FIGURE 4.

Positive allosteric modulation of solute carrier function. (A) Reaction diagram. Displayed is the return step from the substrate-free IF to the substrate-free OF state. This is the rate-limiting reaction in the transport cycle of many, if not most, solute carriers. This reaction was assumed to be a partial reaction in a forwardly biased reaction loop (i.e., in the transport cycle). All other reactions in the loop were lumped together and are represented by the bold curved arrow. The red circle indicates the hypothetical allosteric modulator. The light blue circles indicate the corresponding binding site on the OF state and IF state of the transporter, respectively. In the assumed presence of the modulator the rate of the reaction increased threefold when α = 0.5 (i.e., from 2 s⁻¹ - black arrow - to 6 s⁻¹- red arrow) (B) Plotted is the fold increase (gain) in substrate uptake velocity, which is induced by the hypothetical allosteric modulator, as a function of α. The modulator was assumed to bind with a tenfold higher affinity to the OF state than to the IF state. The colored lines show the gain at the indicated concentrations of the hypothetical allosteric modulator. If α = 0, the modulator is ineffective. (C,D) The same type of plots as in (B) for a hypothetical allosteric modulator, which is 100-fold and 1000-fold more selective for the OF state, respectively. (E) Shown is the normalized increase in the substrate uptake rate as a function of the modulator concentration for three hypothetical modulators with variable selectivity. As seen, the EC₅₀ value for the allosteric effect shifts to the right as the modulator becomes more selective for the OF state. (F) Gain induced by hypothetical allosteric modulators with variable selectivity for the OF over the IF state (ranging from 10- to 10,000 fold) for α = 0.5. At 10 µM of the modulator, the more selective drugs became less efficient. (G) Gain as a function of the selectivity of the hypothetical allosteric modulators for a transporter operating at α = 1. At 10 µM of the modulator the relation between selectivity and the gain in substrate uptake is bell-shaped. (H) Effect of modulators on the transport cycle under the assumption that the slowest reaction in the transport cycle (i.e., 2 s⁻¹) is accelerated to an extent that another reaction becomes rate limiting (20 s⁻¹): the gain (in substrate uptake velocity) is plotted as a function of α for three hypothetical allosteric modulators with differing selectivity for the OF state (i.e., 10-, 100- and 1000-fold). In this instance, the attainable gain is limited, although the modulators are present at a saturating concentration.