Yeast mitochondrial F1-ATPase--effects of metal ions

The effects of a series of metal ions (M) on the velocity (V) of the reaction catalyzed by the yeast mitochondrial F1-ATPase were investigated in N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid and Imidazole buffers (pH 7.45), with total concentrations of ATP ranging from 2 to 8 mM. The biphasic metal ion activation/inhibition effects, observed with a constant total amount of ATP, were found to be consistent with a simple empirical kinetic equation, V1/V = 1 + K1/[M] + [M]/K2, where V1, K1, and K2 are empirical parameters and [M] the concentrations of metal ions not complexed with the ATP. Three alternative kinetic equations, involving the concentrations of either ATP or its metal chelate, consistent with this empirical equation, were shown to be invalid since changing the total amount of ATP did not affect the empirical reaction parameters V1, K1, and K2. The fact that changing the total ATP concentration failed to change the empirical reaction parameters suggests that the inhibition by an excess of ATP, which is observed with a constant total metal ion concentration, is due to chelation of free metal ions that are required for the ATP hydrolysis. When the total amount of added ATP was in excess of that of added magnesium ions it was found that calcium, strontium, and aluminum ions, which liberate free magnesium from the Mg-ATP chelate by complexing the ATP, all activated. These activations apparently confirm the fact that the ATPase requires a free magnesium ion as a cofactor for the hydrolysis of ATP.