TPR- Demo test Bio Qn 50

[bellowbruins]

Which of the following could affect any of the dissociation constants for hemoglobin?

1. Decreasing the temperature
2. Adding NH3
3. Increasing the [O2]

A. I only

B. III only

C. I and II only Correct Answer

D. I, II, and III. Your Answer

Hi can someone explain me why number 3 is not taken into account? Thanks

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[deleted853480]

3 does affect the dissociation Constant. Increasing O2 cause the sigmodal curve to shift rightward via coop binding. Tpr online tests are NOT good!!

 

[NarutoMD]

Cooperative binding just means that hemoglobin affinity for o2 increases with subsequent binding of O2, hence the sigmoidal curve. Factors that decrease hemoglobin's affinity for O2 are products of cellular respiration (low pH, co2, etc) and 2,3bp, thereby allowing release of O2 to tissues. This explains why I and II are correct.

 

[aldol16]

3 does affect the dissociation Constant. Increasing O2 cause the sigmodal curve to shift rightward via coop binding. Tpr online tests are NOT good!!

No it doesn't. The sigmoidal curve doesn't shift. What you're doing when you increase [O2] is moving along the curve - that's why the x-axis is pO2. That's not the same as shifting it.

 

[aldol16]

Hi can someone explain me why number 3 is not taken into account? Thanks

Increasing the pO2 increases the affinity of hemoglobin for O2 via cooperative binding, but it doesn't affect the Kd. You might be tempted to think that Kd is affected because of cooperativity. However, remember that Kd, when you plot binding against pO2, is the pO2 at which half the sites in hemoglobin are occupied. Increasing pO2 would help fill up the hemoglobin sites but it wouldn't affect the dissociation constant itself. In other words, you're only moving along the sigmoid curve.

 

[bellowbruins]

Increasing the pO2 increases the affinity of hemoglobin for O2 via cooperative binding, but it doesn't affect the Kd. You might be tempted to think that Kd is affected because of cooperativity. However, remember that Kd, when you plot binding against pO2, is the pO2 at which half the sites in hemoglobin are occupied. Increasing pO2 would help fill up the hemoglobin sites but it wouldn't affect the dissociation constant itself. In other words, you're only moving along the sigmoid curve.

Please correct me if I am wrong. Here is how I reasoned and thought:

There's a lot of O2 outside and thus, hemoglobin would more likley to bind more than dissociate oxygen. Thus, the curve will shift to the left? and vice versa?

 

[aldol16]

There's a lot of O2 outside and thus, hemoglobin would more likley to bind more than dissociate oxygen. Thus, the curve will shift to the left? and vice versa?

No, there is no shifting of the curve. Do you understand the difference between moving along a curve and shifting the curve? In a y vs. x graph, when you change x, you're moving along the line. You're not shifting the line anywhere. For example, take the simple case of y = x. For x = 1, y = 1. Now when x = 2, what does y equal? y = 2. Are you still on the y = x line? Yes. You've changed x but you've just moved along the line. This is different from a shift in the line, which would be akin to changing the equation to y = x + 1.

So in this case, y is percent hemoglobin bound and x is pO2. As you change pO2 (amount of O2), you move along the curve. You're on the same curve - it's not shifted.