Editing Allosteric regulation (section)

== Types ==

=== Homotropic ===

A homotropic allosteric modulator is a [[substrate (biochemistry)|substrate]] for its target [[protein]], as well as a regulatory molecule of the protein's activity.  It is typically an activator of the protein.<ref name="pmid24456211">{{cite journal | vauthors = Srinivasan B, Forouhar F, Shukla A, Sampangi C, Kulkarni S, Abashidze M, Seetharaman J, Lew S, Mao L, Acton TB, Xiao R, Everett JK, Montelione GT, Tong L, Balaram H | title = Allosteric regulation and substrate activation in cytosolic nucleotidase II from Legionella pneumophila | journal = The FEBS Journal | volume = 281 | issue = 6 | pages = 1613–1628 | date = March 2014 | pmid = 24456211 | pmc = 3982195 | doi = 10.1111/febs.12727 }}</ref> For example, O<sub>2</sub> and CO are homotropic allosteric modulators of hemoglobin. Likewise, in IMP/GMP specific 5' nucleotidase, binding of one GMP molecule to a single subunit of the tetrameric enzyme leads to increased affinity for GMP by the subsequent subunits as revealed by sigmoidal substrate versus velocity plots.<ref name="pmid24456211" />

=== Heterotropic ===

A heterotropic allosteric modulator is a regulatory molecule that is not the enzyme's substrate. It may be either an activator or an inhibitor of the enzyme. For example, H<sup>+</sup>, CO<sub>2</sub>, and [[2,3-bisphosphoglycerate]] are heterotropic allosteric modulators of hemoglobin.<ref>{{cite journal | vauthors = Edelstein SJ | title = Cooperative interactions of hemoglobin | journal = Annual Review of Biochemistry | volume = 44 | pages = 209–32 | year = 1975 | pmid = 237460 | doi = 10.1146/annurev.bi.44.070175.001233 }}</ref> Once again, in IMP/GMP specific 5' nucleotidase, binding of GTP molecule at the dimer interface in the tetrameric enzyme leads to increased affinity for substrate GMP at the active site indicating towards K-type heterotropic allosteric activation.<ref name="pmid24456211" />

As has been amply highlighted above, some allosteric proteins can be regulated by both their substrates and other molecules. Such proteins are capable of both homotropic and heterotropic interactions.<ref name="pmid24456211" />

=== {{anchor|Essential Activators}}Essential activators ===
Some allosteric activators are referred to as "essential", or "obligate" activators, in the sense that in their absence, the activity of their target enzyme activity is very low or negligible, as is the case with N-acetylglutamate's activity on carbamoyl phosphate synthetase I, for example.<ref>{{cite journal | vauthors = Shi D, Allewell NM, Tuchman M | title = The N-Acetylglutamate Synthase Family: Structures, Function and Mechanisms | journal = International Journal of Molecular Sciences | volume = 16 | issue = 6 | pages = 13004–22 | date = June 2015 | pmid = 26068232 | pmc = 4490483 | doi = 10.3390/ijms160613004 | doi-access = free }}</ref><ref>{{cite journal | vauthors = de Cima S, Polo LM, Díez-Fernández C, Martínez AI, Cervera J, Fita I, Rubio V | title = Structure of human carbamoyl phosphate synthetase: deciphering the on/off switch of human ureagenesis | journal = Scientific Reports | volume = 5 | issue = 1 | pages = 16950 | date = November 2015 | pmid = 26592762 | pmc = 4655335 | doi = 10.1038/srep16950 | bibcode = 2015NatSR...516950D }}</ref>