Magnesium Ion-induced Changes in the Binding Mode of Adenylates to Chloroplast Coupling Factor 11

Abstract

The effect of Mg²⁺ on the binding of adenylates to isolated chloroplast coupling factor 1 (CF₁) was studied using CD spectrometry and ultrafiltration. At adenylate concentrations smaller than 100 μM, one mole of CF₁ binds three moles of ATP (or ADP) regardless of the presence of Mg²⁺. In the presence of Mg²⁺, the first two ATP's bind to CF₁ independently with the same binding constant of 2.5×10⁻¹ μM⁻¹, then the third ATP binds with a much higher affinity of 10 μM⁻¹. In the absence of Mg²⁺, the first ATP binds to CF₁ with a binding constant of 2.5×10⁻¹ μM⁻¹, then the other two ATP's bind less easily with the same binding constant of 4.0×10⁻² μM⁻¹. The binding mode of ADP to CF₁ is quite similar to that of ATP. In the presence of Mg²⁺, the binding constants of the first two ADP's are both 7.6×10⁻² μM⁻¹, that of the third ADP being 4.0 μM⁻¹. In the absence of Mg²⁺, the binding constant of the first ADP is 7.6×10⁻² μM⁻¹, the constants of the other two ADP's both being 4.0×10⁻² μM⁻¹. AMP caused a negligible change in CD.