The definition of 'ligand' approved by chemists (see page 233 in ) is: "If it is possible to indicate a central atom in a polyatomic molecular entity, the atoms or groups bound to that atom are called ligands. The term is generally used in connection with metallic 'central' atoms."
The Nomenclature Committee of IUBMB believes that the wider biochemical usage is covered by changing the first sentence - italicizing the words that change the meaning - to read: "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 proteins 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 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 ligate (are ligands of) each calcium ion when this ion is regarded as central. It is the ligand that is said to ligate the central entity, which is said to be ligated.
When the hormone binding to a receptor is called a ligand, the receptor 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.
1. IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN), and Nomenclature Commission of IUBMB (NC-IUBMB),.Newsletter 1992, Arch. Biochem. Biophys., 1992, 294, 322-325; Biochem. Internat., 1992, 26, 567-575; Bioch. J., 1992, 282, I-III; Biol. Chem. Hoppe-Seyler, 1992, 373, 1-4; Chem. Int., 1992, 14, 140-141; Eur. J. Biochem., 1992, 204, 1-3.
2. International Union of Pure and Applied Chemistry (1987) Compendium of chemical terminology (Gold, V., Loening, K. L., McNaught, A. D. & Sehmi, P., eds) Blackwell Scientific Publications, Oxford.