Aerobic catabolism

FIGURE 5.26 Complete oxidation of organic carbon componnds during microbial decomposition leads to production of carbon dioxide accompanied by reduction of oxygen to water. 

The reaction in glycolysis is often referred to as the Embden-Meyerhof pathway. 

The metabolic intermediate (e.g., pyrnvate) nndergoes complete oxidation to CO2, throngh the pathway referred to as tricarboxylic acid cycle (TCA cycle). The following reactions are involved in TCA cycle. The hrst step involves conversion of pyrnvate to acetyl-CoA throngh decarboxylation and prodnction of NADH. The acetyl-CoA (2 carbon) combines with the fonr-carbon componnd oxalacetate, leading to the formation of citric acid (6 carbon). The TCA cycle is also referred to as citric acid cycle. A series of reactions inclnding dehydration, decarboxylation, and oxidation are involved in the conversion of citric acid to carbon dioxide. The electrons released are transferred to enzymes containing the coenzyme NAD+. 

As the NADH is oxidized, the electrons released are removed by specific carriers, and the protons are transported from cytoplasm to outside the cell. Removal of H+ causes an increase in the nmnber of OH ions inside the membrane. These conditions result in a proton gradient (pH gradient) across the membrane. This gradient of potential energy, termed as proton motive force, can be used to do useful work. This potential energy is captured by the cell by a series of complex membrane-bound enzymes, known as the ATPase in the process called oxidative phosphorylation. In 1961, the concept of proton gradient was first proposed as chemiosmotic theory by Peter Mitchell of England, who won the Nobel Prize for this scientific contribution. 

Oxygen reduction in ETS requires addition of four electrons. This reduction occurs in the following steps  

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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. 

Ebert S, P-G Rieger, H-J Knackmuss (1999) Function of coenzyme P420 in aerobic catabolism of 2,4,6-trinitro-phenol and 2,4-dinitrophenol by Nocardiodes simplex FJ2-1 A. J Bacterial 181 2669-2674. 

Schleissner C, ER Olivera, M Eernandez-Valverde, M Luengo (1994) Aerobic catabolism of phenylacetic acid in Pseudomonas putida U biochemical characterization of a specific phenylacetic acid transport system and formal demonstration that phenylacetyl-coenzyme A is a catabolic intermediate. J Bacterial 176 7667-7676. 

Visscher PT, BF Taylor (1993) A new mechanism for the aerobic catabolism of dimethyl sulfide. Appl Environ Microbiol 59 3784-3789. 

To begin with, let us return to the aerobic catabolism of simple sugars such as glucose to yield two molecules of pyruvate -I- two molecules of ATP - - two molecules of NADH. We noted just above that coupling the oxidation of the two molecules of NADH to the electron transport chain yields an additional six molecules of ATP, three for each molecule of NADH, for a total of eight. Now let s ask what happens when we further metabolize the two molecules of pyruvate via the pyruvate dehydrogenase complex and the citric acid cycle. 

Symbiotic system can now carry out aerobic catabolism. Some bacterial genes move to the nucleus, and the bacterial endosymbionts become mitochondria. 

In this chapter, discussion focuses on the TCA cycle and its central role in the aerobic catabolism of carbohydrates. Chapter 14 explains how the free energy present in the reduced coenzymes that are generated by glycolysis and the TCA cycle is conserved as ATP during the companion process of electron transport and oxidative phosphorylation. 

Hochachka and Somero, 1973 Shelukhin etal, 1989). In adapting to cold, the mullet Liza sp. enhances the content of neutral lipids in the muscle by inhibiting aerobic catabolism (Soldatov, 1993). It is therefore no accident that Mongolian grayling from cold water contain more lipids than their counterparts from warmer water (Lapin and Basaanzhov, 1989). 

Figure 2. Aerobic catabolism of methylated sulfides (adapted from Kelly, 1988). 1) DMSO reductase (Hyphomicrobium sp.) 2) DMDS reductase (Thiobacillus sp. 3) trimethylsulfonium-tetrahydrofolate methyltransferase (Pseudomonas sp.) 4) DMS monooxygenase 5) methanethiol oxidase 6) sulfide oxidizing enzymes 7) catalase 8) formaldehyde dehydrogenase 9) formate dehydrogenase 10) Calvin cycle for CO2 assimilation (Thiobacillus sp.) 11) serine pathway for carbon assimilation (Hyphomicrobium sp.).

For the entire cycle, the production of 10 ATP/acetyl-CoA or 20 ATP/Glucose. The aerobic catabolism of glucose can then give a maximum total of 32 ATP/glucose by TCA cycle as summarized in below Table. 9.3... 

Figure 19-1 a Normal and ischemic myocardial metabolism of glucose. A total production of 36 moles of ATP results from the aerobic catabolism of 1 mole of glucose and use of NADH and FADH. in the oxidative phosphorylation process in mitochondria. When oxygen is not available, NADH and FADH levels rise and shut off the tricarboxylic acid (TCA) cycle. Pyruvate is converted to lactate. Only 2 moles of ATP are formed from anaerobic catabolism of 1 mole of glucose. (Adapted from Giuliani, E. R., ei al. Cardiology Fundamentals and Practice, 2nd ed. By permission of the Mayo Foundation, Rochester, MN.)... 

The observed pattern was very similar as compared to the isomeric composition of LABs, attributed to a preferential microbial degradation of The isomeric distribution of ASPE in sediments from different sampling locations and in selected sediment layers was virtually identical (see Figure 2), suggesting their persistence in anaerobic environments, external relative to internal substituted isomers (Takada and Eganhouse, 1998). The observed pattern of ASPE indicated a very similar environmental behaviour as compared to LABs, that are well established molecular marker. In addition, information on the aerobic catabolism of ASPE by soil bacteria presented also the observed pattern as demonstrated for various soil samples (Schmidt et al., 2000). 

Many different groups of bacteria, including Bacillus, Pseudomonas, and Thiobacillus, are capable of denitrification. The primary biochemical pathways for organic substrate oxidation by denitri-fiers are similar to that described for aerobic catabolism. Because most of the denitrifiers are facultative anaerobes, they possess a functional TCA cycle that allows them to metabolize substrates completely to carbon dioxide and water. Many denitrifiers do not produce extracellular enzymes required for hydrolysis of polymers thus, they generally rely on hydrolytic enzymes and fermenters to provide readily available substrates (Ljundahl and Erickson, 1985). 

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