Allosteric effect concerted model

In the context of the Monod-Wyman-Changeux concerted-transition model for allosteric effects, one usually considers the effects of specific site occupancy on the behavior of other binding sites. Thus, a more correct... 

Two theoretical models for allosteric effects have been proposed to explain the mechanism for ligand-protein cooperative interactions the concerted (or symmetry) model of Monod, Wyman, and Changeux and the sequentially induced-fit model of Koshland. The nomenclature associated with allosterism and cooperativity originated from the concerted model. Both models assume that... 

According to the concerted model, an allosteric activator shifts the conformational equilibrium of all subunits toward the R state, whereas an allosteric inhibitor shifts it toward the T state. Thus, ATP (an allosteric activator) shifted the equilibrium to the R form, resulting in an absorption change similar to that obtained when substrate is bound. CTP had a different effect. Hence, this allosteric inhibitor shifted the equilibrium to theT form. Thus, the concerted model accounts for the ATP-induced and CTP-induced (heterotropic), as well as for the substrate-induced (homotropic), allosteric interactions of ATGase. 

In the concerted model, the effects of inhibitors and activators can also be considered in terms of shifting the equilibrium between the T and R forms of the enzyme. The binding of inhibitors to allosteric enzymes is cooperative allosteric inhibitors bind to and stabilize the T form of the enzyme. The binding of activators to allosteric enzymes is also cooperative allosteric activators bind to and stabilize the R form of the enzyme. When an activator, A, is present, the cooperative binding of A shifts the equilibrium between the T and R forms, with the R form favored (Figure 7.6). As a result, there is less need for substrate, S, to shift the equilibrium in favor of the R form, and less cooperativity in the binding of S is seen. 

Fig. 2. Simple two state allosteric model for qE in which the switch between quenched and unquenched conformation is driven by the effect of protonation and de-epoxidation. In this model LHCll can exist in two states, an unquenched conformation and a quenched conformation. Because the affinity of proton and zeaxanthin binding is greater in the quenched state, the ApH and de-epoxidation state will determine the equilibrium between these states. The existence of co-operativity indicates that these two states consist of a group of LHCll subunits which interact and change conformation in concert. The changed conformation, and intersubunit interaction, may give rise to quenching process itself In this model one or moreofthe different LHCll components may be involved.

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