We usually think of ATP as a "high energy" molecule because the hydrolysis of ATP to ADP or AMP releases a lot of energy.1 The standard Gibbs free energy change for the two reactions shown in the large figure aren't terribly relevant because, for simplicity, I've left out a key component of the reaction.
Mg2+ ions are an essential part of the reaction in vivo. They are bound to adjacent phosphate groups as shown below and this ATP:Mg2+ complex has different thermodynamic properties than free ATP.
In addition, the standard Gibbs free energy changes aren't very useful when you're dealing with charged molecules and the ATP hydrolysis reactions have charged molecules—even when some of the negative charges are neutralized by Mg2+ ions. That's why it's better to calculate new values of "standard" Gibbs free energy changes in the presence of Mg2+ ions and an ionic strength that's closer to physiological values.
Robert Alberty and his colleagues have calculated the transformed free energies of hydrolysis for many biochemical compounds (Alberty and Goldberg 1992) and those are the values we should use in biochemistry courses.
The table (below right) shows the standard Gibbs free energy changes for the ATP reactions in the presence of 3 mM2+ and ionic strength = 0.25 M. Note that the values are large and negative. That means a lot of energy is given off. This energy can be captured and used in enzyme catalyzed reactions.
If you've been paying attention to the previous postings in this series—as I'm sure you have—you'll remember that standard Gibbs free energy changes don't mean very much in biochemistry. What really counts is the actual free energy change and that depends on the in vivo concentrations of reactants and products.
ΔGreaction = 0 kJ mol-1.
ATP would be completely useless as a "high-energy" compound if the hydrolysis reaction was a near-equilibrium reaction. That's why these reactions can never be allowed to reach equilibrium. Cells would die if this ever happened.
Most of the reactions that use up ATP are regulated. What this means is that the activity of the enzyme is inhibited if the concentration of ATP falls too much, relative to ADP (or AMP). If the concentration of ATP is always high relative to the products then there's still a large negative ΔG for the reaction and this energy can be used to drive other reactions.
So, we know that the actual Gibbs free energy isn't the same as the standard Gibbs free energy but what is it's actual value? The short answer is that in most cases we don't know. It has to be some large negative value but it's very difficult to measure the concentrations of ATP, ADP, AMP, and inorganic phosphate inside cells.
The situation isn't entirely hopeless since there are some good estimates. Unfortunately the best examples come from rat hepatocytes and erythrocytes1. We don't know if this is typical of all cells (bacteria, plants etc.) or whether mammalian cells are special.
We can calculate the actual Gibbs free energy change for ATP hydrolysis (to ADP) given the known concentrations of reactants and products in rat hepatocytes. The answer is ΔG = -48 kJ mol-1. Thus, the actual Gibbs free energy change is 1½ times the standard Gibbs free energy change. This has important consequences when we try to figure out how ATP is synthesized and where that energy comes from.
There's an easy way to tell the difference between a good introductory biochemistry textbook and the other kind. Check out the standard Gibbs free energy change for the ATP → AMP + PPi reaction. If the value is close to -45 kJ mol-1 then it's probably a good textbook. If the book also mentions that the actual Gibbs free energy of the ATP → ADP + Pi reaction is close to -50 kJ mol-1 then it's almost certain to be a good textbook.
1. I put "high energy" in quotation marks because it's really the Gibbs free energy of the reaction (system) that we're referring to and not an individual molecule.
2. Rat liver biochemistry used to be very popular.
Alberty, R.A. and Goldberg, R.N. (1992) Standard thermodynamic formation properties of adenosie 5′-triphosphate series. Biochem. 31:10610-10615.