The Noncompetitive Inhibitor Ww781 Senses Changes in Erythrocyte Anion Exchanger (Ae1) Transport Site Conformation and Substrate Binding

WW781 binds reversibly to red blood cell AE1 and inhibits anion exchange by a two-step mechanism, in which an initial complex (complex 1) is rapidly formed, and then there is a slower equilibration to form a second complex (complex 2) with a lower free energy. According to the ping-pong kinetic model, AE1 can exist in forms with the anion transport site facing either inward or outward, and the transition between these forms is greatly facilitated by binding of a transportable substrate such as Cl⁻. Both the rapid initial binding of WW781 and the formation of complex 2 are strongly affected by the conformation of AE1, such that the forms with the transport site facing outward have higher affinity than those with the transport site facing inward. In addition, binding of Cl⁻ seems to raise the free energy of complex 2 relative to complex 1, thereby reducing the equilibrium binding affinity, but Cl⁻ does not compete directly with WW781. The WW781 binding site, therefore, reveals a part of the AE1 structure that is sensitive to Cl⁻ binding and to transport site orientation, in addition to the disulfonic stilbene binding site. The relationship of the inhibitory potency of WW781 under different conditions to the affinities for the different forms of AE1 provides information on the possible asymmetric distributions of unloaded and Cl⁻-loaded transport sites that are consistent with the ping-pong model, and supports the conclusion from flux and nuclear magnetic resonance data that both the unloaded and Cl⁻-loaded sites are very asymmetrically distributed, with far more sites facing the cytoplasm than the outside medium. This asymmetry, together with the ability of WW781 to recruit toward the forms with outward-facing sites, implies that WW781 may be useful for changing the conformation of AE1 in studies of structure–function relationships.