Role of metal ions in coupling between chemical catalysis and conformational changes in ATP synthase

Isolated chloroplast ATP synthase (CF₀F₁) was used for determination of the structure-function relation by measuring the effect of divalent metal ions on the properties of ATPase. Mg²⁺ ions were more efficient catalysts than Ca²⁺ ions as indicated by K_cat/K_m of 55.2 and 5.4, respectively. Other activity parameters related to binding, such as the K_m of MATP and K_i of MADP, indicated a stronger binding in the presence of Mg²⁺ as seen from a Mg²⁺/Ca²⁺ ratio of 2.8 and 3.8, respectively. Strong binding of Ca²⁺ ions with a K_d of 0.03 ± 00.6 μM^-1 was detected only in the presence of ADP probably because of the positive interactive effect of CaADP as indicated in the inhibition properties. Mg²⁺ions were more efficient catalysts also in other forms of the enzyme such as in the thylakoid membrane, in isolated CF₀F₁ and in CF₁. The Mg²⁺/Ca²⁺ ratio of K_cat/K_m was 5.3, 10.2 and 1.5 for the thylakoid membrane enzyme, the isolated CF₀F₁ and the soluble CF₁ respectively. This indicated that Ca²⁺ ions became less efficient catalysts in the more intact and integrated enzyme while Mg²⁺ ions were as efficient in all forms of the enzyme. Unlike Mg²⁺, Ca²⁺ ions also did not support proton-coupled ATP synthesis and ATP driven proton pumping. It is suggested that the differences in the ligand structure of these two ions might be the reason for the differential function. An average 0.3Å shorter bond length of octahedral first coordination in Ca²⁺ ions caused a weaker binding of CaATP than that of MgATP. The effect of differential binding is discussed in relation to the binding of the transition state intermediate and to the rate of product release.