Factors Contributing to Large Energies of Hydrolysis of Phosphate Compounds

1. Resonance stabilization of phosphate products. Figure 3.9 depicts the resonance stabilization of the orthophosphate ion, HPO 42- (abbreviated Pi). The multiple resonance forms are of equal energy, but all are not possible when the phosphate group is bound in an ester, such as ATP. Once Pi is released upon hydrolysis, however, the multiple resonance forms increase the overall entropy of the system, an energetically favorable process.

2. Additional hydration of hydrolysis products - Release of Pi allows greater opportunities for hydration. Hydration is an energetically favored state.

3. Electrostatic repulsion between charged products - When both products of hydrolysis are negatively charged (e.g., ADP and Pi in the hydrolysis of ATP), repulsion of the ionized products favors hydrolysis.

4. Enhanced resonance stabilization or tautomerization of product molecules - Hydrolysis is favored when product molecules can adopt multiple molecular forms. For example, pyruvate has two molecular forms, whereas PEP has only one.

5. Release of a proton in buffered solution - A proton is released in some hydrolysis reactions (see Figure 3.7), so hydrogen ion concentration (pH) influences the reaction.

For ATP4- + H 2O <=> ADP3- + HPO 42- + H+,

G = + RT ln {([ADP3-][HPO 42-][H+])/([ATP(-4)][H 2O])}, which can be rearranged as

G = + RTln { ([ADP3-][HPO 42-]) / ([ATP4-])} + RT ln{[H+]/[H 2 O]}

Because RT ln{[H+] / [H 2O]} is relatively constant at pH 7.0 in biological systems, it can be incorporated into to make . Thus,

= +RT ln{[H+] / [H 2O]}

See also: Free Energy and Concentration, ATP as Free Energy Currency (from Chapter 12)