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 V_app 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 K_m values were lower than or equal to the K_i 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 K_m values for the (Ca‐ATP)²⁻ and (Mg‐ATP)²⁻ and in the K_i for free Mg²⁺ ions and ATP. The K_i 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.