Mechanistic Descriptions of Allosteric Regulation

Various models have been offered to describe the mechanisms of allosteric regulation of enzyme activty, of which the symmetry model (Monod et al., 1965) is the simplest. The symmetry model has proven suitable in many cases to explain the experimentally observed characteristics of allosteric enzyme regulation. 

The symmetry model (fig. 2.4) of allostery can describe the cooperative binding of substrate to enzyme (homotropic effect), as well as the influence of effector molecules on the activity of enzymes (heterotropic effect). 

The model assumes the following with regard to the binding of a ligand (=substrate) to the protein  

Allosteric proteins are ohgomeric and composed of symmetric subunits. 

The subunits can exist in two forms, 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 equUibrium with each other, independent of whether the ligand is boimd or not. 

---