Principal Features of Allosteric Regulation

Mechanistic studies and model considerations have revealed the following general 

Allosteric proteins are typically composed of two or more, often symmetric subunits. 

The subunits can exist in active and inactive forms, often termed the T-form and the R-form. The R-form ( relaxed ) is the relaxed, active state the T-form ( tense ) is the less active state. T and R forms are in equilibrium with each other. 

The ligands can bind to both the T and R forms. The two forms differ in their affinity to the ligand. Ligand binding can often be described by a hyperbolic binding curve (see Fig. 2.3). 

Effector molecules that function as activators bind preferentially to the R form of the enzyme and thereby stabilize it. Inhibitors bind preferentially to the T form. In the presence of an inhibitor, more molecules occur in the T form. In this form the enzyme possesses a lower affinity for the substrate and the enzyme is thus less active. 

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