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A follow-up to How does hemoglobin-free blood transport oxygen?

I'm unsure about the use of physiology/metabolism in the title there. The question in mind is whether this reversible binding makes an organism slower, or faster; perhaps more capable of surviving in rarefied atmosphere.

EDIT: I apologize that my comment made it appear this question is only about the Icefish; it is not. There are still other species (for instance, molluscs, and arachnids) which acquire oxygen by means other than haemoglobin.

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I'd love to answer this question, but I don't really understand it. If you are referring to oxygen binding by a carrier protein such as hemoglobin, then the binding has to be reversible otherwise the protein wouldn't be able to deliver oxygen to respiring tissues. –  Alan Boyd Nov 5 '12 at 9:57
    
@AlanBoyd: The answer in the linked question indicates that species the binding is not reversible in Icefish; perhaps other species too. I'm curious about the nature of oxygen binding (reversible or otherwise) does it improve the ability of blood to deliver oxygen/nutrients and in turn what effect that has on the organism's physiology. Could it have evolved to enable the creature to survive adverse environment - as in the case of ice-fish? –  Everyone Nov 5 '12 at 11:31

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The classic example of a hemoglobin adaptation to high altitude is found in the case of the bar-headed goose, Anser indicus.

This bird undertakes very high altitude migratory flight, crossing the Himalaya. It has a number of physiological adaptations that allow this, but it also has a novel form of hemoglobin with an unusually high affinity for O2. A single mutation changes the co-operative oxygen-binding behaviour of the haemoglobin molecule. In terms of the diagram in jello's answer, this shifts the binding curve to the left (i.e. toward that of fetal haemoglobin). This, in turn, means that the molecule binds more oxygen in the lungs, but it also means that it doesn't release the oxygen as readily in the peripheral tissues. On balance the former effect dominates and the efficiency of oxygen delivery is improved.

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I don't understand your question either, but my response is to the question in the title. Oxygen has low saturation in blood plasma, so humans require hemoglobin to serve as a transport protein. Oxygen must bind reversibly, otherwise it gets stuck to hemoglobin.

Humans actually have two proteins, hemoglobin and myoglobin. Hemoglobin (HbA) circulates in the blood, whereas myoglobin is in the muscles. Myoglobin (Mb) has a higher affinity for oxygen than hemoglobin (see image). This makes sense, because the hemoglobin in the blood must unload oxygen at the muscles, and myoglobin must bind it. Hemoglobin's affinity for oxygen is regulated by 2,3-BPG, pH, and [CO2] to ensure that it can unload oxygen at tissues.

Oxygen saturation curves (Curves to the left have higher affinity, meaning that they have a higher oxygen saturation for the same partial pressure of oxygen.)

Furthermore, humans have several types of hemoglobin. Fetal hemoglobin (HbF) has a higher affinity for oxygen than regular hemoglobin, ensuring that the fetus obtains the oxygen that it needs from the maternal blood.

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Curious: If haemocyanin were to replace haemoglobin in a human body, would the person survive? –  Everyone Nov 5 '12 at 17:08
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Ignoring the fact that hemocyanin is only in hemolymph and not red blood cells (as in humans), my guess would be no. Hemoglobin exhibits cooperativity, so that the binding of one oxygen molecule increases the affinity for other oxygen molecules. This leads to the sigmoidal oxygen saturation curve, allowing for more efficient dissociation of oxygen at target locations. –  jello Nov 7 '12 at 8:45

I think I see where you are coming from. There is a misunderstanding here.

Ice-fish have relatively lost their hemoglobin gene over time because they have adapted such a slow metabolism in the cold water where the oxygen is richer (oxygen content in cold water is higher). The don't need any binding of oxygen to a carrier protein like hemoglobin because enough oxygen diffuses in from the water for the needs of their tissue.

There is no binding - reversible or irreversible in their blood - that would imply that the oxygen is binding to a protein like hemoglobin or hemocyanin, which the fish does not have. The oxygen is just diffusing into the fish from the water around it. The circulatory system is helping by flushing the system out and keeping everything fresh as well.

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Just to expand slightly on this answer, for clarity, the oxygen is diffusing into the blood at the gills, and diffusing out of the blood into the peripheral tissues where it is used for respiration. The blood and circulatory system of the icefish is performing the usual function, it just does it without an oxygen-binding protein. –  Alan Boyd Nov 5 '12 at 16:49
    
@AlanBoyd thanks for this. its possible there is diffusion through the gut and the skin too - remember salamanders and most reptiles have some oxygen from their skin too. –  shigeta Nov 5 '12 at 22:28

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