The adenylate energy charge is an index used to measure the energy status ofbiological 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 theATP synthase.[1] ATP can also be produced by “substrate level phosphorylation” reactions (ADP phosphorylation by (1,3)-bisphosphoglycerate, phosphoenolpyruvate, phosphocreatine), by the succinate-CoA ligase and phosphoenolpyruvate carboxylkinase, and byadenylate kinase, an enzyme that maintains the three adenine nucleotides in equilibrium ().
The energy charge is related toATP,ADP andAMP concentrations. It was first defined byAtkinson 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 inmetabolism. Since the adenylate kinase maintains two ADP molecules in equilibrium with one ATP (), Atkinson defined the adenylate energy charge as:[2]
The energy charge of most cells varies between 0.7 and 0.95[1] - oscillations in this range are quite frequent. Daniel Atkinson showed that when the energy charge increases from 0.6 to 1.0, thecitrate lyase andphosphoribosyl pyrophosphate synthetase, two enzymes controllinganabolic (ATP-demanding) pathways are activated,[2][3] while thephosphofructokinase and thepyruvate dehydrogenase, two enzymes controllingamphibolic pathways (supplying ATP as well as important biosynthetic intermediates) are inhibited[4] He concluded that control of these pathways has evolved to maintain the energy charge within rather narrow limits - in other words, that the energy charge, like the pH of a cell, must be buffered at all times. We now know that most if not all anabolic and catabolic pathways are indeed controlled, directly and indirectly, by the energy charge.[5][6][7] In addition to direct regulation of several enzymes by adenyl nucleotides, an AMP-activated protein kinase known asAMP-K phosphorylates and thereby regulates key enzymes when the energy charge decreases. This results in switching off anabolic pathways while switching on catabolic pathways when AMP increases.[8][9]
Life depends on an adequate energy charge. If ATP synthesis is momentarily insufficient to maintain an adequate energy charge, AMP can be converted by two different pathways to hypoxanthine and ribose-5P, followed by irreversible oxidation of hypoxanthine to uric acid. This helps to buffer the adenylate energy charge by decreasing the total {ATP+ADP+AMP} concentration.[10]
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