Figure 7.

The pH profile and apparent affinity of MdtM for Na⁺ and K⁺. (A) The pH profile of MdtM-mediated Na⁺/H⁺ and K⁺/H⁺ antiport activity. Transporter activity at each pH value was calculated as described in Methods. (B) The concentration of Na⁺ and (C) K⁺ required for the half-maximal acridine orange fluorescence dequenching response was estimated from measurements of the antiport activity of wild-type recombinant MdtM as a function of cation concentration at the previously determined pH optimum for each antiport reaction (pH 9.25 for Na⁺/H⁺ exchange and pH 9.0 for K⁺/H⁺ exchange). The [Na⁺]_1/2 and [K⁺]_1/2 values are an indication of the affinity of MdtM for each cation. In each panel, the data represent the mean ± SD of three independent experiments.