The Structure and Hydrolysis of ATP


ATP (adenosine triphosphate) contains the sugar ribose, with the nitrogenous base adenine and a chain of three phosphate groups (the triphosphate group) bonded to it. In addition to its role in energy coupling, ATP is also one of the nucleoside triphosphates used to make RNA.

The bonds between the phosphate groups of ATP can be broken by hydrolysis. When the terminal phosphate bond is broken by addition of a water molecule, a molecule of inorganic phosphate leaves the ATP, which becomes adenosine diphosphate, or ADP. The reaction is exergonic and releases 7.3 kcal of energy per mole of ATP hydrolyzed:

ATP + H2O S ADP + ~P i ∆G = -7.3 kcal/mol (-30.5 kJ/mol)

This is the free-energy change measured under standard conditions. In the cell, conditions do not conform to standard conditions, primarily because reactant and product concentrations differ from 1 M. For example, when ATP hydrolysis occurs under cellular conditions, the actual ∆G is about -13 kcal/mol, 78% greater than the energy released by ATP hydrolysis under standard conditions.

Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, but the term is misleading. The phosphate bonds of ATP are not unusually strong bonds, as “high-energy” may imply; rather, the reactants (ATP and water) themselves have high energy relative to the energy of the products. The release of energy during the hydrolysis of ATP comes from the chemical change of the system to a state of lower free energy, not from the phosphate bonds themselves.

ATP is useful to the cell because the energy it releases on losing a phosphate group is somewhat greater than the energy most other molecules could deliver. But why does this hydrolysis release so much energy? If we reexamine the ATP molecule in the picture below, we can see that all three phosphate groups are negatively charged. These like charges are crowded together, and their mutual repulsion contributes to the instability of this region of the ATP molecule. The triphosphate tail of ATP is the chemical equivalent of a compressed spring.

The structure and hydrolysis of adenosine triphosphate (ATP). Throughout this book, the chemical structure of the triphosphate group seen in (a) will be represented by the three joined yellow circles shown in (b).
Source: Urry, Lisa A.. Campbell Biology (p. 151). Pearson Education. Kindle Edition.


Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition.


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