The tricarboxylic acid cycle, and electron transport

After the glycolysis of carbohydrates and the )3-oxidation of fats has taken place. 

Contrary to what was formerly believed, all bacteria and yeasts use the tricarboxylic acid cycle as the major pathway for terminal oxidation. A small shunt in the cycle is made by some bacteria (e.g. E. coli and Pseudomonas aeruginosa) and fungi (e.g. Aspergillus and yeasts) as follows, /yocitrate is dis-mutated to succinate and glyoxylate the latter is then dimerized to malate which, like succinate, is a normal constituent of the cycle. The bloodstream form of many parasitic trypanosomes, which lack mitochondria, have no tricarboxylic acid cycle. 

The operation of the cycle produces large quantities of the reduced forms of the nucleotides of adenine with nicotinamide (NAD and NADP) and ribo-flavine (FP). The regeneration of these coenzymes is effected by a transfer of electrons from the reduced forms to the oxygen of the atmosphere. In almost every kind of living cell, this transfer is mediated by some or all of the cytochrome respiratory chain (Section 5.4.3). Most of the organisms that lack all cytochromes have insignificant aerobic metabolism. Few enzymatic differences in the cycle have been demonstrated in mammals, but in the rat there is six times more aconitate hydratase in the heart than in skeletal muscle (Dixon and Webb, 1979). 

Rotenone (4.61) an insecticide of vegetable origin, blocks the dehydrogenation of NADH in the respiratory chain, at a dilution of 10 M, by displacing ubiquinone from NADH dehydrogenase (Gutman et al., 1971). It thus prevents the oxidation of pyruvate and glutamate (but not succinate). Fish, but 

Good selectivity is shown by a soil fungicide, sodium / -dimethylamino-benzenediazosulfonate ( Dexon ), which inhibits mitochondrial oxidation of NADH in the fungus Pythium ultimum. Sugar beets, which this fungus infects, have an enzyme in the mitochondria which decomposes this fungicide (Tolmsoff, 1962). 

The operation of the cycle produces large quantities of the reduced forms of the nucleotides of adenine with nicotinamide (NAD and NADP) and riboflavine (FP). The regeneration of these coenzymes is effected 

Inhibitors of the respiratory chain. This chain terminates in cytochrome oxidase for which the cyanide ion is a powerful and specific inhibitor. Antimycin A, obtained from a Streptomyces specifically inhibits the chain between cytochromes-b and -c. Its iron-chelating properties are mentioned under salicylic acid in Section 11.9. 

The point of attack of trialkyItin salts (R Sn ), which are highly active fungicides, is believed to lie in the oxidative phosphorylation of mitochondria (Aldridge, 1958). The tributyl homologue is the mostused because it is the least toxic for mammals (Barnes and Stoner, 1958). 

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Metabolic Strategies 227 Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway 242 The Tricarboxylic Acid Cycle 282 Electron Transport and Oxidative Phosphorylation 305 Photosynthesis 330 Structures and Metabolism of Oligosaccharides and Polysaccharides 356... 

The reticulocyte differs from the erythrocyte in that (1) it contains small amounts of DNA (2) it contains amounts of RNA nearly equal to those in the stem cell (3) it contains intact tricarboxylic acid cycle and electron transport chain and (4) it can synthesize protein, but at a rate much slower than that observed in the stem cell. Some other enzymes that are also present in erythrocytes have been claimed to have higher activities in reticulocytes. Such is the case for glucose-6-phosphate dehydrogenase, 6-phosphoglu-conic dehydrogenase, phosphohexose isomerase, transketolase, aldolase, and lactic dehydrogenase. 

We now explore the remarkable process by which a long-chain saturated fatty acid is converted into two-carbon units (acetate), which can be oxidized to C02 and H20 via the tricarboxylic acid cycle and the electron-transport chain. Fatty acids that enter cells are activated to their CoA derivatives by the enzyme acyl-CoA ligase and transported into the mitochondria for /3 oxidation as we discuss later in this chapter. 

