In biochemistry, a ligand is a small molecule or functional group bound to a surface or another larger macromolecule, like a protein, lipid, or carbohydrate, to serve a biological purpose. Ligands such as antibodies, bound to chromatography matrices, are used to attract target proteins. Ligands such as cofactors, bind to enzymes to help catalyze reactions. Enzyme substrates are also ligands that bind to the active site. Binding is seldom covalent but generally due to ionic, van der waals or hydrogen bonding forces, therefore the bond is usually reversible. This allows the use of ligands in affinity chromatography, one of the most popular protein purification methods in the biotech industry.
Upon binding of a ligand to an enzyme, the active site, or receptor, of the macromolecule is often altered in order to activate or deactivate a chemical reaction or chain of events. Changing the active sites of enzymes, through modification of the encoding gene sequences, is one approach to optimizing enzyme processes.