sentence: The reason the presence of an anion in one position ( at S ) can have the same facilitating effect on PT in opposite directions is due to the rotation of E148 and steric competition between Cl and E148 for S As discussed earlier , the down conformation of E148 is energetically favored in the absence of Cl , but the down rotation of E148 is sterically blocked by the presence of Cl , minimizing the cost of E148 rotation from the pore-facing ( “ down ” ) to the extracellular-facing ( “ up ” ) conformation.The effective rate constant , k , was calculated to obtain the rate constant of the two-step ( rotation and deprotonation ) process : k = k k / k , assuming that the first step quickly reaches quasiequilibrium compared to the second ( k ≫ k ) , where k and k are the forward and the backward rate constants for the first step in the PMF , and k is for the forward rate constant for the second step ( Figure 5 ) .