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Factors contributing to the free energy of hydrolysis of ATP

However, there are a number of factors which make the true situation more complicated. Most important are:

• The inorganic phosphate and the phosphate groups on ATP and ADP are weakly acidic, and have pK values in the physiological range. The activities of the different forms will vary depending on the pH of the reaction medium, and this will alter the value for DG^o.
• HATP^3- <==> ATP^4- + H ⁺; pK₁' = 6.95
• HADP^2- <==> ADP^3- + H ⁺; pK₂' = 6.88
• H₂PO₄^- <==> HPO₄^2- + H ⁺; pK₃' = 7.20
(The effect of these pK values is that a H⁺ is released on ATP hydrolysis, with a stoichiometry which approaches 1 above pK₃'. This proton release can be used to assay the reactions of ATP hydrolysis or synthesis, or follow the kinetics if a recoding pH meter is available.)
• The presence of Mg²⁺ will also modify the value for DG^o, because the different reactants form complexes with bivalent cations. For Mg²⁺, the following equilibria are important:
• Mg²⁺ + ATP^4- <==> MgATP^2-
• Mg²⁺ + ADP^3- <==> MgADP^-
• Mg²⁺ + HPO₄^2- <==> MgHPO₄

Left: Effect of pH on the free energy of hydrolysis of ATP (DG_ATP) at 10 mM Mg²⁺.
Right: Effect of Mg²⁺ concentration on the free energy of hydrolysis of ATP (DG_ATP) at pH 7.0.

Note that for ATP synthesis, the reaction is written in the reverse direction, and the sign of the free energy change is reversed, so that:

The modifying factors which affect the hydrolysis also effect the reverse reaction, so the graphs above can be used to find a value for the free energy of hydrolysis under any set of conditions, and the sign reversed to find the free energy for synthesis of ATP under the same conditions.