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Low Affinity Calcium Binding Sites of the Calcium Transport ATPase of Sarcoplasmic Reticulum Membranes

Hasselbach, Wilhelm, Koenig, Vera
Zeitschrift für Naturforschung 2014 v.35 no.11-12 pp. 1012-1018
Ca2-transporting ATPase, adenosinetriphosphatase, binding sites, calcium, heat, lipid content, lipids, magnesium, sarcoplasmic reticulum
Calcium binding sites having low affinity constants of < 10³ ᴍ⁻¹ were titrated in native sarcoplasmic reticulum vesicles as well as in lipid deprived membranes and in the isolated calcium transport ATPase. Short time calcium binding measurements and the determination of the calcium binding heat allow to distinguish low affinity calcium binding sites located on the external surface of the sarcoplasmic reticulum membranes from those present in the section of the transport molecule directed to the vesicular space. The same number of internal binding sites was found for preparations deprived of their lipid content as well as of preparations depleted of their lipids and of their accessorial proteins. Magnesium interferes with calcium binding to the external as well as to the internal low affinity calcium binding sites. The implications of the existence of the low affinity calcium binding sites in the internal section of the calcium transport ATPase are discussed.