hydrolysis , the bond b/w first 9 second
phosphoryl group yirlds energy
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Throughout this book we will refer to reactions or processes for which ATP supplies energy, and the contribution of ATP to these reactions will commonly be indicated as in Figure 13-8a, with a single arrow showing the conversion of ATP into ADP and Pi, or of ATP into AMP and PPi (pyrophosphate). When written this way, these reactions of ATP appear to be simple hydrolysis reactions in which water displaces either Pi or PPi, and one is tempted to say that an ATP-dependent reaction is "driven by the hydrolysis of ATP." This is not the case. ATP hydrolysis per se usually accomplishes nothing but the liberation of heat, which cannot drive a chemical process in an isothermal system.
Figure 13-8 The contribution of ATP to a reaction is often shown with a single arrow (a), but is almost always a two-step process, such as that shown here for the reaction catalyzed by ATP-dependent glutamine synthetase (b).
Single reaction arrows such as those in Figure 13-8a almost invariably represent two-step processes (Fig. 13-8b) in which part of the ATP molecule, either a phosphoryl group or the adenylate moiety (AMP), is first transferred to a substrate molecule or to an amino acid residue in an enzyme, becoming covalently attached to and raising the free-energy content of the substrate or enzyme. In the second step, the phosphate-containing moiety transferred in the first step is displaced, generating either Pi or AMP. Thus ATP participates in the enzymecatalyzed reaction to which it contributes free energy. There is one important class of exceptions to this generalization: those processes in which noncovalent binding of ATP (or of GTP), followed by its hydrolysis to ADP and Pi, provides the energy to cycle a protein between two conformations, producing mechanical motion, as in muscle contraction or in the movement of enzymes along DNA (discussed below).
The phosphate compounds found in living organisms can be divided arbitrarily into two groups, based on their standard free energies of hydrolysis (Fig. 13-9). "High-energy" compounds have a ΔG°' of hydrolysis more negative than -25 kJ/mol; "low-energy" compounds have a less negative ΔG°' ATP, for which ΔG°' of hydrolysis is -30.5 kJ/mol (-7.3 kcal/mol), is a high-energy compound; glucose-6-phosphate, with a standard free energy of hydrolysis of -13.8 kJ/mol (-3.3 kcal/mol), is a low-energy compound.
The term "high-energy phosphate bond," although long used by biochemists, is incorrect and misleading, as it wrongly suggests that the bond itself contains the energy. In fact, the breaking of chemical bonds requires an input of energy. The free energy released by hydrolysis of phosphate compounds thus does not come from the specific bond that is broken but results from the products of the reaction having a smaller free-energy content than the reactants. For simplicity, we will sometimes use the term "high-energy phosphate compound" when referring to ATP or other phosphate compounds with a large, negative, standard free energy of hydrolysis.