ATP Synthesis by F0F1-ATP Synthase Independent of Noncatalytic Nucleotide Binding Sites and Insensitive to Azide Inhibition

Abstract

ATP hydrolyzing activity of a mutant alpha3beta3gamma subcomplex of F0F1-ATP synthase (DeltaNC) from the thermophilic Bacillus PS3, which lacked noncatalytic nucleotide binding sites, was inactivated completely soon after starting the reaction (Matsui, T., Muneyuki, E. , Honda, M., Allison, W. S., Dou, C., and Yoshida, M. (1997) J. Biol. Chem. 272, 8215-8221). This inactivation is caused by rapid accumulation of the "MgADP inhibited form" which, in the case of wild-type enzyme, would be relieved by ATP binding to noncatalytic sites. We reconstituted F0F1-ATP synthase into liposomes together with bacteriorhodopsin and measured illumination-driven ATP synthesis. Remarkably, DeltaNC F0F1-ATP synthase catalyzed continuous turnover of ATP synthesis while it could not promote ATP-driven proton translocation. ATP synthesis by DeltaNC F0F1-ATP synthase, as well as wild-type enzyme, proceeded even in the presence of azide, an inhibitor of ATP hydrolysis that stabilizes the MgADP inhibited form. The time course of ATP synthesis by DeltaNC F0F1-ATP synthase was linear, and gradual acceleration to the maximal rate, which was observed for the wild-type enzyme, was not seen. Thus, ATP synthesis can proceed without nucleotide binding to noncatalytic sites even though the rate is sub-maximal. These results indicate that the MgADP inhibited form is not produced in ATP synthesis reaction, and in this regard, ATP synthesis may not be a simple reversal of ATP hydrolysis.