Allosteric enzyme interactions. In many allosteric enzymes, the substrate-binding site and the modulator-binding site(s) are on different subunits: catalytic (C) and regulatory (R) subunits, respectively. Binding of a positive (stimulatory) modulator (M) to its specific site on the R subunit is communicated to the C subunit through a conformational change, which renders the C subunit active and capable of binding the substrate (S) with higher affinity. On dissociation of the modulator from the regulatory subunit, the enzyme reverts to its inactive or less-active form.