Concept #1: Protein-Ligand Fractional Saturation

Example #1: If an antibody binds to an antigen (ligand) with a Kd of 5 x 10-8 M, what concentration of antigen will θ = 0.2?

Practice: Which of the following statements about protein-ligand binding is correct?

Practice: Consider the following graph for parts A-C.

A) What is the protein-ligand dissociation constant (Kd) for protein X?

a)  2 µM.      b) 4 µM.      c) 6 µM.      d) 8 µM.

B) What is the protein-ligand dissociation constant (Kd) for protein Y?

a)  2 µM.      b) 4 µM.      c) 6 µM.     d) 8 µM.

C) Which protein has a greater affinity for ligand A?

a) Protein X.       b) Protein Y.

Practice: Match the dissociation constants in the table below to the appropriate curves on the graph.

Practice: Use the table below to answer questions A, B & C below.

A) Which protein has a greater affinity for their ligand?

B) According to the data in the table, what is the dissociation constant (Kd) for Protein 1? 

Protein 1 Kd = ________

C. According to the data in the table, what is the association constant (K a) for Protein 2? 

Protein 2 Ka = ________

Practice: A sample of cells has a total protein-receptor concentration of 10 mM. 25% of the protein-receptors are occupied with ligand when the concentration of free ligand is 15 mM. Calculate the Kd for the receptor-ligand interaction.