Glossary of Pharmacology 

Inhibitory constant (K_i) 

The inhibitory constant (K_i) is a type of equilibrium dissociation constant (K_d) that represents the equilibrium binding affinity for a ligand that reduces the activity of its binding partner. K_i represents the concentration at which the inhibitor ligand occupies 50% of the receptor sites when no competing ligand is present. The smaller the K_i the greater the binding affinity and the smaller the amount of ligand is needed to inhibit its binding partners activity. 

Linked terms:  Equilibrium dissociation constant (K_d), ligand, binding affinity    

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