ATP and Its Role in Bioenergetics

ATP is present in all living cells at concentrations of 10 3 to 10 2 mol L 1 of cell water. The ATP molecule is composed of three parts adenine, D-ribose, and three phosphate groups in ester linkages (Chap. 7). The analogous compounds containing one and two phosphate groups are designated AMP and ADP, respectively. 

ATP has a strong tendency to hydrolyze to ADP and phosphate this is predicted from thermodynamics  

Under biochemical standard-state conditions ([ATP4-] = [ADP3-] = [HPO4-] = 1 mol L-1, [H+] = 10 7molL and 7 =310K), the thermodynamic parameters for this reaction are AG° = -30.5 kJ mol-1, A//° = -20kJmol-1, and A5° = 34 J K-lmol-1. 

The enthalpy change for the reaction is favorable because (1) electrostatic repulsion between the negative charges in ATP exceeds that in the reaction products, (2) the reaction products are resonance stabilized, and (3) the enthalpies of solvation of the products are larger than that for ATP. The entropy change for the reaction is favorable because of the release of a phosphate group. Note that this implies that ATP hydrolysis is strongly temperature-dependent [cf. Eq. 10.7)]. 

ATP is not a high-energy compound in comparison with many other biological compounds. The functions of ATP depend on its having a AG value for hydrolysis that is intermediate in value compared with AG values for hydrolysis of other phosphate esters. Thus, ATP and ADP can act as a donor-acceptor pair for phosphoryl-group transfer. In many cases the free energy of ATP hydrolysis is used to support reactions that would otherwise be thermodynamically unfavorable. This usually occurs via phosphorylation of one of the reactants in an otherwise unfavorable reaction. 

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