ATP thermochemistry

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ATP is used to store and transport chemical energy within cells because of the intermediate energy content of the diphosphate bonds. This energy arises from the unfavourable electrostatic interactions in the ATP tetraanion which are relieved upon hydrolysis, as well as the greater resonance stabilisation present in the phosphate products. The energy is released by cleaving either a phosphate (Pi) or pyrophosphate (PPi) unit from ATP:

ATP + H₂O → ADP + Pi + H⁺ ΔG˚ = -30.54 kJ/mol (-7.3 kcal/mol)

ATP + H₂O → AMP + PPi + H⁺ ΔG˚ = -45.6 kJ/mol (-10.9 kcal/mol)

Note the values given for the Gibbs free energy for this reaction are dependent on a number of factors, including overall ionic strength and the presence of alkaline earth metal ions such as Mg²⁺ and Ca²⁺. Under typical cellular conditions, ΔG is approximately -50 kJ/mol (-12 kcal/mol).

[edit] Reference

Stryer, Lubert (2002). Biochemistry, fifth edition. New York: W.H. Freeman and Company. ISBN 0-7167-1843-X.