Lecture 5

ATP hydrolysis

Factors contributing to the free energy of hydrolysis of ATP

The overall equation for ATP hydrolysis is usually written:
ATP + H2O <=> ADP + inorganic phosphate (Pi)
DGo' = -7.3 kcal. mol-1
Note, the H2O is ommitted from the logarithmic term because it is the solvent with a standard state of 1M.
(Click here for a reactant structures in color)

However, there are a number of factors which make the true situation more complicated. Most important are:

Much work has gone into the measurement of the variation of DGo' for ATP hydrolysis with pH and Mg2+ concentration, which is summarized in tables and graphs available in the literature. Two useful graphs are shown below, which cover the most important physiological range.


Left: Effect of pH on the free energy of hydrolysis of ATP (DGATP) at 10 mM Mg2+.
Right: Effect of Mg2+ concentration on the free energy of hydrolysis of ATP (DGATP) at pH 7.0.

Note that for ATP synthesis, the reaction is written in the reverse direction, and the sign of the free energy change is reversed, so that:

ADP + inorganic phosphate (Pi) <=> ATP + H2O
DGo' = +7.3 kcal. mol-1

The modifying factors which affect the hydrolysis also effect the reverse reaction, so the graphs above can be used to find a value for the free energy of hydrolysis under any set of conditions, and the sign reversed to find the free energy for synthesis of ATP under the same conditions. 

©Copyright 1996, Antony Crofts, University of Illinois at Urbana-Champaign, a-crofts@uiuc.edu