Learning Goal:

Learn how changes in binding free energy affect binding and the ratio of unbound and bound molecules. Similar to the lecture, our example is a drug molecule D and a drug target enzyme E. The drug target enzyme E is commonly found in concentrations [E]_tot of 250 nM, i.e. 2.5 x 10^-7 mol/l. The drug D binds to E to form the complex ED with a dissociation constant of KD = 1.30 M (i.e. 1.30 x 10^-6). The dissociation constant has been determined at room temperature, i.e. for T = 298.15 K.

B.) Which total concentration of the drug D, [D]_tot, is needed to bind 50% of the drug target enzyme E into the complex ED? Provide your answer in units of mol/l and with 3 significant figures.