Concerted transition 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... 

Assuming that the enzymes Ei and E2 obey the concerted transition model, the kinetic equation for the transformed concentrations of S, Pi and P2 is ... 

Cooperative substrate binding results in sigmoidal v versus [S] curves (Fig. 8.1). The Michaelis-Menten model is therefore not appMcable to cooperative enzymes. Two major equihbrium models have evolved to describe the catalytic behavior of cooperative enzymes the sequential interaction and concerted transition models. The reader should be aware that other models have also been developed, such as equilibrium association-dissociation models, as well as several kinetic models. These are not discussed in this chapter. 

Figure 8.5. Diagrammatic representation of the concerted transition model for a two-site cooperative enzyme.

Figure 8.6. Simplified version of the concerted transition model for a two-site cooperative enzyme. In this case the T state of the enzyme is assumed not to bind substrate.

Do we expect this model to be accurate for a dynamics dictated by Tsallis statistics A jump diffusion process that randomly samples the equilibrium canonical Tsallis distribution has been shown to lead to anomalous diffusion and Levy flights in the 5/3 < q < 3 regime. [3] Due to the delocalized nature of the equilibrium distributions, we might find that the microstates of our master equation are not well defined. Even at low temperatures, it may be difficult to identify distinct microstates of the system. The same delocalization can lead to large transition probabilities for states that are not adjacent ill configuration space. This would be a violation of the assumptions of the transition state theory - that once the system crosses the transition state from the reactant microstate it will be deactivated and equilibrated in the product state. Concerted transitions between spatially far-separated states may be common. This would lead to a highly connected master equation where each state is connected to a significant fraction of all other microstates of the system. [9, 10]... 

Theoretical studies on 12 4-substituted phenyl acetates concluded that, in contrast to the proposed interpretation based on 13C NMR chemical shifts and ground-state destabilization calculations, the electrophilicity of the C=0 group increases due to the effect promoted by the electron-withdrawing groups in these systems.11 A DFT investigation of the alkaline hydrolysis of 4-nitrophenyl acetate has shown that a model including seven water molecules proceeds via a concerted transition state.12... 

Calculations on the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism. The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged at a highly asynchronous concerted transition structure. 

Cycloreversion of 4-methoxystyrene from [Re(0)(OCH2CHPhO)-(Tp )] has been investigated and it has been proposed that the reaction proceeds via a significantly asynchronous but concerted transition state. Computation modeling studies as also a Hammett study on cycloreversions of substituted styrene from a series of different [Re(0)(Tp )(diolato)] complexes have also been reported.214... 

For the allosteric model based on a concerted transition between the two conformational states, as considered here in the model for glycolytic oscillations, application of definition (2.27) in the case where the two states of the enzyme differ only by the affinity toward the substrate (0 = 1) leads to expression (2.28) for the Hill coefficient related to saturation of the enzyme by the substrate (Goldbeter, 1976,1977) ... 

The concerted transition (CT) or symmetry model, a departure from prior models of cooperativity, accounted for allosterism but could not explain anticooperativity. This model is based on the following postulates ... 

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