Properties of ATPase Activity in Coupling Factor from Chromatium Strain D Chromatophores

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Abstract
Coupling factor extracted from chromatophores of the photosynthetic bacteria Chromatium strain D was partially purified. The enzyme catalyzed ATPase activity in the presence of Ca2+ and Mg2+. Higher Vapp [apparent velocity] values were obtained when the activity was measured as a function of the divalent cation-ATP complex rather than as a function of the divalent cation or ATP, because the free components competitively inhibited the activity in the presence of the cation-ATP complex. The Km values were lower than or equal to the Ki [inhibition constant] values for free ATP, indicating that the cation-ATP complex is bound tighter than the free ATP to the enzyme. A possible mode of binding of substrate to the active site of the enzyme was suggested. A comparative study indicated no changes in the temperature dependance of ATPase activity when the enzyme was solubilized. Possible conformation changes could cause a decreased in the Km values for the (Ca-ATP)2- and (Mg-ATP)2- and in the Ki for free Mg2+ and ATP. The Ki for free Ca2+ increased on solubilization of the coupling factor. ATPase activity was inhibited by dicyclohexylcarbodiimide in the soluble and the membrane-bound coupling factor.