Biochemistry - Chemical Reactions in Living Cells, Volume 1 - D. Metzler 1980

Bioenergetics of Biochemical Reactions
Tables of ∆G0 Values for Biochemical Compounds
Standard Free Energy of Combustion in the Presence of O2 and NAD+

Due to the exceptional importance of oxidation processes in the METABOLISM of aerobic organisms, it is often very useful to know the standard Free energy of combustion for a given compound. These data can be readily derived from the free energy of formation. For example, ∆G_c of acetate (aq) can be determined as follows:

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Table 3-3 lists The values of ∆G₂ alongside the values of ∆G⁰.

A great many cellular oxidation processes involve the specialized oxidizing agent nicotinamide adenine dinucleotide (NAD⁺) or its relative NADP⁺ (Chapter 8). Therefore, it is convenient to have tabulated values of ∆G⁰ for the Complete oxidation of compounds to CO₂ driven by NAD⁺ rather than O₂¹. These values, designated as ∆G⁰_ox and ∆G'_ox(pH 7), are also presented in Table 3-3. Note that these values are relatively small and, consequently, NAD⁺-linked oxidations yield little energy for The Cell. Consider, for instance, the following reaction:

¹ A similar approach was used by Decker et al. [22].

When the NADH generated from the reduction of NAD⁺ is subsequently reoxidized in Mitochondria [Eq. (3-58)], the cell captures a large amount of energy:

Equations (3-57) and (3-58) sum to the overall combustion of acetate in O₂, and the sum of the two ∆G values equals ∆G_c of acetate^-.

The values of ∆G_ox (Table 3-3) not only provide an immediate measure of the relative energy released during substrate oxidations involving NAD⁺, but they are also exceptionally convenient for estimating the ∆G of Fermentation reactions. Consider, for example, The conversion of glucose to ethanol:

The standard free energy change ∆G' (pH 7) for the fermentation of glucose to ethanol and CO₂ can be obtained directly from Table 3-3:

∆G'(pH7) =—243,8—2∙(—4,6) = —234,6 кДж∙моль^-1 (3-61)

(the ∆G_ox values for H₂O, CO₂, and H⁺ are defined as zero). The same calculation can be performed using ∆G⁰_c values (which is cumbersome because the numbers are excessively large) or ∆G⁰_f values. The latter are likewise quite large, and furthermore require incorporating data for CO₂ and Water into the calculations. Table 3-3 can be used to determine ∆G⁰ values for many of the metabolic reactions discussed later. Data from any Column of the table are suitable for this purpose, but for simplicity, it is best to use the ∆G'_ox values.

Note, however, that for oxidations driven by oxygen or any oxidant other than NAD⁺ (and not included in Table 3-3), the ∆G_ox values cannot be used directly. In such cases, the Procedure is as follows: first, one calculates ∆G⁰ or ∆G' for the reaction under consideration assuming NAD⁺ is the oxidant, and then adds the free energy of oxidation of the resulting NADH by oxygen (or another oxidant). Such values for O₂ are listed in Table 3-7; based on these data, one can also estimate the corresponding values for A number of other oxidants, such as Fe³⁺ and cytochrome c.