Figure 4.

(A) A representative whole-cell current trace generated by the laser-pulse photolysis measurement with an HEK-293 cell expressing the wild-type hGluK2. A pulse of laser was fired at time zero. The concentration of glutamate released was estimated to be 200 μM. The k_obs was calculated from the rising phase of the current to be 900 ± 12 s⁻¹ by using eq 1 (the solid line). Note that the current is plotted opposite to the direction that is recorded. (B) Linear plot of the k_obs data combined from the wild-type and the M867I mutant hGluK2 as a function of glutamate concentration according to eq 2. Each data point represents a single k_obs value obtained at a particular concentration of photolytically released glutamate. Overall, a total of 29 and 36 cells were measured for the wild-type (black circle) and the mutant hGluK2 (red circle). The best fit of k_obs was at n = 2, and the k_op and k_cl values were (1.9 ± 0.1) × 10³ s⁻¹ and (3.3 ± 0.2) × 10² s⁻¹, respectively. See Table 3 and Tables S2 and S4 for a detailed description of nonlinear fits. (C) Linear plot of k_obs as a function of glutamate concentration by eq 2, based on a total of 26 cell measurements, for the rGluK2. The best fitted k_op and k_cl values were (6.4 ± 0.1) × 10³ s⁻¹ and (3.9 ± 0.3) × 10² s⁻¹, respectively, at n = 2. See Table 4 and Tables S3 and S5 for a detailed description of nonlinear fits.