Catabolism completely oxidized

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-... 

Catabolism. This results, eventually, in formation of ammonia and small carbon-contaiiting compounds. The carbon skeletons are used for the synthesis of glucose and triacylglycerol or for complete oxidation to CO2,... 

Figure 8.8 The role of the Krebs cyde in the oxidation of the six common intermediates. The six short arrows indicate the positions in the cycle where the various intermediates from amino acid catabolism feed into the cycle. Eventually they are all converted to acetyl-CoA for complete oxidation by the cycle. The pathway is indicated by the broader arrows.

Comparing this equation with the equation for the complete oxidation of palmitoyl-CoA (see table 18.1, equation 1), we find major differences in carriers and intermediates. The principal electron carrier in the anabolic pathway is the NADPH-NADP+ system in the catabolic pathway, /3 oxidation, the principal electron carriers are FAD-FADH2 and NAD+-NADH. The second striking difference between the two pathways is that malonyl-CoA is the principal substrate in the anabolic pathway but plays no role in the catabolic pathway. These differences reflect the fact that the two pathways do not share common enzymes. Indeed, in animal cells the reactions occur in separate cell compartments biosynthesis takes place in the cytosol, whereas catabolism occurs in the mitochondria. 

In mammalian tissues, complete oxidation of carbohydrate is carried out aerobically via the TCA cycle. The cycle functions catabolically, in conjunction with the electron transport system, to yield ATP, and biosynthetically as a fundamental source of intermediates for other pathways. Although restricted to a small number of species, those cestodes that have been investigated in detail exhibit a complete, or near complete, sequence of... 

The overall scheme for glucose oxidation was given in Eq. (5.13). Reduction of 6 O2 in the body results in the production of 12 molecules of water. However, Eq, (5.13) shows that only 6 H O are produced. Why are 12 H2O not listed The catabolism of one molecule of glucose results in the production of two molecules of pyruvate. Complete oxidation of two pyruvates results ir the use of six molecules of loiiter. Water is used by citrate synthase for the hydrolysis of coenTiyme A from aoetyl-CoA, by sucdnyl thiokinase for the hydrolysis of coetrzyme A from suc-dnyl-CoA, and by fumarase for the hydration of fumarate. 

Catabolism of tyrosine and tryptophan begins with oxygen-requiring steps. The tyrosine catabolic pathway, shown at the end of this chapter, results in the formation of fumaric acid and acetoaceticacid, Iryptophan catabolism commences with the reaction catalyzed by tryptophan-2,3-dioxygenase. This enzyme catalyzes conversion of the amino acid to N-formyl-kynurenine The enzyme requires iron and copper and thus is a metalloenzyme. The final products of the pathway are acetoacetyl-CoA, acetyl-Co A, formic add, four molecules of carbon dioxide, and two ammonium ions One of the intermediates of tryptophan catabolism, a-amino-P-carboxyrnuconic-6-semialdchydc, can be diverted from complete oxidation, and used for the synthesis of NAD (see Niacin in Chapter 9). 

The histidine catabolic pathway is discussed under Folate in Chapter 9. The material reveals that histidine is catabolized to produce glutamate. Glutamate in turn, can be converted to a-ketoglutarate and completely oxidized to CO in the Krebs cycle. In the study depicted in Figure 8,26, the dietary histidine was spiked with I Cjhistidine, The term "spiked" means that only a very small proportion of the histidine contained carbon-14. The metabolic behavior of the radioactive histidine, which can be followed, mirrors the metabolic fate of nonradioactive histidine in the diet. All of the CQz exhaled by the rats can be easily collected, The " COj present in the rat s breath can be measured by use of a liquid scintillation counter. The amount of CO2 produced directly mirrors the proportion of histidine, absorbed from the diet that was degraded the rat s body. 

Another central pathway by which yeasts may catabolize D-glucose is the pentose cycle (see Fig. 6), the initial stages of which are (i) the phosphorylation of D-glucose, followed by (ii) oxidation of D-glucose 6-phosphate to 6-O-phosphono-D-gluconate. The net result of the operation of this cycle is the complete oxidation of D-glucose. 

In the overview of glycolysis we noted that the pyruvate produced must be used up in some way so that the pathway will continue to produce ATP. Similarly, the NADH produced by glycolysis in step 6 (see Figure 21.8) must be reoxidized at a later time, or glycolysis will grind to a halt as the available NAD+ is used up. If the cell is functioning under aerobic conditions, NADH will be reoxidized, and pyruvate will be completely oxidized by aerobic respiration. Under anaerobic conditions, however, different types of fermentation reactions accomplish these purposes. Fermentations are catabolic reactions that occur with no net oxidation. Pyruvate or an organic compound produced from pyruvate is reduced as NADH is oxidized. We will examine two types of fermentation pathways in detail lactate fermentation and alcohol fermentation. 

In the second stage of catabolism, monosaccharides, amino acids, fatty acids, and glycerol are converted by metabolic reactions into molecules that can be completely oxidized. 

In the third stage of catabolism, the two-carbon acetyl group of acetyl CoA is completely oxidized by the reactions of the citric acid cycle. The energy of the electrons harvested in these oxidation reactions is used to make ATP. 

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