Concerted Transition or Symmetry Model

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  

Allosteric enzymes are composed of identical protomers that occupy equivalent positions within the enzyme. A protomer is a structural unit that contains a unique binding site for each specific ligand (e.g., substrate and activator). A protomer does not necessarily correspond to one subunit (a single polypeptide chain). 

Each protomer can only exist in either of two conformational states, R (relaxed, or high substrate binding affinity) or T (taut, or low substrate binding aftinity). The dissociation constant for the R-state protomer-substrate complexes, A r, is lower than that of the T-state protomer-substrate complexes, kj (Fig. 8.5). 

All protomers within the enzyme must be in either the R or T state—mixed conformations are not allowed. The R and T states of the enzyme are in equilibrium with each other. Thus, an equilibrium constant (L) can be written for the R T transition (L = [T]/[R]). 

The binding affinity of a specific ligand depends on the conformation of the enzyme (R or T), and not on neighboring site occupancy. 

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