Hemoglobin and oxygen dissociation curve

[Hemichordate]

I know that the S-shaped oxygen curve is based upon cooperativity in hemoglobin, but not based upon cooperativity in myoglobin since it can only bind to 1 oxygen molecule. However, if we had a hypothetical oxygen binding molecule with say, 12 heme groups instead of the 4 found in hemoglobin and 1 in myoglobin, would that molecule be 1) steeper (slope) than the hemoglobin curve and 2) shifted more toward the right?

---

Members don't see this ad.

 

[Omni]

Having 12 heme groups will start out like horizontal line (even more than the 4 heme group) and then eventually become even steeper. My guess would be that this owuld shift the curve to the far right, decreasing the affinity.

 

[thebillsfan]

good question hemichordate. now why would it shift to the right? because there are more oxygen binding spots so it takes more oxygen to saturate the Hg? is it that simple?

 

[zkbd9]

This is hard to say. On a normal hemoglobin molecule the 4th oxygen group that binds to hemoglobin does so with some difficulty due to overcrowding. Hypothetically, assuming that this overcrowding does not take place in your 12 oxygen hemoglobin, you would think that hemoglobin would have an increased affinity for oxygen at the lungs (we are now binding 12 and not 4 oxygens). This would shift the graph left. It would be steeper though because more oxygen would be released into the systemic capillaries. That's my take on it.

 

[Omni]

This is hard to say. On a normal hemoglobin molecule the 4th oxygen group that binds to hemoglobin does so with some difficulty due to overcrowding. Hypothetically, assuming that this overcrowding does not take place in your 12 oxygen hemoglobin, you would think that hemoglobin would have an increased affinity for oxygen at the lungs (we are now binding 12 and not 4 oxygens). This would shift the graph left. It would be steeper though because more oxygen would be released into the systemic capillaries. That's my take on it.

Actually, it would shift to the right. It would shift so far to the right that it's be practically impossible for the hemoglobin to pick up any oxygen. The reason being is that as you said, there is difficulty for O2 to bind to hemoglobin. However, once it gets just 1 O2 molecule, the remaining three come on pretty easily. That is, after 25% saturation, it becomes increasingly easy to get to 100%. However, think about the 12 heme group hemoglobin. You would need to have it bind to 3 heme groups to get to 25% in order to make the rest of the binding easier. Since heme doesn't have such a high affinity to O2 in the first place, requiring 3-4 heme groups to bind to O2 is asking for too much. However, once you get that at extremely high pressure, the graph may be close to a vertical line past 50%.

Also, look at how the graph shifts from myoglobin (1 heme group) to hemoglobin (4 heme groups). It goes from all the way to the left to further right.