Energy charge

Energy charge is an index used to measure the energy status of biological cells. It is related to ATP, ADP and AMP concentrations. Energy charge 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.

From Stryer:

"Many reactions in metabolism are controlled by the energy status of the cell. One index of the energy status is the energy charge, which is proportional to the mole fraction of ATP plus half the mole fraction of ADP, given that ATP contains two anhydride bonds whereas ADP contains one. Hence the energy charge is defined as:

$\mbox{Energy charge} = \frac{[\mbox{ATP}] + \frac{1}{2} [\mbox{ADP}]} {[\mbox{ATP}] + [\mbox{ADP}] + [\mbox{AMP}]}$
The energy charge can have a value ranging from 0 (all AMP) to 1(all ATP). Daniel Atkinson showed that ATP-generating (catabolic) pathways are inhibited by a high energy charge. In plots of the reaction rates of such pathways versus the energy charge, the curves are steep near an energy charge of 0.9, where they usually intersect. It is evident that control of these pathways has evolved to maintain the energy charge within rather narrow limits. In other words the energy charge like the pH of a cell is buffered. The energy charge of most cells range from 0.8 to 0.95."