Problem:

1. What is the difference between K_d and Θ?

2. Which has the lower K_d: myoglobin or hemoglobin? Think about it in terms of function and answer.

3. When Θ = 40%, which has more O₂ bound: myoglobin or hemoglobin?

Answer:

1. K_d is the dissociation constant. For an association such as that of Mb and O₂, K_d = ([Mb] [pO₂])/[Mb O₂ complex], which means that it indicates the likelihood that a complex will dissociate. A small dissociation constant indicates a high affinity.
Θ is the % of binding sites occupied by a ligand such as O₂. It can be expressed as a function of K_d, where Θ = pO₂/(K_d + pO₂).
The advantage of expressing Θ this way is that [P] and [PL] do not have to be determined.

2. Myoglobin has a lower K_d. If hemoglobin had a lower K_d, it would be unlikely that O₂ would dissociate from hemoglobin and remain associated with myoglobin.

3. Hemoglobin would have more O₂ bound, because it has four binding sites per molecule.

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