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A ligand is a small signaling molecule that is involved in both inorganic and biochemical processes. In coordination chemistry, a ligand enables the formation of a coordination complex, or association of different molecules in solution. Biochemistry generally defines ligands as messenger molecules, such as hormones, substrates, or activation and inhibition factors.
In chemistry, ligands often involve the sharing of electron pairs to form the complexes. Many ligands contain extra lone pairs of electrons that they use to distribute among the other atoms in the complex. As a result, many of these are themselves Lewis bases, or electron pair donors.
Chemistry often classifies ligands based on ligand binding patterns, size, and electric charge. Denticity, a property of chemical ligands, describes the number of bond formations that occur between ligands and the other metals or molecules within a coordination complex. Different bond numbers will result in different overall three-dimensional complex structures. For example, ligands that can achieve four bonds will ultimately yield tetrahedral structures, while those that can only bond to one other molecule, monodentate ligands, can only form a linear structure. Generally, the stability of the complex is dependent on the bonds formed by a single ligand, which increases structure and bond rigidity.
Size and charge are also qualities that vary within ligand chemistry. Not only will they determine how many bonds can be formed with other atoms, but they also determine the type of atoms that will be brought into the coordination complex. Bulk and large size will also alter the angles at which ligands bind to the other atoms in a complex.
In biochemistry, ligands refer to signaling or labeling molecules that bind to certain sites on receptors, enzymes, or other proteins inside a cell. These range from hormones, which lead to signal transduction pathways and signal cascades within a cell, to basic substrates, which bind to enzymes and undergo a single series of chemical reactions. They are often described in terms of their binding affinity, or how strongly they attract and bind their target molecule.
Ligands can also act as labels for certain proteins in post-translational modification processes. They can activate or inhibit different proteins based on their binding states, target proteins to direct them to different regions inside the cell, or label proteins for degradation. In the case of ubiquitin, for example, proteins are tagged with three or four ubiquitin molecules, then other enzymes will bind and degrade them.