| Abstract: | The kinetics of reversible inactivation of chloroplast CF1-ATPase by Mg2+ and ADP was studied. The rate of inactivation obeys the first-order equation and is independent of ADP concentration. An analysis of the dependence of the inactivation rate on Mg2+ concentration demonstrated that the limiting step of inactivation is other than Mg2+ binding, i.e. the subsequent steps which include, in all probability, the conformational changes of the enzyme. The original Mg2+-dependent activity of CF1-ATPase is close to that observed under steady-state conditions in the presence of sulphate and methanol and exceeds the Ca2+-dependent activity approximately 6-fold. Preincubation of CF1-ATPase with Mg2+ results in inhibition of the original activity of the enzyme. This effect is not removed by addition of the ATP-regenerating system (pyruvate kinase + phosphoenol pyruvate) to the preincubation medium but is diminished by sulphite and the non-hydrolyzed analog of ATP--beta, gamma-methyladenosine-5-triphosphate. After addition of AMPPCP to the reaction mixture the initial reaction rate is decreased, while the steady-state rate is increased. It may be concluded that the Mg2+-dependent inactivation of CF1-ATPase is induced by the tightly bound ADP. The latter can be replaced by ATP, which in contrast to ADP does not form an inactive complex with the enzyme. A comparison of experimental results with literature data suggests that the mechanism of "alternating sites" proposed by Boyer et al. for ATP hydrolysis by soluble CF1-ATPase is not realized under the given experimental conditions. |
