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 molecule, ion. 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 bonds, hydrogen bonds. 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 whether the ligand generally binds at a metal site. 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 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 an environment for the ligand and receptor to adapt. Radioligands are radioisotope labeled compounds are used in vivo as tracers in PET studies, the interaction of most ligands with their binding sites can be characterized in terms of a binding affinity. In general, high-affinity binding results in a degree of occupancy for the ligand at its receptor binding site than is the case for low-affinity binding. A ligand that can bind to a receptor, alter the function of the receptor, high-affinity ligand binding implies that a relatively low concentration of a ligand is adequate to maximally occupy a ligand-binding site and trigger a physiological response. The lower the Ki concentration is, the more likely there will be a reaction between the pending ion and the receptive antigen. In the example shown to the right, two different ligands bind to the receptor binding site. Only one of the agonists shown can maximally stimulate the receptor and, thus, an agonist that can only partially activate the physiological response is called a partial agonist
Two agonists with similar binding affinity
Two ligands with different receptor binding affinity.