Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The ATP Cycle and Cellular Bioenergetics
The breakdown of glucose into lactate produces two ultra-high-energy phosphorylated compounds

Two compounds play a crucial role as phosphate group Donors for ADP: 3-phosphoglyceroyl phosphate and phosphoenolpyruvate (Table 14-5). Both of these compounds are formed during the energy-yielding breakdown of glucose to lactate (Fig. 14-5). We will discuss this process, known as Glycolysis, in detail in the next chapter. A significant portion of the Free energy released during the degradation of glucose to lactate is conserved through The formation of 3-phosphoglyceroyl phosphate and phosphoenolpyruvate. Within The Cell, these high-energy phosphorylated compounds do not undergo Hydrolysis; instead, their phosphate groups are transferred to ADP via specific Kinases, resulting in the formation of ATP. For 3-phosphoglyceroyl phosphate (Fig. 14-6), this phosphate transfer reaction, catalyzed by phosphoglycerate kinase, can be represented as follows:

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Fig. 14-5. Two Types of high-energy phosphorylated compounds formed as intermediates during the exergonic breakdown of glucose to lactate. Both compounds are capable of transferring their phosphate group to ADP to form ATP.

In this reaction, only one of the two phosphate groups of phosphoglyceroyl phosphate is transferred to ADP—specifically, the one attached to the carbon of the carboxyl group. (The Cleavage of the second phosphate group, at C-3, is characterized by a small value of ∆G⁰'.) Although this kinase reaction is reversible, under standard conditions its equilibrium lies far to the right, because the ∆G⁰' of hydrolysis of 3-phosphoglyceroyl phosphate (-11.8 kcal/mol) is more negative than the ∆G⁰' of hydrolysis of ATP (-7.3 kcal/mol).

Phosphoenolpyruvate—the second high-energy phosphorylated compound generated during The breakdown of glucose to lactate—also donates its phosphate group to an ADP molecule in a similar reaction (Fig. 14-7) catalyzed by Pyruvate kinase.

Fig. 14-6. Transfer of a phosphate group from 3-phosphoglyceroyl phosphate to ADP.

The equilibrium of this reaction under standard conditions is likewise shifted to the right, since the ∆G⁰' of hydrolysis of phosphoenolpyruvate (-14.8 kcal/mol) is more than double the ∆G⁰' of hydrolysis of ATP. In the cell, this reaction is irreversible. Thus, both phosphoenolpyruvate and 3-phosphoglyceroyl phosphate—two compounds that capture a substantial fraction of the chemical energy released during the anaerobic breakdown of glucose—can transfer a significant share of their energy to ADP molecules, driving the synthesis of ATP.

Fig. 14-7. Transfer of a phosphate group from phosphoenolpyruvate to ADP.