Question

A certain protein can bind one of two ligands: ligand A or ligand B. below is a plot of the fractional saturation (Y) versus ligand for ligand A and ligand B. How do I calculate approximate Kd? for example if the ligand is 200 uM and the fractional saturation is 65%?

Answer 1

Given, [L] = 200 M = 200 x 10^-6 M   , fractional saturation = 65 % = 65 /100 = 0.65

We have the relation as--

fractional saturation = [L] / ([L] + K_d)

=> 0.65 = 200 x 10^-6 / ( 200 x 10^-6 + K_d)

=> 0.65 * ( 200 x 10^-6 + K_d) = 200 x 10^-6

=> 130 x 10^-6 + 0.65 K_d = 200 x 10^-6

=> 0.65 K_d =( 200 x 10^-6 ) - (130 x 10^-6)

=> 0.65 K_d = 10^-6(200 - 130)

=> 0.65 K_d = 70 x 10^-6

=> K_d = 70 x 10^-6 / 0.65

=> K_d = 107.69 x 10^-6

=> K_d = 1.07 x 10^-4