Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Energetics of Biochemical Reactions
Tables of ∆Gº Values for Biochemical Compounds
Role of Metal Ions

Both ATP, ADP, and inorganic pyrophosphate form complexes with Metal Ions. Under physiological conditions, magnesium complexes of ADP and ATP predominate; let us examine the equations for the corresponding reactions along with the changes in Standard Free energy and enthalpy [12] (apparent values for 25 °C and μ = 0.2 are given in kJ∙mol^-1).

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Using the apparent values of ∆G⁰ from equations (3-53) and (3-47), we obtain

The stoichiometry of equation (3-55) is never exactly fulfilled; some amount of Mg²⁺ is released into the solution As a result of MgADP^- dissociation; both protons and Mg²⁺ ions bind to H₂РО^-₄; the medium also contains HATP^3- and HADP^2-, which bind Mg²⁺ weakly [13]. Thus, the observed values of ∆G_obs vary with changes in pH, magnesium concentration, and Ionic strength. Albery [13] and Phillips et al. [14] compiled tables and graphs of apparent ∆G_obs values for various conditions. Fig. 3-2 shows one such plot, constructed using Albery's constants and equations¹. The ∆G_obs determined from this graph for ATP Hydrolysis at pH 7, 25 °C, μ = 0.2, and 1 mM Mg²⁺ (a relatively high concentration [15]) is —30.35 kJ∙mol^-1 (—7.25 kcal∙mol^-1).

We can see that complex formation with Mg²⁺ slightly decreases the phosphoryl group transfer potential of ATP. A significant effect can be exerted by the temporal fluctuations of Mg²⁺ concentration, as well as its spatial heterogeneity within The Cell [16]. In Tissues, magnesium is the major, though by far not the only, cation. The equilibria of polyphosphates (such as ATP) are noticeably influenced by Ca²⁺, Mn²⁺, and even K⁺.

¹ However, the value of ∆G' for ATP hydrolysis = 35.54 kJ∙mol^-1 at [Mg²⁺] = 0 was obtained from the data of [11]. The pK_a values and formation constants for Mg²⁺ complexes were taken from Albery [13]. It is interesting to note that George et al. [15] determined formation constants for these same complexes at infinite dilution and obtained much more negative ∆G⁰_f values than those given in equations (3-53) —(3-55).

FIG. 3-2. pH dependence of the apparent free energy ∆G_obs of ATP hydrolysis at various concentrations of free magnesium ions (numbers by the curves indicate the Mg²⁺ concentration in mM). The curves were computer-generated and are reproduced with the kind permission of Carol M. Harris.