Alternative cycling modes of the Na+/K+-ATPase in the presence of either Na+ or Rb+

► We present steady-state determinations of reaction intermediates and ATPase activity. ► We show occlusion of Rb+ (a K+ congener) during a cycling mode taking place at null [Na+]. ► Phosphoenzyme formed by ATP at null [Na+] differs from the one obtained with Na+. ► Binding of Na+ to transport sites during catalysis is not at random unless rapid equilibrium holds. ► We present a minimal kinetic model that reproduces the results.