Glossary of Pharmacology 

Equilibrium dissociation constant (K_d) 

A measure of the tendency of a larger complex to separate (dissociate) into its smaller parts. For example, when a protein complex separates into its component proteins, or when a salt splits-up into its component ions. The dissociation reaction can be represented as A_xB_y ⇄  A_x + B_y and the equilibrium dissociation constant (K_d) is calculated as K_d=([A_x][B_y])/[A_xB_y]  where [A_x], [B_y], and [A_xB_y] are the concentrations of each component and the fully formed complex at equilibrium.  

In receptor pharmacology, the dissociation constant is commonly used to describe the affinity between a ligand and its receptor. Therefore, K_d can be used as a measure of binding affinity (how tightly a ligand binds to a receptor). The ligand-receptor binding reaction can be represented as L + R ⇄ LR and K_d can be calculated as K_d =([L][R])/[LR]. In the case of ligand-receptor complexes, K_d represents the ligand concentration and should be calculated when 50% of the receptors are bound to ligands. The smaller the K_d, the more tightly bound the ligand is and therefore the higher the affinity between the ligand and the receptor.   

Linked terms:  Binding affinity, receptor, ligand

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