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Soluble Family I, Family II and membrane-integral (M-)Soluble inorganic pyrophosphatases (PPases) all hydrolyse pyrophosphate. Family I enzymes are ubiquitous hexameric or dimeric, utilise two Mg2+ to activate associative water attack and are potently inhibited by fluoride. Family II enzymes are found in bacteria and archaea and are DHH-superfamily dimers, utilise three metals and have a dissociative, metaphosphate-like transition state. Both soluble classes merely use PPi removal to drive biosynthesis. M-PPases are 16-helix membrane homodimers that couple PPi hydrolysis to Na+ and/or H+ pumping. Pumping is reversible and essential in plants, protozoa and many bacteria under energy stress.
Catalytically, M-PPases are 100-fold slower, bind fluoride weakly and seem to activate water only after PPi binding closes the active-site loop and displaces TM11-12 downward. This movement opens the ion gate, which suggests a binding-change model in which the pump stroke precedes hydrolysis, unifies Na+ and H+ transport and obviates the need for the hydrolytic proton to cross the funnel. Thus, while all PPases split the same substrate, they have evolved unrelated structures and distinct chemical strategies and only M-PPases convert PPi bond energy to membrane potential.
Fig. 1 The catalytic cycle of Inorganic Pyrophosphatase. (Kajander T.; et al. 2013)
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