Acceleration of unisite catalysis of mitochondrial F1-adenosinetriphosphatase by ATP, ADP and pyrophosphate--hydrolysis and release of the previously bound [gamma-32P]ATP

Abstract

The effect of ATP, ADP and pyrophosphate (PPi) on hydrolysis and release of [gamma-32P]ATP bound to the high-affinity catalytic site of soluble F1 from bovine heart mitochondria under unisite conditions [Grubmeyer, C., Cross, R. L. & Penefsky, H. S. (1982) J. Biol. Chem. 257, 12092-12100] was studied. In accord with the previous data, it was observed that millimolar concentrations of ATP or ADP added to F1 undergoing unisite hydrolysis of [gamma-32P]ATP accelerated its hydrolysis. PPi also produced a hydrolytic burst of a fraction of the previously bound [gamma-32P]ATP; kinetic data suggested that for production of optimal hydrolysis by PPi of the bound [gamma-32P]ATP, two binding sites with apparent Kd of 27 microM and 240 microM must be filled. The extent of the hydrolytic burst induced by MgPPi was lower than that induced by ADP and ATP. In F1 in which PPi had produced a hydrolytic burst of the bound [gamma-32P]ATP, the addition of ATP induced a second burst of hydrolysis. By filtration experiments and enzyme trapping, it was also studied whether ATP, ADP and PPi produce release of the tightly bound [gamma-32P]ATP. At millimolar concentrations, ATP and ADP brought about release of about 25% of the previously bound [gamma-32P]ATP. At micromolar concentrations, ADP accelerated the hydrolysis of the previously bound [gamma-32P]ATP but not its release. Hence, the hydrolytic and release reactions could be separated, indicating that the two reactions require the occupancy of different sites in F1. With PPi, no release of the tightly bound [gamma-32P]ATP was observed. The ADP induced hydrolysis and release of the F1-bound [gamma-32P]ATP were inhibited by sodium azide to the same extent (60%). Since release of ATP from a high-affinity catalytic site of F1 represents the terminal step of oxidative phosphorylation, the data illustrate that the binding energy of substrates to F1 is critical to the ejection of ATP into the media. The failure of PPi to induce release of [gamma-32P]ATP bound to F1 under unisite conditions is probably due to its lower binding energy.