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Reactions of the Citric Acid Cycle:- The Energy of Oxidations in the Cycle Is Efficiently Conserved

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  ح614-615

2026-06-11

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Reactions of the Citric Acid Cycle:- The Energy of Oxidations in the Cycle Is Efficiently Conserved

We have now covered one complete turn of the citric acid cycle (Fig. 16–13). A two-carbon acetyl group entered the cycle by combining with oxaloacetate. Two carbon atoms emerged from the cycle as CO2from the oxidation of isocitrate and -ketoglutarate. The energy released by these oxidations was conserved in the re duction of three NAD and one FAD and the production of one ATP or GTP. At the end of the cycle a molecule of oxaloacetate was regenerated. Note that the two carbon atoms appearing as CO₂ are not the same two carbons that entered in the form of the acetyl group; additional turns around the cycle are required to release these carbons as CO₂ (Fig. 16–7). Although the citric acid cycle directly generates only one ATP per turn (in the conversion of succinyl CoA to succinate), the four oxidation steps in the cycle provide a large flow of electrons into the respiratory chain via NADH and FADH2 and thus lead to formation of a large number of ATP molecules during oxidative phosphorylation. We saw in Chapter 14 that the energy yield from the production of two molecules of pyruvate from one molecule of glucose in glycolysis is 2 ATP and 2 NADH. In oxidative phosphorylation (Chapter 19), passage of two electrons from NADH to O₂ drives the formation of about 2.5 ATP, and passage of two electrons from FADH2 to O₂ yields about 1.5 ATP. This stoichiometry allows us to calculate the overall yield of ATP from the complete oxidation of glucose. When both pyruvate molecules are oxidized to 6 CO₂ via the pyruvate dehydrogenase com plex and the citric acid cycle, and the electrons are transferred to O₂ via oxidative phosphorylation, as many as 32 ATP are obtained per glucose (Table 16–1). In round numbers, this represents the conservation of 32×30.5 kJ/mol=976 kJ/mol, or 34% of the theoretical maximum of about 2,840 kJ/mol available from the complete oxidation of glucose. These calculations employ the standard free-energy changes; when corrected for the actual free energy required to form ATP within cells, the calculated efficiency of the process is closer to 65%.

FIGURE 16–13 Products of one turn of the citric acid cycle. At each turn of the cycle, three NADH, one FADH2, one GTP (or ATP), and two CO₂ are released in oxidative decarboxylation reactions. Here and in several following figures, all cycle reactions are shown as proceeding in one direction only, but keep in mind that most of the re actions are reversible (see Fig. 16–7).