Yeast Mitochondrial F1-ATPase: The Unusual Kinetics with Excess Magnesium Ions at Low Salt Concentrations

Studies with the yeast mitochondrial F1-ATPase showed that non-Michaelis-Menten kinetics occurred around pH 8 when there was an excess of magnesium over ATP and a low salt concentration. Increasing the salt concentration activated the enzyme and resulted in Michaelis-Menten kinetics. In contrast, increasing the salt concentration had no effect when ATP was in excess of magnesium. The data were interpreted to mean that magnesium ions inhibit by limiting the amount of free ATP and that the salt anions are able to overcome this effect by replacing the ATP. Mechanistically this is consistent with a ligand substitution reaction in which either free ATP or salt anions are required to displace a product from the enzyme.