Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Electron transport, oxidative phosphorylation, and regulation of ATP synthesis
The energy charge serves as another indicator of the cellular energy status

As we saw above, ATP and ADP act as modulators of key regulatory Enzymes involved in Glycolysis, The Citric Acid Cycle, and Oxidative Phosphorylation; ATP Functions as a negative modulator, whereas ADP generally stimulates Carbohydrate Catabolism. Consequently, any shift in the mass-action ratio [ATP]/[ADP][Pi], which is normally quite high, can correspondingly alter The activity of several regulatory enzymes in central Catabolic pathways. However, among these enzymes, There are also those for which AMP serves as a positive modulator. To evaluate The Role of AMP alongside ATP and ADP in Metabolic Regulation, Daniel Atkinson introduced METABOLISM/2.html">THE CONCEPT OF energy charge and used it to characterize the energetic status of Cells. The energy charge is a measure of the fractional saturation of the adenylate pool (i.e., the sum of ATP, ADP, and AMP) with high-energy phosphate groups:

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If the entire adenylate pool is fully phosphorylated—that is, consisting solely of ATP—the energy charge of the system is 1.0; conversely, if the system is energetically "empty" and the entire pool consists solely of AMP, the energy charge is 0. Typically, the cellular energy charge is maintained at approximately 0.9, indicating that the adenylate system is nearly fully "charged." Under certain conditions, allosteric interactions between energy-yielding and energy-utilizing processes are more easily conceptualized in terms of energy charge rather than the mass-action ratio of The ATP system. However, this is a complex issue that defies a simple, definitive solution; it appears that no single, all-encompassing indicator of energetic status exists that can universally account for The regulation of all metabolic systems.



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