Figure 4.

Kinetic traces of Na⁺ binding to human meizoII_RRΔF1 in the 0-100 ms time scale. Shown are the traces obtained at 0, 6.25, 25 and 100 mM Na⁺, as indicated. The binding of Na⁺ obeys a two-step mechanism, with a fast phase completed within the dead time (< 0.5 ms) of the spectrometer that is evident only at high [Na⁺ ], followed by a single-exponential slow phase. The k_obs for the slow phase decreases hyperbolically with increasing [Na⁺ ] (see also Fig. 5). Experimental conditions are: 50 nM meizoII_RRΔF1, 5 mM Tris, 0.1 % PEG, pH 8.0 at 15 °C. The [Na⁺ ] was changed by keeping the ionic strength constant at 400 mM with ChCl. Continuous lines were drawn using the expression a exp (-k_obs t) + b with best-fit parameter values: (0 mM) a = 0±0 V, k_obs = 0±0 s^-1, b = 8.11±0.01 V; (6.25 mM) a = -0.05±0.01 V, k_obs = 276±9 s^-1, b = 8.20±0.01 V; (25 mM) a = -0.18±0.02 V, k_obs = 241±8 s^-1, b = 8.44±0.01 V; (100 mM) a = -0.23±0.01 V, k_obs = 191±8 s^-1, b = 8.50±0.01 V.