Abstract
Energy from cytosolic hydrolysis of inorganic pyrophosphate (PP i) can be conserved through PP i acting as a specific phosphoryl donor or through the substitution of reactions involving PP i for those involving ATP. To assess the relative importance of PP i in this context, the standard free energy change ( ΔG°′) and hence the actual free energy change ( ΔG) values for ATP and PP i hydrolysis have been computed for a simple model cytosol. ΔG°′ ATP and ΔG°′ PP i show dissimilar responses to pH and free Mg 2+, with implications for cell energetics during anaerobiosis. The impact of the estimates on possible in vivo function of two primary H +-translocating phosphoanhydrolases (an ATPase and PPase) at the vacuolar membrane of higher plants is discussed.
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