Properties of ATPase activity in coupling factor from Chromatium strain D chromatophores

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+ ions. Higher Vapp values were obtained when the activity was measured as a function of the divalent cation-ATP complex rather than as a function of either 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 values for free ATP indicating that the cation-ATP complex is bound tighter than the free ATP to the enzyme. Based on these results 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. However, possible conformation changes could have caused a decrease in the Km values for the (Ca-ATP)2- and (Mg-ATP)2- and in the Ki for free Mg2+ ions and ATP. The Ki for free Ca2+ ions increased on solubilization of the coupling factor. ATPase activity was inhibited by dicyclohexylcarbodiimide both in the soluble and in the membrane-bound coupling factor.