<P> In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose . In protein - ligand binding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein . The binding typically results in a change of conformation of the target protein . In DNA - ligand binding studies, the ligand can be a small molecule, ion, or protein which binds to the DNA double helix . The relationship between ligand and binding partner is a function of charge, hydrophobicity, and molecular structure . The instance of binding occurs over an infinitesimal range of time and space, so the rate constant is usually a very small number . </P> <P> Binding occurs by intermolecular forces, such as ionic bonds, hydrogen bonds and Van der Waals forces . The association of docking is actually reversible through dissociation . Measurably irreversible covalent bonding between a ligand and target molecule is atypical in biological systems . In contrast to the definition of ligand in metalorganic and inorganic chemistry, in biochemistry it is ambiguous whether the ligand generally binds at a metal site, as is the case in hemoglobin . In general, the interpretation of ligand is contextual with regards to what sort of binding has been observed . The etymology stems from ligare, which means' to bind' . </P> <P> Ligand binding to a receptor protein alters the chemical conformation by affecting the three - dimensional shape orientation . The conformation of a receptor protein composes the functional state . Ligands include substrates, inhibitors, activators, and neurotransmitters . The rate of binding is called affinity, and this measurement typifies a tendency or strength of the effect . Binding affinity is actualized not only by host - guest interactions, but also by solvent effects that can play a dominant, steric role which drives non-covalent binding in solution . The solvent provides a chemical environment for the ligand and receptor to adapt, and thus accept or reject each other as partners . </P> <P> Radioligands are radioisotope labeled compounds used in vivo as tracers in PET studies and for in vitro binding studies . </P>

Nature of the binding between neurotransmitter and receptor site