Consider a monomeric protein, P, that can bind to two different ligands, A and B. The binding sites for the two ligands overlap, such that the second ligand cannot bind to a protein molecule already complexed with the first ligand, and vice versa.

b) You have an experimental setup in which the PA complex generates a signal, but the PB complex does not. You decide to estimate the KD,A by finding the ligand concentration of A at which the signal is half of the value approached using extremely high ligand concentrations. After collecting the data, you realize that your sample of ligand A was contaminated with ligand B. The initial contamination ratio is b = [B]/[A]. You can assume there is a high concentration of A compared to B. Please derive an expression for [A]50% signal in terms of KD,A, KD,B, and b. Will you calculate an affinity for A that is stronger or weaker than the true A affinity?