- In an inorganic @[email protected], the atoms or groups joined to the @[email protected]
- In biochemistry: if it is possible or convenient to regard part of a polyatomic molecular entity as central, then the atoms, groups or molecules bound to that part are called ligands. Biochemical usage is thus wider, in that the central entity can be polyatomic. Thus H+ may be a ligand for @[email protected] and for citrate as well as for O2− . It may even be a ligand for a univalent entity such as acetate: in other circumstances, AcO− may be the ligand for H+, since the definition makes it clear that the view of which entity is central may change for convenience. Thus, four calcium ions are ligands for calmodulin, when the protein is regarded as central: four carboxylate groups of calmodulin @[email protected] (are ligands of) each calcium ion when this ion is regarded as central. It is the ligand that is said to @[email protected] the central entity, which is said to be ligated. When the hormone binding to a @[email protected] is called a ligand, the @[email protected] is thus regarded as the central entity. Biochemists should bear in mind that the usage in inorganic chemistry has been that ligands bind only single atoms, so they should be cautious in fields such as bioinorganic chemistry where confusion may be possible.