Oxygen Dissociation Curve and Carbon Monoxide

[theyellowking]

What I have trouble understanding is why the curve shifts to the left when carbon monoxide is present. Wouldn't the four heme sites readily bind with the carbon monoxide, decreasing the affinity of the RBC with the oxygen? In other words, why doesn't the presence of CO have the same effect as an increase in pressure of CO2, decrease of pH, or increase in temperature?

I looked up on Wikipedia and this is what I got: The presence of carbon monoxide on one of the 4 heme sites causes the oxygen on the other heme sites to bind with greater affinity.[citation needed]

The fact that a citation is needed is making me a little doubtful of their explanation, so I'm hoping that someone can answer this for me. Thanks!

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[neurodoc]

The Wikipedia explanation is correct, but your question is a good one. CO has a higher affinity than O2 and displaces O2 from Hgb binding sites. This alters the Hgb and increases the O2 binding at the other sites, decreasing the ability of Hgb to deliver O2 to tissues that need it. The CO binds avidly to Hgb, and once bound dissociates with difficulty. That's shifting the curve to the left. Now of course it's possible for CO to continue displacing O2 from the other 3 binding sites, leading to Hgb saturated with CO, but for that to happen you'd have to expose an organism to an atmosphere of 100% CO for a bit of time. In the real world CO poisoning occurs when an organism is exposed to much lower concentrations of CO. The CO binds to one of the Hgb sites and this shifts the dissociation curve enough to the left that tissues suffer hypoxia, even though the Hgb still carries "plenty" of O2.

This is clinically important to understand, as a patient suffering from CO intoxication will not appear cyanotic as in other cases of hypoxia. They will in fact appear to be
flushed and red-faced.

 

[theyellowking]

I see. So even if it increases the oxygen affinity to the site (thereby explaining the "left" shift in the oxygen saturation graph), the oxygen fails to be delivered to the destined tissue?

 

[IslandStyle808]

I see. So even if it increases the oxygen affinity to the site (thereby explaining the "left" shift in the oxygen saturation graph), the oxygen fails to be delivered to the destined tissue?

From what I remember, that the CO causes the hemoglobin to convert the other oxygen binding sites to their relaxed states. Because this happens, oxygen affinity surpasses CO affinity. This is why it is difficult to get 100% CO saturation of hemoglobin. Oxygen will still be delivered none the less, but at a lower level than normal, hence the hypoxia at certain tissues.

EDIT: I just read my costanzo physiology book. The reason that oxygen delivery is lower is not only because of the reduced sites, but also that the hemoglobin is acting like only "half" of the oxygens are on the molecule. Thus it is less willing to unload those oxygens.

 

[ChEMD]

The key point is that higher O2 affinity means the carboxyhemoglobin proteins can and do bind O2, but when they reach peripheral tissues, they won't release it, thus leading to an oxygen deficit in peripheral tissues and ischemia.

 

[Mad Jack]

http://en.wikipedia.org/wiki/Cooperative_binding

http://education-portal.com/academy...binding-of-oxygen-with-hemoglobin.html#lesson

The second link is a really basic example of how cooperative binding and gas transport works. The former is the complex mathy stuff.