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Second Order Binding (of two things)


  • A physical interaction between molecules A and B.
  • A system that contains some initial concentration of molecules A and B (A0 and B0, respectively).


  • Compute the steady state concentrations of free A, free B, and the A:B complex.


  • Write differential equation for change in A:B over time.
  • Solve equation at steady state (that is, no change in concentration of the A:B complex.
0 = + kon * [A] * [B] − koff * [A:B]
  • Solve for KD, the dissociation constant.
Equation 1: K_D = k_{off}/k_{on} = \frac{[A][B]}{[A:B]}
  • Note constraints on system due to conservation of mass.
Equation 2: [A0] = [A] + [A:B]
Equation 3: [B0] = [B] + [A:B]
  • Note system of three unknowns with three equations (1-3 above)! Solve for unknowns A, B, and A:B (takes you through a quadratic).
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