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Abstract

This chapter discusses a few developments that led to the establishment of a concept concerning the energetics and the mechanism of the membranes calcium transporting system, the sarcoplasmic reticulum calcium pump. It has been postulated that there is an existence of adenosine triphosphate (ATP) driven active calcium transport in the muscle's microsomal membranes. This thesis was supported by the following criteria: (1) the observed calcium accumulation only occurs if ATP or other energy yielding substrates are split by the membranes and (2) calcium transport results in an enormous decrease of the product of the ion concentrations of calcium and oxalate or phosphate in the external solution as compared to the internal solution. The ion concentration ratios that are established across the membranes depend on the available free energy of ATP hydrolysis. The chapter discusses the calcium binding to the transport protein and its subunit structure, and phosphate transfer and calcium transport coupling. The assignment of phosphoryl transfer reaction and calcium translocation reduced from experiments near equilibrium are fully supported by the experiments in which calcium translocation transport and phosphoprotein formation are monitored during the reaction cycle unidirectionally driven by ATP. Thus, the coupling between calcium translocation and phosphoryl transfer remains invariant when the reaction conditions change from unidirectionality to equilibrium.