Editing Polyphosphate (section)

== The "high energy" phosphate bond ==
ATP forms [[chelation|chelate]] complexes with metal ions. The [[stability constants of complexes|stability constant]] for the equilibrium
:ATP<sup>4−</sup> + Mg<sup>2+</sup> {{eqm}} MgATP<sup>2−</sup>, log &beta; <math>\approx</math> 4
is particularly large.<ref>{{cite journal |vauthors=Wilson J, Chin A | title = Chelation of divalent cations by ATP, studied by titration calorimetry | journal = Anal Biochem | volume = 193 | issue = 1 | pages = 16–9 | year = 1991 | pmid = 1645933| doi=10.1016/0003-2697(91)90036-S}}</ref> The formation of the magnesium complex is a critical element in the process of ATP hydrolysis, as it weakens the link between the terminal phosphate group and the rest of the molecule.<ref name=Storer/><ref>{{cite journal |vauthors=Garfinkel L, Altschuld R, Garfinkel D | title = Magnesium in cardiac energy metabolism | journal = J Mol Cell Cardiol | volume = 18 | issue = 10 | pages = 1003–13 | year = 1986 | pmid = 3537318 | doi = 10.1016/S0022-2828(86)80289-9 }}</ref>

The energy released in ATP hydrolysis,
:ATP<sup>4−</sup> + H<sub>2</sub>O &rarr; ADP<sup>3−</sup> + P<sub>i</sub><sup>−</sup>
at ΔG <math>\approx</math> -36.8 kJ mol<sup>−1</sup> is large by biological standards. P<sub>i</sub> stands for inorganic phosphate, which is protonated at biological pH. However, it is not large by inorganic standards. The term "high energy" refers to the fact that it is high relative to the amount of energy released in the [[organic chemistry|organic chemical]] reactions that can occur in living systems.