Pyrophosphate hydrolysis in the extremely halophilic archaeon Haloarcula japonica is catalyzed by a single enzyme with a broad ionic strength range

The soluble protein fraction of the extremely halophilic archaeon Haloarcula japonica exhibits substantial inorganic pyrophosphate (PPi) hydrolysis activity in the presence of 2–4 M NaCl (Wakai et al, J Biol Chem 288:29247–29251, 2013), which provides high ionic strength (2–4). In this study, much higher PPi hydrolysis activity was unexpectedly detected, even with 0 M NaCl in the presence of 100–200 mM MgSO 4 , providing a much lower ionic strength of 0.4–0.8, in the same protein fraction. Na + and Mg 2+ ions were required for activity under high and low ionic strength conditions, respectively. A recombinant H. japonica pyrophosphatase (HjPPase) exhibited PPi hydrolysis activity with the same broad ionic strength range, indicating that the activity associated with such a broad ionic strength range could be attributed to a single enzyme. Thus, we concluded that the broad ionic strength range of HjPPase may contribute to adaptation for both Na + and Mg 2+ which are abundant but variable in the unstable living environments of H. japonica .