Fundamentals of Biochemical Engineering, Part 1 - Bailey, J., & Ollis, D. 1989

Stoichiometry and Energetics of Metabolic Transformations
Interconnectedness of metabolic reactions; ATP and NAD
ATP and other phosphates

In Chapters 3 and 4, we examined The kinetics of reactions in systems with isolated Enzymes, as well as the Structure/179.html">Practical Applications OF free enzymes and enzyme preparations. Most of the studies on the Kinetics of Enzymatic catalysis mentioned in Chapter 3, and a large portion of the practical Examples of enzyme use given in Chapter 4, pertain to Hydrolases. By definition, these enzymes cleave or break down large molecules into smaller ones using Water as a second substrate. Such degradation processes are accompanied by a decrease in the Free energy of the system and therefore occur spontaneously in a closed system. In this section, we will discuss the mechanisms that enable an open system—specifically, a living Cell—to couple energy-releasing (exergonic) reactions with reactions and other Functions that require a specific input of energy (endergonic processes).

Earlier, we mentioned the structure and certain properties of adenosine triphosphate (ATP; see Section 2.3.1 and Fig. 2.8), noting in particular that the Enzymatic Hydrolysis of ATP to ADP and inorganic phosphate is accompanied by a large negative change in free energy:

Class="center">АТР + Н₂O → ADP + P_i ∆G°' = -7,3 ккал/моль (5.11)

Here, P_i denotes inorganic phosphate. Thus, a large amount of energy is released during ATP hydrolysis, whereas in the reverse reaction—the attachment of a phosphate group to ADP—energy can be captured and stored as ATP for subsequent use. Let us examine how this latter process is realized under conditions of coupled Chemical Reactions.

METABOLISM/2.html">THE CONCEPT OF a common intermediate can be illustrated using another sequence of reactions that forms part of the Embden-Meyerhof-Parnas metabolic pathway. The conversion of an aldehyde into a carboxylic acid (in an aqueous solution) is accompanied by a free energy decrease of approximately 7,000 cal/mol. As shown in Fig. 5.3, this free chemical energy would be entirely dissipated into the solution in an isolated system. In a living cell, however, this does not happen. During the biochemical oxidation of glucose, the conversion of glyceraldehyde-3-phosphate to the corresponding carboxylic acid (3-phosphoglycerate) is accompanied by the regeneration of ATP from ADP (see reaction 2 in Fig. 5.3). Thus, the decrease in free energy resulting from The oxidation of the aldehyde is coupled with the simultaneous regeneration of ATP through the participation of The Cell's enzymatic system.

1) Oxidation of an aldehyde to a carboxylic acid in an isolated system (in aqueous solution)

RCHO + H₂O → 2H + RCOO^- + H⁺ ∆G₁⁰' ≈ -7 ккал/моль

2) The same reactions coupled with ATP regeneration (glucose oxidation)

RCHO + HPC4^2- + ADP^3- → 2H + RCOO^- + ATP^4- ∆G₂⁰' ≈ 0 ккал/моль

3) Subtracting reaction 1 from reaction 2, we obtain

АDР^3- + НРС4^2- + Н⁺ → АТР^4- + Н₂O ∆G⁰' ≈ +7 ккал/моль

4) Elementary stages of reaction 2:

FIG. 5.3. Examples of reactions with a common intermediate.

Since there is virtually no change in free energy As a result of reaction 2, the free energy released during the oxidation of glyceraldehyde-3-phosphate is evidently transformed into the so-called high-energy (macroergic) phosphate bond of adenosine triphosphate.

The sequence of elementary reactions that actually takes place within the cell is depicted in the lower part of Fig. 5.3. The most essential characteristics of these last two reactions include, firstly, the presence of a common intermediate that is the product of the first reaction and the reactant In the second, and, secondly, the large free energy of hydrolysis (Cleavage of the phosphate group) of the intermediate phosphorylated compound compared to the hydrolysis of ATP. The equilibrium of The final stage is shifted to the right—that is, toward The formation of the reaction product. Thus, this phase of glucose metabolism simultaneously serves as one of several regeneration centers for ATP, which is required for endergonic reactions. ATP regeneration is accomplished by converting a partially metabolized nutrient into a high-energy phosphorylated intermediate, which then transfers its phosphate group to ADP in an enzyme-catalyzed reaction.

The phosphorylation of various compounds, including ADP, serves several purposes for the cell. First, phosphorylated compounds provide a convenient storage depot for a significant portion of the free energy released during the oxidation of cellular fuel. The free energy of hydrolysis for certain compounds, known as phosphate Donors, exceeds the ∆G⁰' of ATP hydrolysis (for example, the ∆G⁰' values for the hydrolysis of phosphoenolpyruvate and 1,3-diphosphoglycerate are —14.8 and —11.8 kcal/mol, respectively). Consequently, the hydrolysis of these compounds can be used to phosphorylate ADP. Similarly, ATP hydrolysis can be coupled with the phosphorylation of certain compounds to form "low-energy" phosphates; the ∆G⁰' cleavage values for the latter are lower than the corresponding value for the ATP hydrolysis reaction (for instance, the ∆G⁰' of hydrolysis for glucose-6-phosphate and glycerol-1-phosphate are —3.3 and —2.20 kcal/mol, respectively).

The second crucial function of phosphorylation in cellular life is related to the inability of highly ionized organic substances to cross cell membranes. Because of this circumstance, charged phosphorylated compounds—which act as intermediates in numerous metabolic pathways—remain trapped inside the cell, which in turn makes it possible to extract the maximum amount of energy and chemical building blocks from nutrients. Typically, dephosphorylation is the final stage of a metabolic pathway; unionized cellular waste products can then be expelled into the surrounding environment.