In
biochemistry, a ligand (latin ligare = to bind) is a
molecule that is able to bind to and form a
complex with a
biomolecule to serve a biological purpose. In a narrower sense, it is an
effector molecule binding to a
site on a target
protein, 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. As opposed to the meaning in
metalorganic and
inorganic chemistry, it is irrelevant, whether or not the ligand actually binds at a
metal site, as it is the case in
hemoglobin. Ligand binding to
receptors alters the
chemical conformation, i.e. the three dimensional shape of the receptor protein. The conformational state of a receptor protein determines the functional state of a receptor. The tendency or strength of binding is called affinity. Ligands include
substrates,
inhibitors,
activators, and
neurotransmitters.
Radioligands are
radioisotope labeled compounds and used
in vivo as
tracers in
PET studies and for
in vitro binding studies.
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