ATP hydrolysis

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ATP hydrolysis is the reaction by which chemical energy that has been stored and transported in the high-energy phosphoanhydridic bonds in ATP (Adenosine triphosphate) is released, for example in the muscles, to produce work. The product is ADP (Adenosine diphosphate) and an inorganic phosphate, orthophosphate (Pi). ADP can be further hydrolyzed to give energy, AMP (Adenosine monophosphate), and another orthophosphate (Pi). ATP hydrolysis is the final link between the energy derived from food or sunlight and useful work such as muscle contraction, the establishment of ion gradients across membranes, and biosynthetic processes necessary to maintain life.

Hydrolysis of the phosphate groups in ATP is especially exergonic, because the resulting orthophosphate group is greatly stabilized by multiple resonance structures, making the products (ADP and P_i) much lower in energy than the reactant (ATP). The high negative charge density associated with the three adjacent phosphate units of ATP also destabilizes the molecule, making it higher in energy. Hydrolysis relieves some of these electrostatic repulsions as well, liberating useful energy in the process.

Hydrolysis of the terminal phosphoanhydridic bond is a highly exergonic process, producing -30.5 kJ mol^-1 energy. This reaction can then be coupled with thermodynamically unfavorable reactions to give an overall negative (spontaneous) ΔG for the reaction sequence. The actual value of ΔG for ATP hydrolysis varies, primarily depending on Mg²⁺ concentration, and under normal physiologic conditions is actually closer to -50 kJ mol^-1.

In humans, approximately 60% of the energy released from the hydrolysis of one mole of ATP produces metabolic heat rather than fuel the actual reactions taking place.^{[citation needed]}