THE COMPUTED FREE-ENERGY CHANGE OF HYDROLYSIS OF INORGANIC PYROPHOSPHATE AND ATP - APPARENT SIGNIFICANCE FOR INORGANIC-PYROPHOSPHATE-DRIVEN REACTIONS OF INTERMEDIARY METABOLISM

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 (DELTAG-degrees') and hence the actual free energy change (DELTAG) values for ATP and PP(i) hydrolysis have been computed for a simple model cytosol. DELTAG-degrees'(ATP), and DELTAG-degrees'(PPi) show dissimilar responses to pH and free Mg2+, 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.