High-energy phosphate
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186272497
2008-01-23T03:35:59Z
Adeez
3919602
'''High-energy phosphate''' can mean one of two things:
* The phosphate-phosphate bonds formed when compounds such as [[adenosine diphosphate]] and [[adenosine triphosphate]] are created.
* The compounds that contain these bonds, which include the nucleoside diphosphates and nucleoside triphosphates, and the high-energy storage compounds of the muscle, the [[phosphagen]]s. When people speak of a '''high-energy phosphate pool''', they speak of the total concentration of these compounds with these high-energy bonds.
High-energy phosphate bonds are [[pyrophosphate]] bonds, acid [[anhydride]] linkages, formed by taking [[phosphoric acid]] derivatives and dehydrating them. As a consequence, the [[hydrolysis]] of these bonds is [[exothermic reaction|exothermic]] under physiological conditions, releasing energy.<br><br>
<table border="1" cellpadding="5">
<caption>Energy released by high energy phosphate reactions</caption>
<tr><th>Reaction</th><th>ΔG [kJ/mol]</th></tr>
<tr>
<td>ATP + H<sub>2</sub>O → ADP + P<sub>i</sub> </td><td align="right">-36.8</td></tr>
<tr>
<td>ADP + H<sub>2</sub>O → [[Adenosine monophosphate|AMP]] + P<sub>i</sub> </td><td align="right">-36.0</td></tr>
<tr>
<td>ATP + H<sub>2</sub>O → AMP + PP<sub>i</sub> </td><td align="right">-40.6</td></tr>
<tr>
<td>PP<sub>i</sub> → 2 P<sub>i</sub> </td><td align="right">-31.8</td></tr>
<tr>
<td>AMP + H<sub>2</sub>O → A + P<sub>i</sub> </td><td align="right">-12.6</td></tr>
</table>
Except for PP<sub>i</sub> → 2 P<sub>i</sub>, these reactions are, in general, not allowed to go uncontrolled in the human cell but are instead coupled to other processes needing energy to drive them to completion. Thus, high-energy phosphate reactions can:
* provide energy to cellular processes, allowing them to run;
* couple processes to a particular nucleoside, allowing for regulatory control of the process;
* drive the reaction ''to the right'', by taking a reversible process and making it irreversible.
The one exception is of value because it allows a single hydrolysis, ATP + 2H<sub>2</sub>O → AMP + PP<sub>i</sub>, to effectively supply the energy of hydrolysis of two high-energy bonds, with the hydrolysis of PP<sub>i</sub> being allowed to go to completion in a separate reaction. The AMP is regenerated to ATP in two steps, with the equilibrium reaction ATP + AMP ↔ 2ADP, followed by regeneration of ATP by the usual means, [[oxidative phosphorylation]] or other energy-producing pathways such as [[glycolysis]].
Often, high-energy phosphate bonds are denoted by the character '~'. In this notation, ATP becomes A-P~P~P.
== References ==
* McGilvery, R. W. and Goldstein, G., ''Biochemistry - A Functional Approach'', W. B. Saunders and Co, 1979, 345-351.
[[Category:Phosphates]]