Modulation by Divalent Metal Ions of the Autocatalytic Reactivity of Adenosinetriphosphatase from Chloroplasts

A nonlinear, pre-steady-state initial rate of ATP hydrolysis is obtained on the addition of a divalent metal ion-ATP complex to a heat-activated coupling factor 1 isolated from chloroplasts. The acceleration of the initial rate follows first-order kinetics. The observed first-order kinetic constant (k_obsd) changes with the concentration of the substrate, reaching half-maximal value at the K_m for ATP hydrolysis. Preincubation of the enzyme with divalent metal ions decreases the k_obsd from 1 to 0.04 s-¹. Saturation of the divalent metal ion effect was obtained at the micromolar range. It is suggested that the autocatalysis is a result of early stages in ATP hydrolysis which induce conformational changes in the enzyme. Binding of divalent metal ions in the absence of ATP slows down this change.