Figure 2.
A free-energy diagram presentation for an (A) allosteric system, (B) nonallosteric system, and (C) silent coupling system. ΔGa and ΔGx are the free energies associated with substrate (A) and effector (X) binding in the absence of other ligands. ΔGx/a is the free energy for the binding of X when A is present. ΔGa/x is the free energy for the binding of A when X is present. ΔGax is the free energy associated with allosteric coupling. This value is zero in both a nonallosteric system and a silent coupling system. Structural representations have been added in each of the three panels. As diagrammed, in both the allosteric and the silent coupling systems, binding of one ligand alters the second ligand-binding site. To see this figure in color, go online.