The 10-state model is biased toward antiport when driven solely by a pH gradient. We varied the second protonation value, pK_a², to examine how this parameter affects transport in the 10-state model. Here, pK_a² = 4, blue; 5, orange (overlapped with blue); 7, yellow; 7.5, purple; 8, green; and 9, cyan. Unlike the 8-state model, where inverting the rates for an efficient antiporter can produce an equally efficient symporter, the 10-state model is asymmetric. The requirement that EmrE sequentially bind protons, requiring pK_a² ≤ pK_a¹, restricts the sample space (gray area excluded). Although the blue and orange traces (pK_a² = 4 and 5, respectively) retain the behavior of the 8-state model, symport is not observed for pK_a² ≥ 7.0 for any value of R_off, and the transporter approaches stoichiometric transport at high values of pK_a², reflecting a strong bias toward antiport for the 10-state model under these particular conditions and constraints.