Abstract
Abstract Equations are set up to calculate the observed standard free energy change for the hydrolysis of ATP (namely, ΔGobs, the quantity evaluated on the basis of total ATP, ADP, and orthophosphate concentrations at equilibrium, irrespective of the state of ionization or complex formation, or both) as a function of pH and Mg2+ ion concentration. The variation of ΔGobs is analyzed in terms of ΔG, the standard free energy change with respect to the 1 molal standard states for the H+ and Mg2+ ions, and the driving force due to the prevailing H+ and Mg2+ ion concentrations with data extrapolated to zero ionic strength so that the standard free energy, enthalpy, and entropy changes are free from minor contributions arising from unspecific ionic interactions. The variation of ΔG is then discussed in terms of the component enthalpy and entropy changes for the individual hydrolysis equilibria which predominate over the pH range 6 to 9 as first the ATP, then the ADP, and finally the orthophosphate are converted into Mg2+ complexes. The calculations, based on data for the glutamine synthetase and hydrolysis reactions, lead to a revised value for ΔGobs at pH 7.5, 25°, and µ = 0.2 of -10.7 kcal per mole. Series of values are listed in an appendix for various ionic strengths at 25° and 37° over the pH range 6 to 9 with Mg2+ concentrations up to 50 mm, from which ΔGobs for other reaction conditions can be obtained by interpolation.
Highlights
In order to correlate these standard free energy changes and their component enthalpy and entropy changes with molecular properties it is essential that the standard states of all of the species participating as reactants and
Estimates of the experimental uncertainty have been made for each stage in the calculation. Assuming as they did that in the dilute solutions used the activity coefficient of glutamine can be taken as unit, it follows that Kobs for ATP hydrolysis, under the conditions of temperature, pH, ionic strength, and Mg”+ ion concentration used for the glutamine synthetase reaction, is given by the expression, TABLE
We have shown previously (6) that in the absence of Mg2+, while AGobs becomes more negative by about 4 kcal per mole over the pH range 6 to 9, the component terms n,RTlnH and AG vary much more, by some 10 kcal per mole, the smaller variation of AGobs being due to extensive compensation between these terms
Summary
Assuming as they did that in the dilute solutions used the activity coefficient of glutamine can be taken as unit, it follows that Kobs for ATP hydrolysis, under the conditions of temperature, pH, ionic strength, and Mg”+ ion concentration used for the glutamine synthetase reaction, is given by the expression, TABLE
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