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

Stoichiometry and Energetics of Metabolic Transformations
Respiration
Respiratory Chain

Putting aside carbon compound METABOLISM for a moment, let us trace the reactions in which hydrogen atoms are oxidized to Water—the very process that primarily provides aerobic Cells with energy. In each single turn of the TCA cycle, four pairs of hydrogen atoms are released; three of these pairs are transferred to NAD, while one, generated during the dehydrogenation of succinic acid as shown in Fig. 5.7, is transferred to flavin adenine dinucleotide (FAD) (see Fig. 2.9). The reducing equivalents released in the TCA cycle are used partly in biosynthetic reactions and partly in the regeneration of ATP.

In our subsequent Structure/133.html">Discussion of ATP regeneration during Respiration, we will focus particularly on the situation where all the hydrogen atoms generated during glucose breakdown participate in the Respiratory Chain reactions. It is precisely this pathway that generates the bulk of The Cell's energy reserves in the form of ATP. Fig. 5.8 summarizes The sequence of reactions in the Respiratory Chain and their connection to the complete breakdown of Pyruvate via acetyl-CoA in the TCA (Krebs) cycle. In this diagram, the symbols FP₁ and FP₂ denote two different Flavoproteins, which are FAD-containing Enzymes involved in electron transport. Electrons from NADH (NADH = FP₁) are transferred to coenzyme Q (designated as Q in the figure); during this process, The transfer of each pair of electrons is accompanied by the phosphorylation of one molecule of ADP to ATP. Electrons released in the TCA cycle during succinate dehydrogenation are transferred via FAD (in FP₂) directly to coenzyme Q.

From this point onward, The Fate of all electrons is identical: they all interact with a series of Cytochromes—heme-containing Proteins designated in Fig. 5.8 by the symbols b, c, a, and a₃. During this interaction, each pair of electrons generates two molecules of ATP. The process of ATP regeneration in the respiratory chain is called Oxidative Phosphorylation.

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FIG. 5.8. Phosphorylation of ADP in the respiratory chain involving high-energy electrons released during The oxidation of CARBOHYDRATES, Fatty acids, and Amino Acids (FP₁ — NADH; FP₂ — succinate dehydrogenase; Q — coenzyme Q; b, c, a, and a₃ — respective cytochromes). [Reprinted from Lehninger A., Bioenergetics, 2d ed., p. 74, W. A. Benjamin, Inc., Palo Alto, CA, 1974.]

Ultimately, the hydrogen atoms react with dissolved oxygen, yielding water as the second end product of oxidation. By tracking the changes in Free energy at individual Stages of the respiratory chain, one can easily see that ATP is regenerated whenever the decrease in free energy exceeds the 7.3 kcal/mol required for ADP phosphorylation.

Thus, all reactions occurring in the respiratory chain are ultimately described by the following equations:

NADH + H⁺ + ¹/₂O₂ + 3ADP + 3P_i→ NAD⁺ + 4H₂O + 3ATP (5.20)

and

FADH₂ + ¹/₂O₂ + 2ADP + 2Pi → FAD + 3H₂O + 2ATP      (5.21)

The energy-supplying potential of respiration vastly exceeds that of Glycolysis, since the Standard Free Energy change ∆G⁰' for the reaction

Glucose + 6O₂ → 6СO₂ + 6Н₂O      (5.22)

is —686 kcal/mol. We can examine how efficiently this rich energy source is utilized in living aerobic organisms. For comparison with reaction (5.22), let us assume that glucose is completely oxidized to СO₂ and Н₂O via the EMP pathway, the TCA cycle, and the respiratory chain. However, we should keep in mind that the EMP pathway and the TCA cycle are amphibolic; consequently, some intermediates are constantly siphoned off into biosynthetic metabolic pathways. Therefore, glucose does not always undergo precisely these transformations, and the calculations below merely estimate the upper limit of ATP yield from glucose in an aerobic cell.

By adding equation (5.12) (the EMP pathway) to twice equation (5.19) (the TCA cycle), ten times equation (5.20) [oxidation of all NADH + H⁺ generated by the EMP pathway (twice) and the TCA cycle reactions (four times)], and twice equation (5.21) [oxidation of FADH₂], we obtain

С₆Н₁₂O₆ + 38ADР + 38Р_i + 6O₂ → 6СO₂ + 38АТР + 44Н₂O      (5.23)

Since the standard free energy change for ATP Hydrolysis is —7.3 kcal/mol, the free energy change for reaction (5.23) is approximately

∆G⁰' ≈ (38 mol ATP/mol glucose) (7.3 kcal/mol ATP) =

   =-277 kcal/mol glucose      (5.24)

This value is 19 times greater than the energy captured by the cell during glycolysis. As in glycolysis, the efficiency of THE RESPIRATORY PROCESS is very high:

When adjusted for non-standard intracellular concentrations, we arrive at a remarkably high efficiency exceeding 70%. Most of the energy not captured during ATP regeneration is dissipated as heat, which must be dissipated in one way or another to keep the Temperature within physiological limits tolerable for the cell.

Because chemical engines are not typically studied in practical Thermodynamics, an analogy with the classical example of gas compression in a piston cylinder may be helpful here. If a gas is compressed rapidly, a large amount of energy is lost as heat and thus cannot be harnessed during subsequent expansion. A completely analogous process is the highly inefficient combustion of air and glucose. Compression of a gas via very slow piston movement, which minimizes heat release, approximates an ideal reversible process. Similarly, processes in the living cell approach reversibility and maximum energy-extraction efficiency because glucose oxidation occurs in multiple stages, each characterized by a relatively small change in free energy.

The respiratory chain reactions described above are typical of a standard Introduction/5.html">Eukaryotic Cell. In prokaryotes, electron transport to oxygen may be coupled to the phosphorylation of fewer than three ADP molecules. Furthermore, the cytochrome pathway may vary somewhat depending on the prokaryotic species, and phosphorylation efficiency can be influenced by cell growth conditions. To evaluate the Efficiency of Energy capture in the respiratory chain, a special parameter is used: the P/O ratio, defined as the number of phosphorylation events per atom of oxygen consumed.

Later in this chapter, we will learn how the substrate oxidation state and the biosynthetic demands of the cell jointly impose certain constraints on the stoichiometry of amphibolic pathways. First, however, we will conclude our Overview of metabolic pathways as Energy Sources by briefly considering Photosynthesis—the energy-conversion process without which life on Earth would be impossible.