The binding of protein A to two other proteins, B and C, has been studied with fluorescence anisotropy, and the results of the measurements have been plotted in the following schematic diagram. Given equal kₒn rate constants, which complex do you think has a longer half-life? What is the value of K?ₐ (the association constant) for this complex?
A) A-B; 10⁵ L/mol
B) A-B; 10-⁵ L/mol
C) A-C; 1.5 × 10⁵ L/mol
D) A-C; 6.7 × 10⁴ L/mol
E) A-C; 6.7 × 10-⁴ L/mol

Question

The binding of protein A to two other proteins, B and C, has been studied with fluorescence anisotropy, and the results of the measurements have been plotted in the following schematic diagram. Given equal kₒn rate constants, which complex do you think has a longer half-life? What is the value of K?ₐ (the association constant) for this complex? 
A) A-B; 10⁵ L/mol 
B) A-B; 10-⁵ L/mol 
C) A-C; 1.5 × 10⁵ L/mol 
D) A-C; 6.7 × 10⁴ L/mol 
E) A-C; 6.7 × 10-⁴ L/mol

Quizplus · Accepted Answer

The A-B complex formation has a lower Kd (10 µM) as shown by the curves. Accordingly, it should have a lower kₒff (given identical kₒn values), and therefore a longer half-life. The Kₐ is the inverse of Kd.

The Binding of Protein a to Two Other Proteins, B