Scheme of OAT-mediated uptake (bold arrows indicate inwardly directed organic anion transport). Step 1, binding of the substrate anion from the extracellular medium to the binding site of the transporter in outside-open conformation (Co). Step 2, switch of the substrate-transporter complex to inside-open conformation (Co→Ci). Step 3, substrate dissociation into the intracellular medium. Step 4, switch of “empty” transporter back to the Co conformation. Steps 1 and 2 are assumed to determine, respectively, substrate potency (Km) and efficacy (Vmax).