Enhancement of the Rate of Pyrophosphate Hydrolysis by Nonenzymatic Catalysts and by Inorganic Pyrophosphatase

To estimate the proficiency of inorganic pyrophosphatase as a catalyst, P-31 NMR was used to determine rate constants and thermodynamics of activation for the spontaneous hydrolysis of inorganic pyrophosphate (PPi) in the presence and absence of Mg2+ at elevated temperatures. These values were compared with rate constants and activation parameters determined for the reaction catalyzed by Escherichia coli inorganic pyrophosphatase using isothermal titration calorimetry. At 25 degrees C and pH 8.5, the hydrolysis of MgPPi2- proceeds with a rate constant of 2.8 x 10(-10) s(-1), whereas E. coli pyrophosphatase was found to have a turnover number of 570 s(-1) under the same conditions. The resulting rate enhancement (2 x 10(12)-fold) is achieved entirely by reducing the enthalpy of activation (Delta Delta H-double dagger = -16.6 kcal/mol). The presence of Mg2+ ions or the transfer of the substrate from bulk water to dimethyl sulfoxide was found to increase the rate of pyrophosphate hydrolysis by as much as similar to 10(6)-fold. Transfer to dimethyl sulfoxide accelerated PPi hydrolysis by reducing the enthalpy of activation. Mg2+ increased the rate of PPi hydrolysis by both increasing the entropy of activation and reducing the enthalpy of activation.