Question

Suppose that a ligand A binds to a protein with a dissociation constant of 5 x 10–6 M, and ligand B binds to the same protein in the absence of ligand A with a dissociation constant of 2 x 10–7 M. When the protein is saturated with ligand A, however, the dissociation constant for ligand B is 2 x 10–5 M. Answer parts (a), (b), and (c). (a) Based on this information, complete the linked functions diagram (thermodynamic box) below.

(b) What is the factor, C, by which binding of one ligand changes the dissociation constant for the other ligand?
(c) What is the dissociation constant for ligand A when the protein is saturated with ligand B?

Answer 1

When one ligand binds to a protein, it affects the dissociation constant of the other ligand. The factor C can be calculated to represent the change in the dissociation constant. The dissociation constant for ligand A when the protein is saturated with ligand B can be determined using the same approach.

Step-by-step explanation:

The information provided suggests that ligand A and ligand B bind to the same protein, but their dissociation constants change depending on the concentration of the other ligand. Based on this, we can say that there is a linked function between the binding of one ligand and the dissociation constant of the other ligand. The thermodynamic box diagram can be completed by connecting the dissociation constants for ligands A and B with arrows going in both directions.

The factor, C, by which the binding of one ligand changes the dissociation constant for the other ligand can be calculated by taking the ratio of their dissociation constants when the protein is saturated with the other ligand. In this case, C = KB (protein saturated with A) / KB (protein not saturated with A).

To determine the dissociation constant for ligand A when the protein is saturated with ligand B, we need to use the same approach. Under these conditions, the dissociation constant for ligand A is KA (protein saturated with B).