Figure 11
A protein (triangle) with three ligand binding sites (A, B, and C) illustrates a possible subsaturating effect. Qac is set to equal 1, so that there is no direct allosteric coupling between the binding of A and the binding of C. If Qab and Qbc are both activating, then at subsaturating concentrations of B (i.e., partial occupancy of the B site), the binding of C will increase the occupancy of B, which in turn will result in allosteric activation of A binding. If Qab and Qbc are both inhibiting, then at subsaturating concentrations of B (i.e., partial occupancy of the B site), the binding of C will remove bound B, which in turn, removes allosteric inhibition of A binding. Thus, in both scenarios, at subsaturating concentrations of B, increases in the concentration of C appear to allosterically activate the binding of A, despite that Qac is equal to 1. No allosteric responses of A binding to the presence of C are expected when B is absent or at a concentration sufficiently high to saturate the B binding site. Therefore, across a concentration range of B, the influence of C on A binding would go from no influence at low [B], to activation at intermediate [B], to no influence at high [B]49,50,51.
