Ligand (biochemistry)

The ligand is a substance referred to in biochemistry and related sciences, which can bind to a target protein, such as a receptor. The binding of the ligand is usually reversible, and it is possible in particular by ionic bonds, hydrogen bonds, van der Waals forces and hydrophobic effects. The affinity of a ligand to the target protein can be determined with the aid of ligand binding assays.


Ligands are not enforced by the target molecule in contrast to substrates, but can affect its three-dimensional structure and its function. Ligands that activate a receptor are called agonists, whereas ligands, antagonists will inhibit or deactivate a receptor but do not initiate pharmaceutical effect even called. Ligands, which lead to a receptor to an opposite effect, are called inverse agonists. With respect to the binding site on the target protein, a distinction orthosteric of allosteric ligands. Concerted act co- ligands. A high-affinity binding follows the key - lock principle.

The following are the types of chemical bonding:

  • Noncovalent, reversible: most common type of binding, determined by the association and dissociation
  • Noncovalently, pseudoirreversibel: very slow dissociation; Ex methyllycaconitine
  • Covalent, irreversible: the ligand contains a reactive functional group; Ex MAO inhibitor such as tranylcypromine. Typically, the target protein loses its function
  • Covalent, reversible: eg a series of tear gases; Here binding of an electrophile by reaction with the thiol group of cysteine ​​in TRPA1, the delivery is done by elimination reaction and leaves the target protein functional.
  • Covalently fragmented: a rare special case. The ligand binds to the target protein via a reaction center of the ligand covalently and eliminated by a different reaction center ( " perforation " ) a part of the molecule, which then dissociates. This has been experimentally realized for a DAT ligand covalently bound in the DAT a small molecule fragment leaves according to this principle. Bulkier molecules like cocaine, access to the DAT is blocked, while smaller molecules such as dopamine by DAT continues to be recorded.