By the mid-1950s, therefore, it had become clear that oxidation in the tricarboxylic acid cycle yielded ATP. The steps had also been identified in the electron transport chain where this apparently took place. Most biochemists expected oxidative phosphorylation would occur analogously to substrate level phosphorylation, a view that was tenaciously and acrimoniously defended. Most hypotheses entailed the formation of some high-energy intermediate X Y which, in the presence of ADP and P( would release X and Y and yield ATP. A formulation of the chemical coupling hypothesis was introduced by Slater in 1953,... 

The metabolic machinery responsible for the heterotrophic respiration reactions is contained in specialized organelles called mitochondria. These reactions occur in three stages (1) glycolysis, (2) the Krebs or tricarboxylic acid cycle, and (3) the process of oxidative phosphorylation also known as the electron transport chain. As illustrated in... 

Marine organisms concentrate metals in their tissues and skeletal materials. Many of these trace metals are classified as micronutrients because they are required, albeit in small amounts, for essential metabolic functions. Some are listed in Table 11.4, illustrating the role of metals in the enzyme systems involved in glycolysis, the tricarboxylic acid cycle, the electron-transport chain, photosynthesis, and protein metabolism. These micronutrients are also referred to as essential metals and, as discussed later, have the potential to be biolimiting. 

This type of effect can occur in all tissues and is caused by a metabolic inhibitor such as azide or cyanide, which inhibits the electron transport chain. Inhibition of one or more of the enzymes of the tricarboxylic acid cycle such as that caused by fluoroacetate (Fig. 6.7) also results in inhibition of cellular respiration (for more details of cyanide and fluoroacetate see chap. 7). 

Pyruvate enters the tricarboxylic acid cycle after conversion into acetyl-CoA via the PDHC. The tricarboxylic acid cycle generates NADH from NAD+, and the NADH then enters the mitochondrial electron-transport chain. 

The tricarboxylic acid (TCA) cycle (also known as the citric acid cycle and the Krebs cycle) is a collection of biochemical reactions that oxidize certain organic molecules, generating CO2 and reducing the cofactors NAD and FAD to NADH and FADH2 [147], In turn, NADH and FADH2 donate electrons in the electron transport chain, an important component of oxidative ATP synthesis. The TCA cycle also serves to feed precursors to a number of important biosynthetic pathways, making it a critical hub in metabolism [147] for aerobic organisms. Its ubiquity and importance make it a useful example for the development of a kinetic network model. 

Actively respiring fungal cells possess a distinct mitochondrion, which has been described as the power-house of the cell (Fig. 4.2). The enzymes of the tricarboxylic acid cycle (Kreb s cycle) are located in the matrix of the mitochondrion, while electron transport and oxidative phosphorylation occur in the mitochondrial inner membrane. The outer membrane contains enzymes involved in lipid biosynthesis. The mitochondrion is a semiindependent organelle as it possesses its own DNA and is capable of producing its own proteins on its own ribosomes, which are referred to as mitoribosomes. 

Oxidation The series of biochemical reactions in which fatty acids are degraded to acetyl-CoA, which then enters the tricarboxylic acid cycle for the production of energy in the form of reducing equivalents and GTP. Each round of p-oxidation shortens the fatty acid by two carbons and, in addition to acetyl-CoA, produces NADH and FADH, which are fed into the electron transport system for the production of ATP. 

When yeast cells grow aerobically, they can oxidize the pyruvate entirely to carbon dioxide and water. These reactions take place in structures called mitochondria. The reactions form a cyclic scheme termed the tricarboxylic acid cycle (TCA cycle) or Krebs cycle in which in one cycle, the substrate, pyruvate, is converted entirely to carbon dioxide and water (Fig. 3-10). Both substrate level phosphorylation and electron transport mediated phosphorylation occur during the process. In electron... 

Fig. 3-10 The biochemical pathway of the tricarboxylic acid cycle (TCA cycle). Pyruvate, generated from glycolysis (Fig. 3-8), enters the cycle as acetyl-CoA (acetyl-coenzyme A), and is then degraded through a series of reactions to a 4-carbon compound, oxaloacetate. The energy resulting from the reactions is stored as ATP, which is produced through electron transport and oxidative phosphorylation (see Fig. 3-11).

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