Un ligando (del Latín ligandum, ligando) es una sustancia (usualmente una molécula pequeña) que forma un complejo con una biomolécula. En un sentido más estricto, es una molécula que envía una señal al unirse al centro activo de una proteína. En enzimas o proteínas no enzimáticas, especialmente de proteínas reguladoras o transportadoras, se denomina ligando a aquella molécula que se une al centro activo de la proteína para que ésta pueda realizar su función (transportar, o inhibir una reacción metabólica). Por ejemplo, el ligando de la hemoglobina es el oxígeno, ya que es la molécula que se une al centro activo de la proteína para que ésta pueda transportarlo. La unión ligando–proteína provoca un cambio conformacional en la proteína, muy importante para determinar la afinidad de una proteína con un sustrato.

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

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