Drug identity can trigger a switch from efflux to influx. Four rate constants may vary depending on the identity of the transported drug (⁠$koffED$⁠, $koffEHD$⁠, $kAAED$⁠, and $kAAEHD$⁠). We varied these rate constants over a physiological range to explore whether different combinations of these parameters (to mimic transport of different drugs) can induce both symport (T_r < 1, pink) and antiport (T_r > 1, orange) of different drugs by the same transporter. (A) Using pK_a values estimated for WT EmrE (pK_a¹ = 8.2 and pK_a² = 7), antiport dominates the parameter space regardless of drug-dependent rate constants when transport is driven by a pH gradient alone. (B) By lowering the pK_a values (pK_a¹ = 7.0, pK_a² = 5.0) for proton binding by the transporter, different values of the drug-dependent rate constants can result in either symport or antiport, even though the transporter-specific parameters are held constant, reflecting the potential for a single transporter to perform both proton-coupled antiport and symport of different substrates.