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Glossary of Pharmacology 

Equilibrium dissociation constant (Kd) 

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 AxBy   Ax + By and the equilibrium dissociation constant (Kd) is calculated as Kd =([Ax][By])/[AxBy]  where [Ax], [By], and [AxBy] 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, Kd 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 Kd can be calculated as Kd =([L][R])/[LR]. In the case of ligand-receptor complexes, Krepresents the ligand concentration and should be calculated when 50% of the receptors are bound to ligands. The smaller the Kd, 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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