Phosphoryl Group Transfers and ATP:- The Free-Energy Change for ATP Hydrolysis Is Large and Negative
Figure 13–1 summarizes the chemical basis for the relatively large, negative, standard free energy of hydrolysis of ATP. The hydrolytic cleavage of the terminal phosphoric acid anhydride (phosphoanhydride) bond in ATP separates one of the three negatively charged phosphates and thus relieves some of the electrostatic repulsion in ATP; the Pi (HPO4-2 ) released is stabilized by the formation of several resonance forms not possible in ATP; and ADP2 , the other direct product of hydrolysis, immediately ionizes, releasing H into a medium of very low [H ] (~10-7 M). Because the concentrations of the direct products of ATP hydrolysis are, in the cell, far below the concentrations at equilibrium (Table 13–5), mass action favors the hydrolysis reaction in the cell.
Although the hydrolysis of ATP is highly exergonic (ΔGo=-30.5 kJ/mol), the molecule is kinetically stable at pH 7 because the activation energy for ATP hydrolysis is relatively high. Rapid cleavage of the phosphoanhydride bonds occurs only when catalyzed by an enzyme.
The free-energy change for ATP hydrolysis is -30.5kJ/mol under standard conditions, but the actual free energy of hydrolysis (ΔG) of ATP in living cells is very different: the cellular concentrations of ATP, ADP
FIGURE 13–1 Chemical basis for the large free-energy change associated with ATP hydrolysis. 1The charge separation that results from hydrolysis relieves electrostatic repulsion among the four negative charges on ATP. 2The product inorganic phosphate (Pi) is stabilized by formation of a resonance hybrid, in which each of the four phosphorus–oxygen bonds has the same degree of double-bond character and the hydrogen ion is not permanently associated with any one of the oxygens. (Some degree of resonance stabilization also occurs in phosphates involved in ester or anhydride linkages, but fewer resonance forms are possible than for Pi.) 3The product ADP2- immediately ionizes, releasing a proton into a medium of very low [H+] (pH 7). A fourth factor (not shown) that favors ATP hydrolysis is the greater degree of solvation (hydration) of the products Piand ADP relative to ATP, which further stabilizes the products relative to the re actants.
and Pi are not identical and are much lower than the 1.0M of standard conditions (Table 13–5). Furthermore, Mg2 in the cytosol binds to ATP and ADP (Fig. 13–2), and for most enzymatic reactions that involve ATP as phosphoryl group donor, the true substrate is MgATP2-. The relevant ΔG0 is therefore that for MgATP2 hydrolysis. Box 13–1 shows how ΔG for ATP hydrolysis in the intact erythrocyte can be calculated from the data in Table 13–5. In intact cells, ΔG for ATP hydrolysis, usually designated ΔGp, is much more negative than
ΔG0, ranging from 50 to 65 kJ/mol. ΔGp is often called the phosphorylation potential. In the following discussions we use the standard free-energy change for ATP hydrolysis, because this allows comparison, on the same basis, with the energetics of other cellular reactions. Remember, however, that in living cells ΔG is the relevant quantity—for ATP hydrolysis and all other reactions—and may be quite different from ΔG0 .
