Methemoglobin question

[dragon72]

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question: why the conversion from normal hemoglobin to methemoglobin causes the dissociation curve to shift left instead of right. If it shift left then affinity for oxygen would increase and that is wrong. I don 't understand why.

Thanks in advance,
Andy

 

[Moxxie]

question: why the conversion from normal hemoglobin to methemoglobin causes the dissociation curve to shift left instead of right. If it shift left then affinity for oxygen would increase and that is wrong. I don 't understand why.

Thanks in advance,
Andy

Not all of the hemoglobin in the blood will be converted to methemoglobin when oxidation takes place, but the formation of metHb will essentially remove a portion of the normal Hb population that could bind to oxygen. Consequently, the hemoglobin that can bind with oxygen does so with increased affinity. Think of this like receptors and substrates - lots of substrate (O2) is available with less receptors (Hb) for binding. One of the interesting things about Hb is that the more affinity it has for oxygen (ie can pick up O2 molecules more efficiently), the less likely it is to actually give up this oxygen later - the O2 is tightly bound.

So when excess methemoglobin is formed, the remaining Hb binds oxygen aggressively (thus shifting the dissociation curve to the left), but this oxygen won't actually get to the tissues it needs to because the Hb won't give it up later. This is why patients with methemoglobinemia are usually cyanotic. This might be what you were having trouble with - patients with excess methemoglobin don't get enough oxygen, yet their (intact) Hb is saturated with it - it's pretty confusing.

I hope that this makes sense. I haven't had physiology in over two years, but I remember being confused about it so I studied it pretty hard. If I'm wrong, though - don't hesitate to correct me!

 

[starsweet]

Um... is this being studied for the PCAT? I hope this is for class.

 

[Pharmwannab]

Er, we never even touched on hemoglobin binding curves in biology 🙁 Some in biochem, but we never even looked at methemoglobin. I seriously hope this stuff isn't on the PCAT.

 

[Moxxie]

Er, we never even touched on hemoglobin binding curves in biology 🙁 Some in biochem, but we never even looked at methemoglobin. I seriously hope this stuff isn't on the PCAT.

I kind of assumed that this wasn't necessarily a PCAT-related questions (even though this is the PCAT forum). I didn't learn this stuff until I was in comparative physiology (a senior/grad level course at my school).