Equilibrium of the ATP:Glutamine Synthetase Adenylyltransferase Reaction

The activity of glutamine synthetase from Escherichia coli is regulated by an enzymecatalyzed adenylylation of a tyrosyl residue. The equilibrium of the adenylyl‐group transfer has been measured. Apparent ΔG°=−;1.8 kcal/mole (Keq= 19.6 at 25 °C, 10 mM MgSO4), implying that ΔG° for the hydrolysis of the tyrosyl‐O‐AMP bond is about 10 kcal/mole. Apparent enthalpy of the transfer reaction was estimated from the temperature dependence of the equilibrium constant; ΔH°app=−1.7 kcal/mole, ΔS°app∼ 0 cal deg−1 mole−1.Solutions for the distribution of the various ionic forms of ATP and pyrophosphate (protonated and magnesium‐complexed) present in the reaction mixtures have been obtained by computer. A model reaction written with MgATP, magnesium pyrophosphate, and the subunit of glutamine synthetase as the thermodynamic reactants provides the best interpretation of equilibrium data at various conditions of magnesium and hydrogen‐ion concentrations.