In elementary school, I happened to ask my teacher:

What is $\ce{CO}$ they talk about these days?

I was then taught that Carbon monoxide "competes" with the oxygen in our blood (we were taught that oxygen gets to our cells via blood at that time) and replaces it and thus, doesn't let enough O get to cells and this will be fatal in extreme cases.

I didn't do any research about it after that because I later made the implication that $\ce{CO}$ is able to replace O and react with Hb instead; but

I recently became doubtful about my previous lessons and decided to do a research to see what really was going on:

Carbon monoxide mainly causes adverse effects in humans by combining with hemoglobin to form carboxyhemoglobin (HbCO) in the blood. This prevents hemoglobin from releasing oxygen in tissues, effectively reducing the oxygen-carrying capacity of the blood, leading to hypoxia. Additionally, myoglobin and mitochondrial cytochrome oxidase are thought to be adversely affected. Carboxyhemoglobin can revert to hemoglobin, but the recovery takes time because the HbCO complex is fairly stable. Wikipedia, CO poisoning

Wikipedia states that the problems are:

  1. $\ce{CO}$ is able to combine with Hb to form HbCO.

  2. This compound is fairly stable, so we'll have a hard time removing $\ce{CO}$ from hemoglobin.

Other sources, such as How stuff works, Answers.com and CDC.gov have given either only more explicit explanations of wikipedia, or the explanations weren't "chemical-biology" enough.

I haven't seen a single reference in which the first assumption was declined, or agreed with. Is what I'd learnt non-scientific?


Each hemoglobin molecule has 4 subunits (4 hemes each having one iron atom). Each time a molecule (such as oxygen or CO) binds to one subunit, it changes the binding properties of the other subunits. This is referred to as positive "cooperative binding" in the case of hemoglobin, because binding one molecule makes the other sites bind more easily.

Initially, considering hemoglobin with no CO or oxygen bound, CO competes with oxygen for the same binding sites (ligating one of the iron atoms). CO has a binding constant 300 times lower than oxygen, meaning a factor of 300 lower CO concentration will result in an equal percent of bound protein.

Considering CO has reached a level such that 2 out of the 4 sites can have CO bound, the remaining 2 sites would still provide enough oxygen carrying capacity for a person to live if the sites were function as normal; however, the 2 remaining sites then bind oxygen too tightly and don't release it.

Other iron-containing proteins such as myoglobin can also bind CO and are also inhibited.

See Pharmacokinetics and Mechanisms of Action of Carbon Monoxide for more infomation.

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  • $\begingroup$ +1 Thanks Dave. Can you give me a reference to study more about the issue? $\endgroup$ – M.A.R. Jan 24 '15 at 16:48
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    $\begingroup$ @MARamezani here is a reference, it was more difficult to find a reference than I thought. nershco.com/case_studies/co%202.pdf You should read about cooperative binding of hemoglobin subunits generally also. $\endgroup$ – DavePhD Jan 24 '15 at 17:09

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