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I recently learnt the basic mechanism of haemoglobin: oxygen forms coordinate bonds with the $\ce{Fe^2+}$ ion in haem groups, and this reaction is reversible to allow oxygen to be released. I also learnt that carbon monoxide does the same thing as oxygen, however, this reaction is said to be irreversible due to larger bond strengths etc. meaning that it can't be released.

What isn't clear to me is what happens to carbon monoxide if it does bind to a haem group. If the reaction is irreversible, then how is carbon monoxide removed from the body if you were to breathe it in?

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  • $\begingroup$ That's just a matter of what you'd call irreversible in this case it's just a lot harder to unbind then O2 $\endgroup$ – Mithoron Apr 16 '18 at 15:35
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The reaction is not irreversible, but hemoglobin adheres to $\ce{CO}$ about 200 times more tightly than it does to $\ce{O2}$. Thus, part of the treatment for $\ce{CO}$ poisoning is to expose the patient to oxygen, which will slowly displace $\ce{CO}$ from carboxyhemoglobin.

For more on the chemistry involved, see this qestion.

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  • $\begingroup$ Thanks for clearing that up. My exam specification says that the haem-carbon monoxide bond is irreversible, which I thought was a bit strange. $\endgroup$ – user60221 Apr 16 '18 at 7:05
  • $\begingroup$ You might politely point out to the examiner the references citing reversibility, e.g. Wikipedia, and that people do recover from CO poisoning more rapidly than it would take for RBC's to be replaced (their lifetime is on the order of three months), depending on how open the examiner is to criticism. $\endgroup$ – DrMoishe Pippik Apr 16 '18 at 15:06
  • $\begingroup$ It's not a single examiner, it's a full exam board. For me to want to mention this to them, I'd have to ask my teacher to send some form of notice to them. That of course would take time etc. haha $\endgroup$ – user60221 Apr 16 '18 at 15:40
  • $\begingroup$ Sorry about that. As a teacher, I occasionally noticed mistakes in a text and would point them out to students, but an exam with an error is downright, frustratingly, unfair. $\endgroup$ – DrMoishe Pippik Apr 16 '18 at 15:52
  • $\begingroup$ Maybe it was intentional. For the A Levels we only need to know the concept of coordination chemistry, so maybe haemoglobin is just an example for us to know; they might've said it's irreversible and left it at that. Just like we need to know that cisplatin is a chemotherapy drug but we don't need to know its full mechanism of action. $\endgroup$ – user60221 Apr 16 '18 at 16:56

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