Glycolysis energy yield

The energy yield from glycolysis for the anaerobic decomposition of glucose to 2 mol of lactic acid may be calculated as follows ... 

G. Energy yields from glycolysis depend on the system used to regenerate NAD". ... 

We saw in Chapter 14 that the energy yield from the production of two molecules of pyruvate from one molecule of glucose in glycolysis is 2 ATP and 2 NADH. In oxidative phosphorylation (Chapter 19), passage of two electrons from NADH to 02 drives the formation of about 2.5 ATP, and passage of two electrons from FADH2 to 02 yields about 1.5 ATP. This stoichiometry allows us to calculate the overall yield of ATP from the complete... 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

Since two molecules of ATP are converted to ADP in the first part of the glycolysis process, there is a net gain of two molecules of ATP. The second part of the glycolysis process also yields two molecules of NADH + H+ per molecule of glucose. Subsequently, the energy-yielding conversion of the two molecules of ATP back to ADP and the oxidation of NADH,... 

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. 

In oyster heart, in Mytilus, and other bivalves as well, aspartate occurs at high (about 15 /imol g 1) concentrations in the normoxic state. In anoxia (figure 3.9) the bulk of the aspartate is fermented to succinate, a process that is particularly important during early states of anoxia. At this time, it is probably redox-coupled to glycolysis. Thus the energy yield increases by 1 mole ATP per mole aspartate fermented to succinate, or by 2 moles ATP per mole aspartate if the fermentation continues to the level of propionate, and the maximum total energy yield becomes 8 moles ATP per mole glucose plus aspartate fermented to propionate. 

In the above discussion, the energy yields of the different anaerobic pathways of metabolism are expressed in terms of moles of ATP per mole of substrate fermented. This is the traditional way to express the energetic efficiency of fermentations and for some purposes it is informative and useful. However, it is important to recall that in vivo these pathways are linked to ATP utilizing pathways (usually ATPases). At steady state, rates of ATP synthesis by these fermentations equal rates of ATP utilization. For classical glycolysis, for example... 

Step 10, the final step in glycolysis, is the irreversible conversion of phosphoenolpyruvate to pyruvate, catalyzed by pyruvate kinase. This is the second energy-yielding step in the glycolytic pathway, and produces ATP Mg2+ is required here, too. 

The complete oxidation of muscle glycogen to CO substantially increases the energy yield, but this aerobic process is a good deal slower than anaerobic glycolysis. However, as the distance of a run increases, aerobic... 

One turn of the citric acid cycle results in the production of two CO2 molecules, three NADH molecules, one FADH2 molecule, and one ATP molecule. Oxidative phosphorylation yields three ATP molecules per NADH molecule and two ATP molecules per FADH2 molecule. The only exception to these energy yields is the NADH produced in the cytoplasm during glycolysis. Oxidative phosphorylation )delds only two ATP molecules per cytoplasmic NADH molecule. The reason for this is that energy must be expended to shuttle electrons from NADH in the cytoplasm to FADHj in the mitochondrion. 

When glycolysis occurs under anaerobic conditions, it is followed by fermentation reactions, such as the lactate and alcohol fermentations. These reactions reduce pyruvate—or a molecule produced from pyruvate—and simultaneously oxidize the NADH produced in glycolysis. As a result, the net energy yield from glycolysis under anaerobic conditions is only two ATP. No further ATP energy is harvested from the oxidation of the NADH. It is simply reoxidized in the fermentation reactions. 

Under aerobic conditions, the energy yield of glycolysis is much greater because the high-energy electrons carried by NADH are shuttled into mitochondria and used in oxidative phosphorylation to produce more ATP. 

The main drawback of the glycerol-3-phosphate shuttle is that only two ATP are produced for each cytoplasmic NADH. The reason is that the electrons are shuttled to FADHj, which )delds only two ATP by oxidative phosphorylation. (The energy yield of the oxidation of mitochondrial NADH is three ATP.) Thus the total energy yield from glycolysis imder aerobic conditions in muscle and nerve cells is two ATP, produced by substrate level phosphorylation, plus four ATP (two ATP per NADH), produced by oxidative phosphorylation. This provides an energy yield of six ATP per glucose. 

In summary, the energy yield of glycolysis depends on the conditions present (aerobic versus anaerobic) and the type of cell. The following table summarizes the energy gains from glycolysis under various conditions. 

Glycolysis is tightly regulated in coordination with other energy-yielding pathways to assure a steady supply of ATP. Hexokinase, PFK-1, and pyruvate kinase are all subject to allosteric regulation that controls the flow of carbon through the pathway and maintains constant levels of metabolic intermediates. 

Fermentation is defined as an energy-yielding metabolic pathway that involves no net change in oxidation state. Anaerobic glycolysis is a type of fermentation. The lactic acid fermentation (conversion of glucose to lactate) is important in the manufacture of cheese. Another important fermentation involves cleavage of pyruvate to acetaldehyde and C02, with the acetaldehyde then reduced to ethanol by alcohol dehydrogenase in the reaction that follows ... 

---