Acid Cycle and Oxidative Phosphorylation

The citric acid cycle, also called the Krebs cycle or the tricarboxylic add (TCA) cyde, is in the mitochondria. Although oxygen is not directly required in the cyde, the pathway will not occur anaerobically because NADH and FADH will accumulate if oxygen is not available for the electron transport chain. 

The primary function of the dtric acid cycle is oxidation of acetyl CoA to carbon dioxide. The energy released from this oxidation is saved as NADH, FADHj, and guanosine triphosphate (GTP). The overall result of the cyde is represented by the following reaction  

Notice that none of the intermediates of the citric add cyde appear in this reaction, not as reactants or as products. This emphasizes an important (and frequently misunderstood) point about the cycle. It does not represent a pathway for the net conversion of acetyl CoA to citrate, to malate, or to any other intermediate of the cyde. The only fate of acetyl CoA in this pathway is its oxidation to CO,. Therefore, the dtric acid cycle does not represent a pathway by which there can be net synthesis of glucose from acetyl CoA 

The cyde is central to the oxidation of any fuel that yields acetyl CoA, induding glucose, fritty acids, ketone bodies, ketogenic amino acids, and alcohol There is no hormonal control of the cyde, as activity is necessary irrespective of the fed or fasting state. Control is exerted by the energy status of the cell. 

Isocitrate dehydrogenase, the major control enzyme, is inhibited by NADH and activated 

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The combustion of the acetyl groups of acetyl-CoA by the citric acid cycle and oxidative phosphorylation to produce COg and HgO represents stage 3 of catabolism. The end products of the citric acid cycle, COg and HgO, are the ultimate waste products of aerobic catabolism. As we shall see in Chapter 20, the oxidation of acetyl-CoA during stage 3 metabolism generates most of the energy produced by the cell. 

FIGURE 18.16 Compartmentalization of glycolysis, the citric acid cycle, and oxidative phosphorylation. 

Coordinate Regulation of the Citric Acid Cycle and Oxidative Phosphorylation... 

In eukaryotes, the cytoplasm, representing slightly more than 50% of the cell volume, is the most important cellular compartment. It is the central reaction space of the cell. This is where many important pathways of the intermediary metabolism take place—e.g., glycolysis, the pentose phosphate pathway, the majority of gluconeogenesis, and fatty acid synthesis. Protein biosynthesis (translation see p. 250) also takes place in the cytoplasm. By contrast, fatty acid degradation, the tricarboxylic acid cycle, and oxidative phosphorylation are located in the mitochondria (see p. 210). 

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... 

Carbohydrate metabolism in a typical plant cell is more complex in several ways than that in a typical animal cell. The plant cell carries out the same processes that generate energy in animal cells (glycolysis, citric acid cycle, and oxidative phosphorylation) it can generate hexoses from three- or four-carbon compounds by glu-coneogenesis it can oxidize hexose phosphates to pentose phosphates with the generation of NADPH (the ox-... 

Figure 20-8 Perspective of the metabolic scheme whereby carbohydrates, fats, and proteins in foodstuffs are oxidized to C02, showing the link between glycolysis, the citric acid cycle, and oxidative phosphorylation...

Glycolysis is a set of reactions that take place in the cytoplasm of prokaryotes and eukaryotes. The roles of glycolysis are to produce energy (both directly and by supplying substrate for the citric acid cycle and oxidative phosphorylation) and to produce intermediates for biosynthetic pathways. 

Answer Lactate and alanine are converted to pyruvate by their respective dehydrogenases, lactate dehydrogenase and alanine dehydrogenase, producing pyruvate and NADH + H+ and, in the case of alanine, NH. Complete oxidation of 1 mol of pyruvate to C02 and H20 produces 12.5 mol of ATP via the citric acid cycle and oxidative phosphorylation (see Table 16-1). In addition, the NADH from each dehydrogenase reaction produces 2.5 mol of ATP per mole of NADH reoxidized. Thus oxidation produces 15 mol of ATP per mole of lactate. Urea formation uses the equivalent of 4 mol of ATP per mole of urea formed (Fig. 18-10), or 2 mol of ATP per mol of NH4. Subtracting this value from the energy yield of alanine results in 13 mol of ATP per mole of alanine oxidized. 

How many molecules of ATP are produced per molecule of (a) pyruvate, (b) NADH, (c) glucose, and (d) phosphoenolpyruvate, by a cell homogenate in which glycolysis, the citric acid cycle, and oxidative phosphorylation are all completely active ... 

In the third stage, ATP is producedfrom the complete oxidation of the acetyl unit of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation offuel molecules. Acetyl CoA brings acetyl units into the citric acid cycle [also called the tricarboxylic acid (TCA) cycle or Krebs cycle], where they are completely oxidized to CO2. Four pairs of electrons are transferred (three to NAD+ and one to FAD) for each acetyl group that is oxidized. Then, a proton gradient is generated as electrons flow from the reduced forms of these carriers to O2, and this gradient is used to synthesize ATP. 

In the third stage, ATP is produced from the complete oxidation of the acetyl unit of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules. 

Oxidative phosphorylation is the process by which NADH and FADHj are oxidized and ATP is produced. Two molecules of ATP are produced when FADHj is oxidized, and three molecules of ATP are produced when NADH is oxidized. The complete oxidation of one glucose molecule by glycolysis, the citric acid cycle, and oxidative phosphorylation yields thirty-six molecules of ATP versus two molecules of ATP for anaerobic degradation of glucose by glycolysis and fermentation. 

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