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Any one of the special bacterial enzymes involved in regulatory functions. End-product inhibition is a bacterial control mechanism whereby the end product of a biosynthetic pathway can react with the first enzyme of the pathway and prevent its activity. This end-product inhibition is a device through which a cell conserves its economy by shutting off the synthesis of building blocks when too many are present.
A model to explain the action of allosteric enzymes suggests that the enzyme molecule is a complex consisting of identical subunits, each of which has a site for binding the substrate and another for binding the regulatory substance. These subunits interact in such a way that two conformational forms may develop. One form is in a relaxed condition (R state) and has affinity for the substrate and activator; the other form is in a constrained or taut condition (T state) and has affinity for the inhibitor. The forms exist in a state of equilibrium, but this balance can be readily tipped by binding one of the reactants. The substrate and activator are bound by the relaxed form; when this happens, the balance is tipped in favor of that state. Conversely, the inhibitor will throw the balance toward the constrained state. The balance is thus tipped one way or the other, depending on the relative concentrations of substrate and inhibitor. Since the two states require subunit interaction for their maintenance, it can be seen why dissociation of the subunits leads to a simple monomeric enzyme which no longer exhibits allosteric effects. The model also shows how the binding sites may interact in either a cooperative or antagonistic manner. See Enzyme