Question

A sample of cells with a total receptor concentration of 50 mM was incubated with a ligand concentration of 100 mM. Following the incubation period, the concentration of unbound receptors was determined to be 15 mM. What is the Kd for the receptor-ligand interaction?

A) 28 mM

B) 43 mM

C) 143 mM

D) 217 mM

E) 333 mM

Answer 1

Initial receptor concentration = 50 mM

Initial ligand concentration = 100 mM

Now, after incubation, the equilibrium receptor concentrations is 15 mM.

Now, we can write the reaction of formation of receptor-ligand complex as follows by denoting L as ligand , R as receptor and LR as receptor-ligand complex:

Hence, we can create the following ICE table as follows:

Initial, mM	100	50	0
Change, mM	-x	-x	+x
Equilibrium, mM	100-x	50-x	x

It is given that that the equilibrium receptor concentration is 15 mM.

Hence, using the ICE table we can write

Hence, the equilibrium concentrations are

Now, the dissociation constant, K_d is expressed as follows:

Hence, the correct choice is A) 28 mM.