Two Subunits Each Having One Site Allosteric Effect

TWO SUBUNITS EACH HAVING ONE SITE ALLOSTERIC EFFECT 

This model extends one aspect of the model treated in Section 3.2. The adsorbing polymers are still identical, independent, and for convenience also assumed to be localized. Each polymer consists of two subunits that are distinguishable, say the right- and left-hand ones. On each subunit there is one adsorption site, and each subunit can attain one of two conformational states, L and H. This is perhaps the simplest model that manifests the so-called allosteric effect. 

Originally the term allosteric was used for regulatory enzymes, where an effector binds to a site other than the active site and thereby changes the properties of the active site. Now it is also used whenever two ligands on two different sites of the same protein can communicate through the medium, which is the protein itself. Literally, the model of section 3.2 is also allosteric in the sense that adsorption on one site affects the adsorption on a different site. In the model treated in this section, the communication between the two sites is transmitted through the interaction energies between the two subunits. 

We shall examine in this section the details of the mechanism of the communication between the two ligands on the two subunits. 

---