Glycolysis under aerobic conditions

Canceling out common terms on both sides of the equation gives the overall equation for glycolysis under aerobic conditions ... 

Pyruvate is typically considered the last molecule produced in glycolysis. Under aerobic conditions pyruvate is transformed into acetyl-CoA, which then enters the citric acid cycle. Under anaerobic conditions, however, something else must be done to oxidize all the NADH formed in glycolysis. 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

This is true of skeletal muscle, particularly the white fibers, where the rate of work output—and therefore the need for ATP formation—may exceed the rate at which oxygen can be taken up and utilized. Glycolysis in erythrocytes, even under aerobic conditions, always terminates in lactate, because the subsequent reactions of pymvate are mitochondrial, and erythrocytes lack mitochondria. Other tissues that normally derive much of their energy from glycolysis and produce lactate include brain, gastrointestinal tract, renal medulla, retina, and skin. The liver, kidneys, and heart usually take up... 

Under aerobic conditions, the pyravate is oxidized to CO and H O via the tricarboxylic acid or Krebs cycle and the electron transport system. The net yield for glycolysis followed by complete oxidation is 38 moles ATP per mole glucose, although there is evidence that the yield for bacteria is 16 moles ATP per mole glucose (Aiba et al., 1973). Thus, 673 kcal are liberated per mole glucose, much of which is stored as ATP. 

The glycolytic pathway, or glycolysis, is a metabolic sequence in which glucose is broken down to pyruvic acid. The subsequent fate of pyruvate then depends upon whether or not the organism is aerobic or anaerobic Under aerobic conditions, pyruvate is oxidized via oxidative phosphorylation under anaerobic conditions, pyruvate is converted further into compounds such as lactate or ethanol, depending upon the organism. 

Cyanide ion exerts an inhibitory action on certain metabolic enzyme systems, most notably cytochrome oxidase, the enzyme involved in the ultimate transfer of electrons to molecular oxygen. Because cytochrome oxidase is present in practically all cells that function under aerobic conditions, and because the cyanide ion diffuses easily to all parts of the body, cyanide quickly halts practically all cellular respiration. The venous blood of a patient dying of cyanide is bright red and resembles arterial blood because the tissues have not been able to utilize the oxygen brought to them. Cyanide intoxication produces lactic acidosis, the result of an increased rate of glycolysis and production of lactic acid. ... 

The two molecules of NADH formed by glycolysis in the cytosol are, under aerobic conditions, reoxidized to NAD+ by transfer of their electrons to the electron-transfer chain, which in eukaryotic cells is located in the mitochondria. The electron-transfer chain passes these electrons to their ultimate destination, 02 ... 

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate. As mentioned earlier, some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to C02) produce lactate from glucose even under aerobic conditions. The reduction of pyruvate is catalyzed by lactate dehydrogenase, which forms the l isomer of lactate at pH 7 ... 

The NADH formed in glycolysis must be recycled to regenerate NAD+, which is required as an electron acceptor in the first step of the payoff phase. Under aerobic conditions, electrons pass from NADH to 02 in mitochondrial respiration. 

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-... 

Under aerobic conditions additional ATP is produced, following glycolysis, when ... 

Hochachka and Somero, 1977). Even glycogen and glucose are used intensively in red muscle of tuna under aerobic conditions (so-called aerobic glycolysis). In contrast, the sluggish scorpion fish and whiting can perform only relatively slow movements using the white muscle. 

Two ATPs are used in glycolysis and four ATPs are synthesized for each molecule of glucose so that the net yield is two ATPs per glucose. Under aerobic conditions, the two NADH molecules arising from glycolysis also yield energy via oxidative phosphorylation. 

Fig. 8.2 Glycolysis and related pathways. Glycolysis is a central metabolic machinery in which one mole of glucose is catabolized to two moles of pyruvate, NADH, and ATP. Under aerobic conditions, pyruvate is further oxidized by mitochondrial system. In erythrocytes DHAP is a dead-end product however, in brain it can be converted into direction of lipid synthesis. Glycolysis and the pentose phosphate pathway (pentosePP) are interconnected via fructose-6-P and glyceral-dehyde-3-P. A high level of NADPH favors lipid synthesis via pentose phosphate shunt (pentosePP). At TPI inhibition (TPI deficiency), glyceraldehyde-3-Pcan be produced via G6PDH as well, to contribute to the glycolytic flux. a-GDH catalyzes the...

The characteristic feature of carbohydrate breakdown in cestodes is the production of a range of complex end-products, usually organic acids, even under aerobic conditions (Table 5.4). This contrasts with predominantly aerobic organisms, such as most free-living metazoa, where the end-product of glycolysis is almost exclusively lactic acid formed from pyruvic acid. Lactic acid is produced as a result of rapid muscular contraction carried out essentially under anaerobiosis and its production ensures a rapid expenditure of energy without the limitation due to the rate of diffusion of oxygen. The anaerobic phase is followed by an aerobic phase, where pyruvic acid is metabolised to acetyl-coenzyme A which is in turn oxidised completely to... 

The regulation of these two means of ATP production is very different. Under aerobic conditions (see answer to Problem 9), glycolysis is inhibited by the relatively high [ATP], as acetyl-CoA units derived from fat feed into the citric acid cycle and ATP is produced by oxidative phosphorylation. Under anaerobic conditions, glycolysis is stimulated and metabolism of fats does not occur at an appreciable rate because [citrate] and [acetyl-CoA] are low and 02 (the final acceptor of electrons in oxidative phosphorylation) is absent. 

Answer The addition of oxygen to an anaerobic suspension allows cells to convert from fermentation to oxidative phosphorylation as a mechanism for reoxidizing NADH and making ATP. Because ATP synthesis is much more efficient under aerobic conditions, the amount of glucose needed will decrease (the Pasteur effect). This decreased utilization of glucose in the presence of oxygen can be demonstrated in any tissue that is capable of aerobic and anaerobic glycolysis. 

When grown under aerobic conditions, the yeast produces two ATP molecules from one molecule of glucose by substrate-level phosphorylation in glycolysis. The two molecules of pyruvate produced can then be completely oxidized to CO2, and each yields a further 15 molecules of ATP. This leads to a slow decrease in the concentration of glucose, a steady production of CO2, and relatively little change in the amount of ATP. Also, the two molecules of NADH can be reoxidized to NAD+ by the electron-transport system. (This produces yet more ATP, as discussed in Chap. 14.)... 

The first metabolic pathway that we encounter is glycolysis, an ancient pathway employed by a host of organisms. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules ofpyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) inasmuch as it evolved before the accumulation of substantial amounts of oxygen in the atmosphere. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acidfermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP, as will be discussed in Chapters 17 and 18. 

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