyHB?

 

[SciencewithDrJ]

Dissociation of both human deoxyhemoglobin (deoxy-Hb) at alkaline pH exposes previously buried tyrosine residues to the environment and alters their PK.