Are the states that occlude rubidium obligatory intermediates of the Na/K-ATPase reaction?

KAUFMAN SB, GONZALEZ LEBRERO R, SCHWARZBAUM PJ, NOERBY JG, GARRAHAN PJ, ROSSI RC

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Año: 1999 p. 20779 - 20790

0021-9258

In the Albers-Post model, occlusion of K1 in the E2conformer of the enzyme (E) is an obligatory step ofNa1/K1-ATPase reaction. If this were so the ratio (Na1/K1-ATPase activity)/(concentration of occluded species)should be equal to the rate constant for deocclusion. Wetested this prediction in a partially purified Na1/K1-ATPase from pig kidney by means of rapid filtration tomeasure the occlusion using the K1 congener Rb1. Assumingthat always two Rb1 are occluded per enzyme,the steady-state levels of occluded forms and the kineticsof deocclusion were adequately described by the Albers-Post model over a very wide range of [ATP] and[Rb1]. The same happened with the kinetics of ATP hydrolysis.However, the value of the parameters that gavebest fit differed from those for occlusion in such a waythat the ratio (Na1/K1-ATPase activity)/(concentrationof occluded species) became much larger than the rateconstant for deocclusion when [Rb1] <10 mM. Thispoints to the presence of an extra ATP hydrolysis that isnot Na1-ATPase activity and that does not involve occlusion.A possible way of explaining this is to posit thatthe binding of a single Rb1 increases ATP hydrolysiswithout occlusion.