Two Proteins Bind to the Same Ligand, and Protein a Has

Multiple Choice

Two proteins bind to the same ligand, and protein A has half of its binding sites occupied when the ligand concentration is 0.5 mM, while protein B has half of its binding sites occupied when the ligand concentration is at 0.3 mM. Which protein binds the ligand more strongly, and what is the dissociation constant for that protein-ligand interaction?

A) Protein A binds more strongly, and K_d = 0.3 mM.
B) Protein A binds more strongly, and K_d = 0.5 mM.
C) Protein B binds more strongly, and K_d = 0.3 mM.
D) Protein B binds more strongly, and K_d = 0.5 mM.
E) Both proteins bind the ligand with the same affinity, and K_d = 0.4 mM.