Is the correct answer choice false in this question? Cooperative Binding

[BrownEMS]

1. The four subunits of hemoglobin exhibit cooperative binding. As pO2 of blood changes entering the pulmonary veins, the KM of hemoglobin towards this substrate:

• (A) Increases
• (B) Decreases
• (C) Remains Unchanged
• (D) Could be higher or lower, depending on the saturation

The answer is C. At first, this makes sense because changing the substrate concentration should not affect KM, however this fails to take into account the "right shift" that occurred at your tissues:

When blood reaches your tissues, the sigmoidal curve shifts to the right to allow oxygen to diffuse into cells. This means the KM was increased (less affinity for oxygen). Once blood reaches your lungs (and pO2 increases), you expect a left shift in the sigmoidal curve, thus KM was decreased from its previous value (higher O2 affinity). So wouldn't the answer be B.

---

Members don't see this ad.

 

[aldol16]

The writers of the question obviously didn't take into account the changing of Km due to the blood being in the lungs vs. the rest of the body. The question is written to suggest that the writer is asking whether Km changes as a result of the pO2 changing.

 

[andelJ94]

The writers of the question obviously didn't take into account the changing of Km due to the blood being in the lungs vs. the rest of the body. The question is written to suggest that the writer is asking whether Km changes as a result of the pO2 changing.

Can Km change as a result of binding? Is it not the other way round? Thanks!

 

[aldol16]

Can Km change as a result of binding? Is it not the other way round? Thanks!

This is what the question is asking you. Km does not change as a result of substrate binding. It can change if an allosteric inhibitor binds. But not with substrate binding.