Editing Cooperativity (section)

==Cooperative binding==
{{Main article|Cooperative binding}}
When a substrate binds to one enzymatic subunit, the rest of the subunits are stimulated and become active.  
[[Ligands]] can either have positive cooperativity, negative cooperativity, or non-cooperativity.

[[File:Hemoglobin saturation curve.svg|thumb|The sigmoidal shape of hemoglobin's oxygen-dissociation curve results from cooperative binding of oxygen to hemoglobin.]]
An example of positive cooperativity is the binding of [[oxygen]] to [[hemoglobin]]. One [[oxygen]] molecule can bind to the [[ferrous iron]] of a heme molecule in each of the four chains of a [[hemoglobin]] molecule. Deoxy-hemoglobin has a relatively low affinity for [[oxygen]], but when one molecule binds to a single heme, the [[oxygen]] affinity increases, allowing the second molecule to bind more easily, and the third and fourth even more easily. The [[oxygen]] affinity of 3-oxy-hemoglobin is ~300 times greater than that of deoxy-hemoglobin. This behavior leads the affinity curve of [[hemoglobin]] to be [[sigmoid function|sigmoidal]], rather than [[hyperbolic function|hyperbolic]] as with the monomeric [[myoglobin]]. By the same process, the ability for [[hemoglobin]] to lose [[oxygen]] increases as fewer oxygen molecules are bound.<ref name="Whitford_2005" /> ''See also [[Oxygen-hemoglobin dissociation curve]].

Negative cooperativity means that the opposite will be true; as [[ligand]]s bind to the [[protein]], the [[protein]]'s affinity for the ligand will decrease, i.e. it becomes less likely for the ligand to bind to the protein. An example of this occurring is the relationship between [[glyceraldehyde-3-phosphate]] and the [[enzyme]] glyceraldehyde-3-phosphate dehydrogenase.

[[Allosteric regulation#Types of allosteric regulation|Homotropic]] cooperativity refers to the fact that the molecule causing the cooperativity is the one that will be affected by it. [[Allosteric regulation#Types of allosteric regulation|Heterotropic]] cooperativity is where a third party substance causes the change in affinity. Homotropic or heterotropic cooperativity could be of both positives as well as negative types depend upon whether it support or oppose further binding of the ligand molecules to the enzymes.<ref>{{cite journal | vauthors = Hussain R, Kumari I, Sharma S, Ahmed M, Khan TA, Akhter Y | title = Catalytic diversity and homotropic allostery of two Cytochrome P450 monooxygenase like proteins from Trichoderma brevicompactum | journal = Journal of Biological Inorganic Chemistry | volume = 22 | issue = 8 | pages = 1197–1209 | date = December 2017 | pmid = 29018974 | doi = 10.1007/s00775-017-1496-6 | s2cid = 25685603 }}</ref>