Understanding the Energy Source for Na⁺-Ca²⁺ Exchange after Dephosphorylation Steps of the Na⁺-ATPase Activity of Na⁺, K⁺-ATPase

Abstract

Living cells maintain a steady different ionic compositions between the intra and extracellular compartments. This implies electrochemical gradients for many ionic species which are of large magnitudes; in turn, it requires mechanisms that keep a delicate balance between inward and outward ionic fluxes across the cellular membrane. Dissipating fluxes in favor of an electrochemical gradient are opposed by energy consuming fluxes of the same magnitude. In the case of the so called “pumps” the free energy comes directly from ATP hydrolysis. In the co- and countertransport systems the energy arises from the gradient dissipation of other ionic species (Ullricht, 1979; Tanford, 1983; Aroson, 1985). A paradigmatic case is given by Ca²⁺ ions. With an electronegative cell interior the intracellular Ca²⁺ concentration is about 10⁴ times smaller than that at the extracellular fluid. Two mechanisms work in parallel to account for this large electrochemical gradient. The Ca²⁺ pump and the Na⁺-Ca²⁺ exchange. The first takes the required energy from ATP. The second extrudes Ca²⁺ at the expenses of the free energy stored in the gradient of Na⁺ across the membrane. Actually, the Na⁺ gradient is as a generalized energy donor for co- and countertransport of solutes as it is the ATP for ionic pumps. Therefore, the knowledge of the mechanism for the Na⁺ electrochemical gradient generation is of paramount importance for all non-pumped energy requiring transports mechanism. This chapter deals with a detailed study of some partial reactions of the complex Na⁺-K⁺ transport cycle which structure, transport and biochemical identity is the Na⁺,K⁺ATPase (Glynn, 1985; Norby & Klodos, 1988; Froehlich & Fendler, 1991).