Mass Action, Affinity and Dissociation

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  • Law of mass action:
  • When a drug (D) combines with a receptor (R), it does so at a rate which is dependent on the concentration of the drug and the concentration of the receptor
  • [D] + [R] <--> [DR]
  • k1 - the rate for association
  • k2 - the rate for dissociation
  • KD - dissociation constant = k1/k2 = [D][R]/[DR]
  • KA - association constant = k2/k1 = [DR]/[D][R]
  • Affinity:
  • How tightly a drug binds to its receptor
  • Measured using the dissociation constant KD
  • KD is the same as the concentration of drug when 50% of receptors are occupied
  • Higher KD = lower affinity of the drug