The Role of Water in ATP Hydrolysis Energy Transduction by Protein Machinery by Makoto Suzuki

Author:Makoto Suzuki
Language: eng
Format: epub, pdf
Publisher: Springer Singapore, Singapore

12.2.3 The Free Energy of ATP Hydrolysis and the Mechanism of Its Generation

12.2.3.1 ATP Hydrolysis in Solution

ATP is the “molecular unit of currency” of intracellular energy transfer (Knowles 1980). When the phosphate–phosphate bonds (phospho-anhydride bonds) in ADP and ATP are hydrolyzed, a free energy of 7.3 kcal/mol is released at a standard state of 1 M (Netlson and Cox 2005). This bond is therefore called the high-energy phosphate bond. Molecular machines, such as ABC transporters and motor proteins (e.g., F1–Fo motor, myosin, kinesin), use ATP as an energy source. According to standard biochemistry textbooks, the following three factors are responsible for the “high-energy” character of the phospho-anhydride bond: (1) the resonance stabilization effect, (2) electrostatic repulsions, and (3) the hydration effect. It has been hypothesized that the third factor favors ATP hydrolysis due to the greater degree of solvation (hydration) of the Pi and ADP products relative to that of ATP, which further stabilizes the products relative to the reactants. However, quantum chemical calculations combined with a theory of solution, such as the continuum model (Colvin et al. 1995; Hayes et al. 1978), 3D-RISM (Yoshida 2014), and energy representation (Takahashi et al. 2017), have provided more accurate insight into this effect.

Table 12.1 shows the hydrolysis energy of pyrophosphate as a model of ATP in different ionized conditions. The second and third columns indicate the data from Colvin et al. (1995) and this work, respectively. In both studies, the solvent effect was evaluated using a polarizable continuum model (PCM). Our data accurately reproduce the experimental data as a whole, although the hydrolysis energy of reaction 1 was overestimated. Table 12.2 shows the decomposition of the hydrolysis energy into the intra- and intermolecular energies. The total hydrolysis energy is governed by the electronic energy and the electrostatic solvation energy, while the contributions of thermal and non-electrostatic solvation energies are small. The electronic energy and electrostatic solvation energy tended to cancel each other out, resulting in a small exothermic (negative) hydrolysis energy of ~10 kcal/mol. Even in an intrinsically endothermic reaction (positive contribution of the electronic energy), such as reaction 2, the total reaction energy was ultimately exothermic due to the hydration effect. For the extremely exothermic reactions, such as reactions 3–5, the excessive reaction energies were reduced by hydration to ~10 kcal/mol. As a result, the moderately exergonic ATP hydrolysis reaction becomes capable of directly coupling with energetically unfavorable (endergonic) reactions in cells.Table 12.1Hydrolysis energy ΔG0 (kcal/mol) of pyrophosphate

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