The effect of ionic strength on the conformation and stability of S1 and S1-nucleotide-phosphate analog complexes in solution was studied. It was found that increasing concentration of KCI enhances the reactivity of Cys707 (SH1 thiol) and Lys84 (reactive lysyl residue) and the nucleotide-induced tryptophan fluorescence increment. In contrast, high KCI concentration lowers the structural differences between the intermediate states of ATP hydrolysis in the vicinity of Cys707, Trp510 and the active site, possibly by increasing the flexibility of the molecule. High concentrations of neutral salts inhibit both the formation and the dissociation of the M**.ADP.Pi analog S1.ADP.Vi complex. High ionic strength profoundly affects the structure of the stable S1.ADP.BeFx complex, by destabilizing the M*.ATP intermediate, which is the predominant form of the complex at low ionic strength, and shifting the equilibrium to favor the M**.ADP.Pi state. The M*.ATP intermediate is destabilized by perturbation of ionic interactions possibly by disruption of salt bridges. Two salt-bridge pairs, Glu501-Lys505 in the Switch II helix and Glu776-Lys84 connecting the catalytic domain to the lever arm, seem most appropriate to consider for participating in the ionic strength-induced transition of the open M*.ATP to the closed M**.ADP.Pi state of S1.
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