Mass Action, Affinity and Dissociation

• 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
• K_D - dissociation constant = k₁/k₂ = [D][R]/[DR]
• K_A - association constant = k₂/k₁ = [DR]/[D][R]

• Affinity:

• How tightly a drug binds to its receptor
• Measured using the dissociation constant K_D
• K_D is the same as the concentration of drug when 50% of receptors are occupied
• Higher K_D = lower affinity of the drug