Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cellular Bioenergetics
As a result of the phosphate group transfer from ATP to an acceptor molecule, energy is imparted to that molecule.

ATP can now transfer its phosphate group to various acceptor molecules to yield low-energy phosphorylated compounds, primarily phosphoric acid esters (Table 14-5). These reactions are also catalyzed by Kinases. Hexokinase, for example, catalyzes The transfer of a phosphate group from ATP to D-glucose

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whereas glycerol kinase catalyzes the reaction

Fig. 14-8. Two low-energy phosphorylated compounds. They are ester derivatives formed by The addition of a phosphoric acid residue to a hydroxyl group.

In both cases, one of the hydroxyl groups of the acceptor molecule is phosphorylated to form a phosphoric acid ester (Fig. 14-8). Since The values of ∆G⁰' for the Hydrolysis of glucose-6-phosphate (∆G⁰' = -3.3 kcal/mol) and glycerol-3-phosphate (∆G⁰' = -2.2 kcal/mol) are less than the ∆G⁰' for ATP hydrolysis, both of these reactions proceed from left to right at standard initial reactant and substrate concentrations of 1.0 M, in accordance with the equations given above.

Glucose-6-phosphate and glycerol-3-phosphate contain more energy than free (unphosphorylated) glucose and glycerol. We can therefore view them as "energy-rich" (energized) forms of glucose and glycerol. They can participate in other enzymatic reactions where they serve as activated building blocks for the synthesis of larger molecules. Glucose-6-phosphate, for instance, acts as an activated precursor in Glycogen Biosynthesis, while glycerol-3-phosphate is used as an activated building block in Lipid Biosynthesis. Thus, part of the Free energy initially released during The breakdown of glucose to lactate—and stored in the form of 3-phosphoglyceryl phosphate and phosphoenolpyruvate—can be transferred to glycerol, glucose, and certain other phosphate acceptors, with ATP serving as an intermediate carrier of chemical energy in the form of phosphate groups.

Fig. 14-9. Transfer of phosphate groups from super-high-energy phosphorylated compounds (phosphate Donors) via ATP to various acceptor compounds, yielding low-energy phosphorylated derivatives of these molecules. This kinase-catalyzed Phosphate group transfer ultimately results in a net loss of free energy under cellular conditions. Creatine phosphate serves as a reserve store of high-energy phosphate groups in Muscle and Nerve Cells.

The enzymatic phosphate-transfer reactions occurring within The Cell are outlined in Fig. 14-9. An important feature of this transfer is that nearly all super-high-energy phosphorylated compounds donate their phosphate groups to low-energy phosphate acceptors via ATP, meaning the transfer takes place in two sequential steps; both steps are catalyzed by specific kinases.