Ligand (biochemistry)

Ligand (biochemistry)

In biochemistry and pharmacology, a ligand (from the Latin ligandum, binding) is a substance (usually a small molecule), that forms a complex with a biomolecule to serve a biological purpose. In a narrower sense, it is a signal triggering molecule, binding to a site on a target protein.

The binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and van der Waals forces. The docking (association) is usually reversible (dissociation). Actual irreversible covalent binding between a ligand and its target molecule is rare in biological systems. In contrast to the meaning in metalorganic and inorganic chemistry, it is irrelevant whether the ligand actually binds at a metal site, as is the case in hemoglobin.

Ligand binding to a receptor (receptor protein) alters its chemical conformation (three dimensional shape). The conformational state of a receptor protein determines its functional state. Ligands include substrates, inhibitors, activators, and neurotransmitters. The tendency or strength of binding is called affinity.

Radioligands are radioisotope labeled compounds and used in vivo as tracers in PET studies and for in vitro binding studies.