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Name: Natalie
Status: student
Grade: other
Location: Outside U.S.
Country: United Kingdom
Date: Winter 2012-2913

How does Haemoglobin change shape when it is carried with oxygen and CO2?

Hi Natalie,

Thanks for the question. When hemoglobin binds oxygen, the iron atom (a ferrous ion, Fe2+) gets smaller and it moves into the plane of porphyrin ring. I will refer you to pictures on Wikipedia and the internet of hemoglobin and the porphyrin ring. What happens is that the oxygen (specifically O2) oxidizes the iron 2+ to make it 3+. This oxidation causes the iron ion to become smaller and thus fit into the porphyrin ring. When oxygen is released, the iron ion pops back outside the plane of the porphyrin ring. There should be an animation of this somewhere on the internet, perhaps in a biochemistry course webpage. Not to be forgotten, there is also a distal histidine amino acid residue that participates in the above described motion.

I hope this helps. Please let me know if you have any more questions. Thanks Jeff

This is an excellent and scientifically important question! In fact, hemoglobin was one of the first proteins for which we discovered its precise three dimensional structure; a close relative, myoglobin, was the first, and earned two scientists the 1962 Nobel Prize in Chemistry. The detailed structure of hemoglobin gave scientists insight into the way oxygen and carbon dioxide are carried in the blood stream.

Hemoglobin is a large protein composed of four smaller subunit proteins held together by weak chemical bonds. Inside each subunit is a small, tightly bound chemical structure called a 'heme' group, which is not a part of normal proteins. The heme group is a complex chemical structure called a porphyrin ring which holds a single iron ion at its center This iron ion is bound both to the porphyrin ring and the protein subunit, with one 'free' binding site for oxygen or water. When bound to oxygen, the iron is 'pulled' into the ring, causing a very slight shift in the conformation of the protein subunit. This in turn causes small shifts in the shape of the other three subunits, making them more likely to accept an oxygen molecule.

Hemoglobin shape is also affected by the amount of carbon dioxide present as well as the pH of the blood (which itself is tied to carbon dioxide concentration). This can cause hemoglobin to shift from a T ('taut') conformation to a R ('relaxed') state. The T state is favored when carbon dioxide levels are high and pH is low, as you might find in tissues which need oxygen. The T state binds less strongly to oxygen and releases it to the tissue. Conversely, when carbon dioxide levels are low and pH is high, as you might find in the lungs, the R state is favored, which increases the chances of binding oxygen. This is a simple way for oxygen to be successfully carried from the lungs to tissues that need it.

S. Unterman Ph.D.

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