BIOMECHANICAL MODEL OF THE P-TYPE ION PUMPS OF THE CELL

The mechanisms of the Na/K pump and of the primary Ca pumps of the cel l have not yet been clarified. A biomechanical model of these so-calle d p-type ion pumps is proposed here. It is based on the assumption tha t the Na+ and Ca2+ ions are occluded by a contracting protein chain co operating with the ATPase section of the pump. After transfer of the c hain into the region of high Naf or Ca2+ concentration, the ions are r eleased through stretching of the chain by the ATPase. In the backward transfer of the chain, a retrograde transport of Na+ ions is prevente d through occlusion of K+ ions by another region of the same chain. In the case of Ca2+ ions, a similar effect is expected from hydrated Mg2 + ions. The two sections of the chain discriminate between the electri cal field strength at the surface and the polarizability of the ions. The most likely mechanism for the transfer of the ion-binding chain is considered to involve a thermally induced transition of a pump dimer between two almost equivalent stable orientations in the membrane.