ATP-Hydrolyzing Excitation State of the reconstituted α_3β_3 Complex of ATP Synthase from the Thermophilic Bacterium PS3: Structural Characteristics Shown by Time-Resolved Small-Angle X-Ray Scattering with Synchrotron Radiation

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The ATP-hydrolyzing excitation state of the α3β3 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 α3β3 complex at a steady state during ATP hydrolysis when the α3β3 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 α3β3 complex dissociated into α1β1 heterodimers with apparent first-order reaction kinetics after all the ATPs were converted to ADPs. In contrast, when the α3β3 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.

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