Energetics of pyruvate phosphate dikinase catalysis

The present study was carried out to determine the energetics of Clostridium symbiosum pyruvate phosphate dikinase (PPDK) catalyzed interconversion of adenosine 5'-triphosphate (ATP), orthophosphate (Pi), and pyruvate (pyr) with adenosine 5'-monophosphate (AMP), inorganic pyrophosphate, and phosphoenolpyruvate (PEP) [E.ATP <==> E-PP.AMP <==> E-PP.AMP.Pi <==> E-P.AMP.PPi <==> E-P.pyr <==> E.PEP where E-PP and E-P represent the pyrophosphoryl and phosphoryl enzyme intermediates]. Thermodynamic techniques were used along with steady-state and pre-steady-state kinetic techniques to determine the rate constants for the substrate/product binding and release steps and the rate constants for the forward and reverse chemical steps. These values were used along with estimates of the cellular concentrations of the substrates and products to construct the free energy profile for the enzymatic reaction under physiological conditions. The energy profile obtained with the Mg2+/NH4(+)-activated enzyme revealed well-balanced transition states and well-balanced internal ground state energies (i.e., within 1 kcal/mol of each other). Examination of the energetics of the reaction steps leading from ATP to phosphohistidine formation in E-P suggested the use of intrinsic binding energy in the synthesis of a high energy P-N linkage. Comparison of the energy profiles of the Mg2+/NH4(+)-vs Co2+/NH4(+)-activated enzymes revealed cofactor selectivity at each of the phosphosphoryl transfer steps.