Definition
Binding affinity describes how tightly a ligand binds its target. It is usually expressed as the equilibrium dissociation constant, Kd, the concentration at which half the receptors are occupied at equilibrium; a lower Kd means tighter binding. In competition assays, a test compound displaces a labelled reference ligand, and its IC50 is converted to an inhibition constant, Ki, using the Cheng–Prusoff equation. Methods include radioligand binding, surface plasmon resonance, which also yields association and dissociation rates, and isothermal titration calorimetry. Affinity is not the same as functional potency or efficacy: a ligand can bind tightly but act as an antagonist. Peptide modifications such as cyclisation and lipidation can change affinity.