Aerobic cells

Fig. 7.7. The proposed free metallome profile of the cytoplasm of the aerobic cell (8) and of the combined metallome of the organism . P represents pump. Compare Figs. 5.3 and 4.2.

The O2 molecule is essential to all aerobic forms of life, but many anaerobic organisms (e.g. anaerobic bacteria such as Clostridia spp.) are killed after only brief exposures to molecular O2. However, it is well established that even aerobic organisms, including man and other animals, show signs of oxygen toxicity when exposed to O2 tensions above those normally found in air (i.e. >21% O2). Such toxicity does not normally occur because aerobic cells possess protective enzymes that prevent either the formation or the accumulation of oxygen metabolites. It is only when these protective systems be-... 

Because all aerobic cells generate 02 , which will dismutate (either spontaneously or enzymically) into H202, then, provided a suitable transition-metal salt is available for reaction (5.9), there is the possibility that OH formation may occur in biological systems. 

Misonidazole [27 l-methoxy-3-(2-nitroimidazol-l-yl)-2-propanol] and the model compound l-methyl-2-nitroimidazole have been used as radiosensitizers in the treatment of certain types of human tumors. One important property of these compounds is that they are more toxic to hypoxic cells than to aerobic cells, indicating that reductive metabolism of the drug is involved in the toxicity. Results of a number of studies suggest that intracellular thiols play a significant role in the hypoxic cell toxicity, and it was found that reduction products formed stable thio ethers with GSH (for literature see References 181-183). The reaction mechanism of thio ether formation has not been fully established. It has been suggested that the 4-electron reduction product was involved in thio ether formation181,184,185, and that the hydroxylamine rather than the nitroso derivative was the reactant. On the other hand, an intermediate nitroso derivative is expected to give a sulfenamide cation (see Scheme 1) which easily allows thio ether formation. 

Most individual biochemical reactions are reversible and are therefore quite correctly considered to be chemical equilibria, but cells are not closed systems fuel (e.g. a source of carbon and, in aerobic cells, oxygen) and other resources (e.g. a source of nitrogen and phosphorus) are continually being added and waste products removed, but their relative concentrations within the cell are fairly constant being subject to only moderate fluctuation. Moreover, no biochemical reaction exists in isolation, but each is part of the overall flow of substrate through the pathway as a whole. 

Porphyrin rings containing iron are also a feature of the cytochromes. Several cytochromes are responsible for the latter part of the electron transport chain of oxidative phosphorylation that provides the principal source of ATP for an aerobic cell (see Section 15.1.2). Their function involves alternate oxidation-reduction of the iron between Fe + (reduced form) and Fe + (oxidized form). The individual cytochromes vary structurally, and their classification (a, b, c, etc.) is related to their absorption maxima in the visible spectrum. They contain a haem system that is covalently bound to protein through thiol groups. 

Oxidative phosphorylation produces most of the ATP made in aerobic cells. Complete oxidation of a molecule of glucose to C02 yields 30 or 32 ATP (Table 19-5). By comparison, glycolysis under anaerobic conditions (lactate fermentation) yields only 2 ATP per glucose. Clearly, the evolution of oxidative phosphorylation provided a tremendous increase in the energy efficiency of catabolism. Complete oxidation to C02 of the coenzyme A derivative of palmitate (16 0), which also occurs in the mitochondrial matrix, yields 108 ATP per palmitoyl-... 

Here AH2 is an oxidizable organic compound such as an alcohol or a pair of one-electron donor molecules. Catalases, which are found in almost all aerobic cells,194b may sometimes account for as much as 1% of the dry weight of bacteria. The enzyme catalyzes the breakdown of H202to water and oxygen by a mechanism similar to that employed by peroxidases. If Eq. 16-7 is rewritten with H202 for AH2 and 02 for A, we have the following equation ... 

Hydrocarbons yield more energy upon combustion than do most other organic compounds, and it is, therefore, not surprising that one important type of food reserve, the fats, is essentially hydrocarbon in nature. In terms of energy content the component fatty acids are the most important. Most aerobic cells can oxidize fatty acids completely to C02 and water, a process that takes place within many bacteria, in the matrix space of animal mitochondria, in the peroxisomes of most eukaryotic cells, and to a lesser extent in the endoplasmic reticulum. 

It is important not to confuse the reactions of Eq. 17-42 as they occur in an aerobic cell with the tightly coupled pair of redox reactions in the homolactate fermentation (Fig. 10-3 Eq. 17-19). Tire reactions of steps a and c of Eq. 17-42 are essentially at equilibrium, but the reaction of step b may be relatively slow. Furthermore, pyruvate is utilized in many other metabolic pathways and ATP is hydrolyzed and converted to ADP through innumerable processes taking place within the cell. Reduced NAD does not cycle between the two enzymes in a stoichiometric way and the "reducing equivalents" of NADH formed are, in large measure, transferred to the mitochondria. The proper view of the reactions of Eq. 17-42 is that the redox pairs represent a kind of redox buffer system that poises the NAD+/NADH couple at a ratio appropriate for its metabolic function. 

