FIGURE 5.

Sodium and potassium binding and conformational changes of the isoforms measured with RH421. A, equilibrium titration of sodium binding to the E₁ conformation. The inset shows a standard experiment and fluorescence changes associated with ion binding and release to α₂β₂ (sodium binding to E₁, sodium release and conformational transition to E₂P, and potassium binding). B, equilibrium titration of potassium binding to E₂P. C, stopped-flow trace of the E₁Na3 → E₂P transition of α₂β₂ fitted to a double exponential function. The average of 15 traces is shown. D, stopped-flow trace of the α₂β₂ E₂(Rb2)ATP → E₁Na3ATP transition fitted to a single exponential function.