Mechanism of the conformational change in sodium pump reported by eosin

The conformational change in sodium pump reported by eosin has been reinvestigated because of apparent discrepancies between results obtained in other laboratories with this reversibly-bound, fluorescent reporter group and results obtained in our laboratory with covalently bound fluorescein. The study demonstrates that both probes report the same reaction. Data obtained by following the conformational change in unphosphorylated enzyme with either fluorescein or eosin can be superimposed and fit by an equivalent site mechanism [Smirnova, I. N., & Faller, L. D. (1995) Biochemistry 34, 8657-8667] with the same numerical values of the rate constants for the conformational change and of the dissociation constants for the transported ions. The result is synthesis of data from several laboratories obtained with different probes over more than a decade into a simple and quantitative interpretation of Na+ and K+ interaction with the enzyme and regulation of a protein conformational change that explains the stoichiometry of transport. Therefore, the study supports both the hypothesis that conformational changes function in transport and our proposal that the ion binding sites in one conformation of the enzyme are identical and independent. A corollary to be tested in the future is that ions are transported by a concerted mechanism.