Looking at the other end of the respiratory chain, Otto WarburgC/d noted in 1908 that all aerobic cells contain iron. Moreover, iron-containing charcoal prepared from blood catalyzed nonenzymatic oxidation of many substances, but iron-free charcoal prepared from cane sugar did not. Cyanide was found to inhibit tissue respiration at low concentrations similar to those needed to inhibit nonenzymatic catalysis by iron salts. On the basis of these investigations, Warburg proposed in 1925 that aerobic cells contain an iron-based Atmungsferment (respiration enzyme), which was later called cytochrome oxidase. It was inhibited by carbon monoxide. 

Large metabolic charts have been designed to display all the major biochemical pathways. Such charts present a bewildering array of interconnected pathways, making it difficult to appreciate relationships between different pathways. The overall operational aspects of metabolism may be clarified by simpler block diagrams that omit details and focus on functional relationships. Such a functional block diagram for a typical heterotrophic aerobic cell is shown in figure 11.4. The metabolism of such a system is symbolized by two functional blocks ... 

This chapter is mainly concerned with the contribution of the tricarboxylic acid cycle to carbohydrate metabolism. The TCA cycle is the main source of electrons for oxidative phosphorylation, and thereby the major energetic sequence in the metabolism of aerobic cells or organisms. It serves as the main distribution center of metabo-... 

Kalckar, who showed that aerobic cells make ATP from ADP and Pj by a process that depends on respiration. At that time it was unclear that this oxidative phosphorylation occurred in mitochondria or that it involved NADH. Resolution of these questions had to await development of methods for preparing mitochondria free of other cellular constituents, and for presenting NADH on the matrix side of the inner membrane. When these methods were devised in... 

Krebs cycle The second metabolic pathway in aerobic cell respiration converts carbohydrates and lipids (sugars and fats) into carbon dioxide and water and produces energy-rich compounds, including some ATP. 

Since mitochondria are the site of high oxidative metabolism, they are under continual oxidative stress. In fact, it has been estimated that approximately 2 percent of mitochondrial 02 consumption generates ROS. The mitochondrial electron transfer chain is one of the main sources of ROS in aerobic cells, due to electron leakage from energy-transducing sequences leading to the formation of superoxide radicals. 

The reaction shown in Eq. 2 differs from the one described previously with enzymes from aerobic cells (Gunsalus (50)) by the absence of requirements for lipoic acid and DPN. Until recently, the electron carrier participating in this reaction was unknown. 

These organelles are the sites of energy production of aerobic cells and contain the enzymes of the tricarboxylic acid cycle, the respiratory chain, and the fatty acid oxidation system. The mitochondrion is bounded by a pair of specialized membranes that define the separate mitochondrial compartments, the internal matrix space and an intermembrane space. Molecules of 10,000 daltons or less can penetrate the outer membrane, but most of these molecules cannot pass the selectively permeable inner membrane. By a series of infoldings, the internal membrane forms cristae in the matrix space. The components of the respiratory chain and the enzyme complex that makes ATP are embedded in the inner membrane as well as a number of transport proteins that make it selectively permeable to small molecules that are metabolized by the enzymes in the matrix space. Matrix enzymes include those of the tricarboxylic acid cycle, the fatty acid oxidation system, and others. 

The basic properties of oxygen are largely responsible for the destructive power of free radicals in aerobic cells and tissues. The quantitative importance of oxygen-derived free radicals can be realized with the fact that about 250 grams of oxygen are consumed every day by a human organism [19]. Of this, about 2-5% would be converted to superoxide. While this review focuses on the deleterious... 

The oxidative power of peroxyl radicals, although sometimes increased by electron withdrawal, is smaller than that of RO radicals, but thanks to their longer lifetime [48], peroxyl radicals are ideal candidates for propagating oxidative chain reactions in biological membranes of aerobic cells, as discussed below. Traces of unprotected iron will therefore be sufficient to maintain a substantial free radical production from pre-formed hydroperoxides, even in the absence of important reductant concentrations [49]. 

In aerobic cells, polyunsaturated fatty acids of membrane phospholipids easily undergo such oxidative chain reactions [111,112]. This is because the double bonds of the polyunsaturated structure are repeatedly connected to each other by c/s-methylene units. Such bis-allylic structures enable electron delocalization on five carbon atoms, making the initial hydrogen abstraction on... 

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