ATP-HYDROLYZING EXCITATION STATE OF THE RECONSTITUTED ALPHA(3)BETA(3), COMPLEX OF ATP SYNTHASE FROM THE THERMOPHILIC BACTERIUM PS3 - STRUCTURAL CHARACTERISTICS SHOWN BY TIME-RESOLVED SMALL-ANGLE X-RAY-SCATTERING WITH SYNCHROTRON-RADIATION

The ATP-hydrolyzing excitation state of the alpha(3) beta(3) complex of the ATP synthase from the thermophilic bacterium PS3 was investigated using time-resolved small-angle X-ray scattering with synchrotron radiation. The results showed the presence of the alpha(3) beta(3) complex at a steady state during ATP hydrolysis when the alpha(3) beta(3) hexamer reacted with Mg-ATP. The radius of gyration of the complex in the steady state was significantly larger than that of the Mg-AMP-PNP-hexamer complex, indicating a conformational change to an expanded structure during catalysis, This alpha(3) beta(3) complex dissociated into alpha(1) beta(1) heterodimers with apparent first-order reaction kinetics after all the ATPs were converted to ADPs. In contrast, when the alpha(3) beta(3) complex reacted with Mg-ADP, the complex dissociated into dimers with apparent first-order reaction kinetics without showing the steady state of the complex, The dimers, however, re-associated into the hexamer when Mg-ATP was added. The results were well-explained by a computer simulation based on non-linear chemical dynamics, in which a reaction mechanism that incorporates the dynamic structure of the hexamer in the steady state was considered.