Editing Cooperative binding (section)

=== Multi-site molecules ===
Although most proteins showing cooperative binding are multimeric complexes of homologous subunits, some proteins carry several binding sites for the same ligand on the same polypeptide. One such example is [[calmodulin]]. One molecule of calmodulin binds four calcium ions cooperatively.<ref name=Teo1973>{{cite journal | vauthors = Teo TS, Wang JH | title = Mechanism of activation of a cyclic adenosine 3':5'-monophosphate phosphodiesterase from bovine heart by calcium ions. Identification of the protein activator as a Ca2+ binding protein | journal = The Journal of Biological Chemistry | volume = 248 | issue = 17 | pages = 5950–5 | date = September 1973 | doi = 10.1016/S0021-9258(19)43493-5 | pmid = 4353626 | doi-access = free }}</ref> Its structure presents four [[EF hand|EF-hand domains]],<ref name=Babu1980>{{cite journal | vauthors = Babu YS, Sack JS, Greenhough TJ, Bugg CE, Means AR, Cook WJ | title = Three-dimensional structure of calmodulin | journal = Nature | volume = 315 | issue = 6014 | pages = 37–40 | year = 1985 | pmid = 3990807 | doi = 10.1038/315037a0 | bibcode = 1985Natur.315...37B | s2cid = 4316112 }}</ref> each one binding one calcium ion. The molecule does not display a square or tetrahedron structure, but is formed of two lobes, each carrying two EF-hand domains. [[File:CalmodulinConformation.png|thumb|right|Cartoon representation of the protein Calmodulin in its two conformation: "closed" on the left (derived from [[Protein Data Bank|PDB]] id: 1CFD) and "open" on the right (derived from [[Protein Data Bank|PDB]] id: 3CLN). The open conformation is represented bound with 4 calcium ions (orange spheres).]]