Reversibility of the ATP:Glutamine Synthetase Adenylyltransferase Reaction

The reversibility of adenylylation of glutamine synthetase from E. coli by adenylyltransferase was demonstrated. Several positive effectors (Gln, 2-hydroxyethyl-S-cysteine, Trp and Met) stimulate the back reaction in the same manner as the forward reaction. The apparent Michaelis constant for PP(i) is 2.2 mM at pH 7.35. The pH optimum of the back reaction is 6.5-7 while the pH optimum of the forward reaction is 7.6. The apparent equilibrium constant in the presence of 10 mM Mg(2+) at pH 7.36 is 8.5 in favor of adenylylated glutamine synthetase and PP(i). The equilibrium constant is strongly dependent from pH and from Mg(2+) concentration. There is a difference of about 0.5 to 1 kcal/mole free energy between the adenylyl-O-tyrosine bond and the pyrophosphate bond of adenosine triphosphate (ATP). It follows from these considerations that the adenylyl-O-tyrosine bond is an "energy-rich phosphate bond."