Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cell Bioenergetics
Why is the standard free energy of ATP hydrolysis relatively large?

What Structural Features of the ATP molecule account for the fact that the hydrolytic Cleavage of its terminal phosphate group releases considerably more Free energy than, for instance, the Hydrolysis of glucose-6-phosphate? To answer this question, one must consider The properties of both the substrate and the reaction products, because The change in Standard Free Energy is the difference between the Free energy of the starting Materials and that of the products. The magnitude of the standard free energy of hydrolysis of ATP is determined by three main structural factors. The first of these is the degree of dissociation of ATP itself and its hydrolysis products. At pH 7.0, ATP is almost completely ionized—that is, it exists as the ATP^4- anion. Hydrolysis of ATP yields not one, but three products: ADP^3-, HPO^4-₂, and H⁺. The overall equation for the hydrolysis of ATP is as follows:

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The standard state of the system is defined as one in which the concentrations of ATP^4-, ADP^3-, and HPO^4-₂ are all 1.0 M. However, the hydrogen ion concentration at pH 7.0 (the standard pH value for calculating ∆G⁰') is only 10^-7 M. Thus, the hydrogen ion concentration is extremely low compared with the standard concentrations of all other components (1.0 M); consequently, According to the law of mass action, at pH 7.0 the equilibrium of the ATP hydrolysis reaction must be strongly favored in the forward direction. In contrast, the hydrolysis of glucose-6-phosphate at pH 7.0 does not result in The formation of any appreciable amounts of H⁺ ions:

The second reason for the relatively large magnitude of ∆G⁰' for ATP hydrolysis is that at pH 7.0, ATP molecules bear four negative charges located quite close to one another, resulting in strong electrostatic repulsion between them (Fig. 14-2). When the terminal phosphoanhydride bond is cleaved during hydrolysis, this electrostatic strain within the ATP molecule is relieved through the spatial Separation of the negatively charged hydrolysis products, ADP^3- and HPO^4-₂. Because these products carry charges of the same sign, they have little tendency to recombine with each other to re-form an ATP molecule. The situation is quite different when glucose-6-phosphate undergoes hydrolysis. One of its products, glucose, is entirely uncharged. Consequently, no repulsive forces arise between glucose and the other hydrolysis product, the HPO^4-₂ ion, making their tendency toward recombination much stronger.

Finally, the third important reason for the large negative value of ∆G⁰' of ATP hydrolysis is that two of the reaction products, namely ADP^3- and HPO^4-₂, are Resonance hybrids—that is, structural forms characterized by enhanced stability because a fraction of their electrons occupy configurations of significantly lower energy than those in the intact ATP molecule. Therefore, when ATP is hydrolyzed, the electrons in the products ADP^3- and HPO^4-₂ can drop to lower energy levels than they occupied in the unhydrolyzed ATP molecule. As a result, the separation of the ADP^3- and HPO^4-₂ anions leads to a decrease in free energy compared to that possessed when they were combined as the ATP^4- anion.

High-energy phosphorylated compounds—that is, compounds whose hydrolysis is accompanied by a substantial decrease in standard free energy—are frequently described as containing a "high-energy phosphate bond" (denoted in structural formulas by the symbol ~). Although this term has long been used by biochemists, it is somewhat misleading. It can be misinterpreted to mean that the Energy is stored within the bond itself, which is not the case. It is well established that the rupture of any chemical bond requires an input of energy. The free energy released during the hydrolysis of phosphoric acid esters is not due to the breaking of a specific phosphate bond, but rather to the fact that the products of hydrolysis contain less free energy than the reactants. Nevertheless, the designation "high-energy phosphorylated compounds" remains entirely appropriate when applied to ATP and other phosphorylated compounds characterized by a high standard free energy of hydrolysis (∆G⁰').

It remains to mention one more very important circumstance regarding Free Energy Changes in biochemical reactions. Although under standard conditions the value of ∆G⁰' for ATP hydrolysis is —7.3 kcal/mol, the actual free energy change of ATP hydrolysis in intact Cells differs significantly from this value. This is because the concentrations of ATP, ADP, and P_i in living cells are, first, variable and, second, vastly lower than the standard concentration of 1.0 M. The true free energy change of ATP hydrolysis at nonstandard concentrations can be determined by calculation. Box 14-2 provides an example of the calculation of ∆G for ATP hydrolysis in intact erythrocytes based on the data in Table 14-4. This value, designated as ∆G_p, turns out to be substantially greater than ∆G⁰'; for most cells, it ranges from —12 to —16 kcal/mol. The quantity ∆G_p is frequently referred to as the phosphorylation potential; we will discuss it in more detail later.