V-ATPase of Thermus thermophilus Is Inactivated during ATP Hydrolysis but Can Synthesize ATP *

The ATP hydrolysis of the V 1 -ATPase of Thermus thermophilus have been investigated with an ATP-regenerating system at 25 °C. The ratio of ATPase activity to ATP concentration ranged from 40 to 4000 μ m ; from this, an apparent K m of 240 ± 24 μ m and a V max of 5.2 ± 0.5 units/mg were deduced. An apparent negative cooperativity, which is frequently observed in case of F 1 -ATPases, was not observed for the V 1 -ATPase. Interestingly, the rate of hydrolysis decayed rapidly during ATP hydrolysis, and the ATP hydrolysis finally stopped. Furthermore, the inactivation of the V 1 -ATPase was attained by a prior incubation with ADP-Mg. The inactivated V 1 -ATPase contained 1.5 mol of ADP/mol of enzyme. Difference absorption spectra generated from addition of ATP-Mg to the isolated subunits revealed that the A subunit can bind ATP-Mg, whereas the B subunit cannot. The inability to bind ATP-Mg is consistent with the absence of Walker motifs in the B subunit. These results indicate that the inactivation of the V 1 -ATPase during ATP hydrolysis is caused by entrapping inhibitory ADP-Mg in a catalytic site. Light-driven ATP synthesis by bacteriorhodopsin-V o V 1 -ATPase proteoliposomes was observed, and the rate of ATP synthesis was approximately constant. ATP synthesis occurred in the presence of an ADP-Mg of which concentration was high enough to induce complete inactivation of ATP hydrolysis of V o V 1 -ATPase. This result indicates that the ADP-Mg-inhibited form is not produced in ATP synthesis reaction.