CO binds competitively or allosterically?

[stronghold]

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USMLE Rx made me mad! In one questions it states that CO binds Hb competitively, while in another questions it states allosteric binding! Which one is the right answer?

 

[Anastomoses]

It binds allosterically.

Edit: Oops. carbon monoxide...I don't know why I thought you asked about carbon dioxide. CO definitely binds competitively. Goljan likens it to oxygen having its seat taken.

 

[loveoforganic2]

I'm quite sure it's a competitive binder at the oxygen binding site - this is why hyperbaric O2 works as a treatment. That said, I would carefully read those explanations - the O2 binding site on hemoglobin are allosteric modulators of the other O2 binding sites within that hemoglobin molecule. High affinity carbon monoxide binding at the O2 binding site greatly increases the affinity of the remaining O2 molecules via positive allostery

 

[stronghold]

Thank you very much. I understand it now!