Interaction of mitochondrial F1-ATPase with trinitrophenyl derivatives of ATP and ADP. Participation of third catalytic site and role of Mg2+ in enzyme inactivation.

Relatively high ATP concentrations show an unexpected lack of inhibition of the hydrolysis of low concentrations of trinitrophenyl ATP (TNP-ATP) by mitochondrial F1-ATPase. In striking contrast low TNP-ATP concentrations markedly inhibit the hydrolysis of much higher ATP concentrations. The three catalytic sites undergoing sequential conformational changes have different conformations at any instant of catalysis, and only two need to be filled for rapid, steady-state ATP hydrolysis. The remaining site has low affinity for ATP (Kd 2 mM) but about 10(4) greater affinity for TNP-ATP (Km and Kd about 0.2 microM). Thus 500 microM ATP does not prevent binding of less than 1 microM TNP-ATP. As the site binding the TNP-ATP undergoes sequential conformational changes the TNP-ATP undergoes sequential conformational changes the TNP-ATP is hydrolyzed and products are released. The results give strong support to the view that all three catalytic sites proceed equivalently in ATP as well as TNP-ATP hydrolysis. The conformation that has the lowest affinity for ATP has over a 10-fold greater affinity for ADP (Kd 150 microM) and may be akin to the conformation to which ADP binds during net ATP synthesis by the ATP synthase. The recognition of these features was made possible by new information obtained from detailed studies of the interactions of Mg2+, TNP-ADP, TNP-ATP, ATP, and noncatalytic sites on initial and steady-state hydrolysis rates.