Citric acid cycle stoichiometry

TABLE 16-1 Stoichiometry of Coenzyme Reduction and ATP Formation in the Aerobic Oxidation of Glucose via Glycolysis, the Pyruvate Dehydrogenase Complex Reaction, the Citric Acid Cycle, and Oxidative Phosphorylation... 

Because the carbon atoms of acetate molecules that enter the citric acid cycle appear eight steps later in oxaloacetate, it might seem that this pathway could generate oxaloacetate from acetate and thus generate phosphoenolpyruvate for gluconeogenesis. However, as an examination of the stoichiometry of the citric acid cycle shows, there is no net conversion of acetate to ox-... 

No. While oxygen is not involved in any of the steps of the citric acid cycle, C02 is liberated at two of them, giving rise to part of the stoichiometry of respiration. 

We can now estimate how many molecules of ATP are formed when glucose is completely oxidized to GO2. The number of ATP (or GTP) molecules formed in glycolysis and the citric acid cycle is unequivocally known because it is determined by the stoichiometries of chemical reactions. In contrast, the AT P yield of oxidative phosphorylation is less certain because the stoi chiometries of proton pumping, ATP synthesis, and metabolite-transport processes need not be integer numbers or even have fixed values. As stated earlier, the best current estimates for the number of protons pumped out of the matrix by NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase per electron pair are four, two, and four, respectively. The synthesis of a molecule of ATP is driven by the flow of about three protons through ATP synthase. An additional proton is consumed in transporting ATP from the matrix to the cytoplasm. Hence, about 2.5 molecules of cytoplasmic ATP are generated as a result of the flow of a pair of electrons from NADH to O2. For electrons that enter at the level of Q-cytochrome c oxidoreductase, such as those from the oxidation of succinate or cytoplasmic NADH, the yield is about 1.5 molecules of ATP per electron pair, Hence, as tallied in Table 18.4, about 30 molecules of ATP are formed... 

The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the final oxidative pathway for carbohydrates, lipids, and amino acids. It is also a source of precursors for biosynthesis. The authors begin Chapter 17 with a detailed discussion of the reaction mechanisms of the pyruvate dehydrogenase complex, followed by a description of the reactions of the citric acid cycle. This description includes details of mechanism and stereospecificity of some of the reactions, and homologies of the enzymes to other proteins. In the following sections, they describe the stoichiometry of the pathway including the energy yield (ATP and GTP) and then describe control mechanisms. They conclude the chapter with a summary of the biosynthetic roles of the citric acid cycle and its relationship to the glyoxylate cycle found in bacteria and plants. 

The transformation of pyruvate to carbon dioxide is achieved by the several steps in a cyclical series of reactions known as the tricarboxylic acid (TCA) cycle. The name of the cycle comes from the first step where acetyl-CoA is condensed with oxaloacetic acid to form citric acid, a tricarboxylic acid. Once citrate is formed the material is converted back to oxaloacetate through a series of 10 reactions, as illustrated in Fig. 5.22, with the net production of 2 molecules of carbon dioxide and reducing equivalents in the form of 4 molecules of NADH + H and 1 molecule of FADH2, together with 1 mole of ATP. The overall stoichiometry of the TCA cycle from pyruvate is ... 

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