<P> A morpheein is a homo - oligomeric structure that can exist as an ensemble of physiologically significant and functionally different alternate quaternary assemblies . Transitions between alternate morpheein assemblies involve oligomer dissociation, conformational change in the dissociated state, and reassembly to a different oligomer . The required oligomer disassembly step differentiates the morpheein model for allosteric regulation from the classic MWC and KNF models . </P> <P> Porphobilinogen synthase (PBGS) is the prototype morpheein . </P> <P> Ensemble models of allosteric regulation enumerate an allosteric system's statistical ensemble as a function of its potential energy function, and then relate specific statistical measurements of allostery to specific energy terms in the energy function (such as an intermolecular salt bridge between two domains). Ensemble models like the ensemble allosteric model and allosteric Ising model assume that each domain of the system can adopt two states similar to the MWC model . The allostery landscape model introduced by Cuendet, Weinstein, and LeVine allows for the domains to have any number of states and the contribution of a specific molecular interaction to a given allosteric coupling can be estimated using a rigorous set of rules . Molecular dynamics simulations can be used to estimate a system's statistical ensemble so that it can be analyzed with the allostery landscape model . </P> <P> Allosteric modulation is used to alter the activity of molecules and enzymes in biochemistry and pharmacology . For comparison, a typical drug is made to bind to the active site of an enzyme which thus prohibits binding of a substrate to that enzyme causing a decrease in enzyme activity . Allosteric modulation occurs when an effector binds to an allosteric site (also known as a regulatory site) of an enzyme and alters the enzyme activity . Allosteric modulators are designed to fit the allosteric site to cause a conformational change of the enzyme, in particular a change in the shape of the active site, which then causes a change in its activity . In contrast to typical drugs, modulators are not competitive inhibitors . They can be positive (activating) causing an increase of the enzyme activity or negative (inhibiting) causing a decrease of the enzyme activity . The use of allosteric modulation allows the control of the effects of specific enzyme activities; as a result, allosteric modulators are very effective in pharmacology . </P>

What does it mean when an enzyme has an allosteric activator