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J. Biol. Chem., Vol. 277, Issue 8, 5910-5921, February 22, 2002

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The Occlusion of Rb⁺ in the Na⁺/K⁺-ATPase
I. THE IDENTITY OF OCCLUDED STATES FORMED BY THE PHYSIOLOGICAL OR THE DIRECT ROUTES: OCCLUSION/DEOCCLUSION KINETICS THROUGH THE DIRECT ROUTE^*

Rodolfo M. González-Lebrero^§, Sergio B. Kaufman, Mónica R. Montes^¶, Jens G. Nørby, Patricio J. Garrahan^**, and Rolando C. Rossi

From the  Instituto de Química y Fisicoquímica Biológicas and Departamento de Química Biológica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Junín 956, 1113 Buenos Aires, Argentina, and  Institute of Biophysics, University of Aarhus, Ole Worms Allé 185, DK-8000 Aarhus, Denmark

Occlusion of K⁺ or its congeners in the Na⁺/K⁺-ATPase occurs after K⁺-dependent dephosphorylation (physiological route) or in media lacking ATP and Na⁺ (direct route). The effects of P_i or ATP on the kinetics of deocclusion of the K⁺-congener Rb⁺ formed by each of the above mentioned routes was independent of the route of occlusion, which suggests that both routes lead to the same enzyme intermediate. The time course of occlusion via the direct route can be described by the sum of two exponential functions plus a small component of very high velocity. At equilibrium, occluded Rb⁺ is a hyperbolic function of free [Rb⁺] suggesting that the direct route results in enzyme states holding either one or two occluded Rb⁺. Release of occluded Rb⁺ follows the sum of two decreasing exponential functions of time, corresponding to two phases with similar sizes. These phases are not caused by independent physical compartments. The rate constant of one of the phases is reduced up to 30 times by free Rb⁺. When Rb⁺ is the only pump ligand, the kinetics of occlusion and deocclusion through the direct route are consistent with an ordered-sequential process with additional independent step(s) interposed between the uptake or the release of each occluded Rb⁺.

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