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A plasma membrane-bound adenosine triphosphatase with specific activities up to 0.2 μmol min −1  (mg protein) −1  at 80°C was detected in the thermoacidophilic crenarchaeon  Acidianus ambivalens  (DSM 3772). The enzymatic activity exhibited a broad pH-optimum in the neutral range with two suboptima at pH 5.5 and 7.0, respectively. Sulfite activation resulted in only one pH optimum at 6.25. In the presence of the divalent cations Mg 2+  and Mn 2+  the ATPase activity was maximal. Remarkably, the hydrolytic rates of GTP and ITP were substantially higher than for ATP. ADP and pyrophosphate were only hydrolyzed with small rates, whereas AMP was not hydrolyzed at all. Both activities could be weakly inhibited by the classical F-type ATPase inhibitor N, N′dicyclohexylcarbodiimide, whereas azide had no influence at all. The classical inhibitor of V-type ATPases, nitrate, also exerted a small inhibitory effect. The strongly specific V-type ATPase inhibitor concanamycin A, however, showed no effect at all. The P-type ATPase inhibitor vanadate had no inhibitory effect on the ATPase activity at pH 7.0, whereas a remarkable inhibition at high concentrations could be observed for the activity at pH 5.5. Arrhenius plots for both membrane bound ATPase activities were linear up to 95°C, reflecting the enormous thermostability of the enzyme.

Functional Characterization of an Extremely Thermophilic ATPase in Membranes of the Crenarchaeon Acidianus ambivalens