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The adenylate energy charge is an index used to measure the energy status of biological cells.
ATP or Mg-ATP is the principal molecule for storing and transferring energy in the cell : it is used for biosynthetic pathways, maintenance of transmembrane gradients, movement, cell division, etc... More than 90% of the ATP is produced by phosphorylation of ADP by the ATP synthase. ATP can also be produced by “substrate level phosphorylation” reactions -bisphosphoglycerate, phosphoenolpyruvate, phosphocreatine], by the succinate-CoA ligase and phosphoenolpyruvate carboxylkinase, and by adenylate kinase, an enzyme that maintains the three adenine nucleotides in equilibrium.
The energy charge is related to ATP, ADP and AMP concentrations. It was first defined by Atkinson and Walton who found that it was necessary to take into account the concentration of all three nucleotides, rather than just ATP and ADP, to account for the energy status in metabolism. Since the adenylate kinase maintains two ADP molecules in equilibrium with one ATP , Atkinson defined the adenylate energy charge as :
Energy charge = + 1 2 + + {\displaystyle {\mbox{Energy charge}}={\frac {+{\frac {1}{2}}}{++}}}