Regulation of the thermoalkaliphilic F1-ATPase from Caldalkalibacillus thermarum

The crystal structure has been determined of the F-1-catalytic domain of the F-ATPase from Caldalkalibacillus thermarum, which hydrolyzes adenosine triphosphate (ATP) poorly. It is very similar to those of active mitochondrial and bacterial F-1-ATPases. In the F-ATPase from Geobacillus stearothermophilus, conformational changes in the e-subunit are influenced by intracellular ATP concentration and membrane potential. When ATP is plentiful, the epsilon-subunit assumes a "down" state, with an ATP molecule bound to its two C-terminal alpha-helices; when ATP is scarce, the alpha-helices are proposed to inhibit ATP hydrolysis by assuming an "up" state, where the alpha-helices, devoid of ATP, enter the alpha(3)beta(3)-catalytic region. However, in the Escherichia coli enzyme, there is no evidence that such ATP binding to the epsilon-subunit is mechanistically important for modulating the enzyme's hydrolytic activity. In the structure of the F-1-ATPase from C. thermarum, ATP and a magnesium ion are bound to the alpha-helices in the down state. In a form with a mutated epsilon-subunit unable to bind ATP, the enzyme remains inactive and the epsilon-subunit is down. Therefore, neither the gamma-subunit nor the regulatory ATP bound to the epsilon-subunit is involved in the inhibitory mechanism of this particular enzyme. The structure of the alpha(3)beta(3)-catalytic domain is likewise closely similar to those of active F-1-ATPases. However, although the beta(E)-catalytic site is in the usual "open" conformation, it is occupied by the unique combination of an ADP molecule with no magnesiumion and a phosphate ion. These bound hydrolytic products are likely to be the basis of inhibition of ATP hydrolysis.