Adenine nucleotides and the transfer of energy
The breaking of one phosphoanhydride bond releases 30.5 kJ/mol of energy. Thus, the dephosphorylation of the energy-rich ATP to ADP yields a free energy of 30.5 kJ/mol or even 61 kJ/mol when two phosphate groups are released (as shown below).
- ATP + H2O → ADP + Pi ΔG = -30.5 kJ/mol
- ADP + H2O → AMP + Pi ΔG = -30.5 kJ/mol
- ATP + H2O → AMP + 2Pi ΔG = -61 kJ/mol
To restore the energy, an endergonic reaction, in which ATP is created from ADP and a free phosphate is needed. This reaction can be carried out by the membrane embedded ATP synthase (also called complex V). The energy which is used to generate ATP from ADP and Pi is hereby available in the form of hydrogen ions (H+), which are moved down an electrochemical gradient, e.g. from the intermembrane space into the mitochondrial matrix.
ATP and the free energy, which can be stored or generated by its conversion into ADP and AMP, is useful in many cellular processes, especially cellular respiration. It serves as an energy source for glycolysis, photosynthesis, fatty acid oxidation, anaerobic respiration, active transport mechanisms across the cell membrane - e.g. in the Electron transfer pathway -, and synthesis of macromolecules such as DNA.