Energy Yield from Glucose Oxidation

6 Energy Yield from Glucose Oxidation The overall equation of glucose oxidation may be written as 

By simple calculation it can be shown that 36 molecules of ATP can be obtained overall, per glucose molecule input, from the NADH, FADH and GDP involved in the operation of the Krebs cycle. To this can be added the two molecules of ATP formed per glucose molecule in the earlier stages of glycolysis, making a total of 38 molecules of ATP (Table 11.20). 

Only about 5% of the ATP generated comes from substrate-level phosphorylation in glycolysis, and the remainder is obtained by the oxidative phosphorylation process. If the oxygen supply becomes temporarily inadequate, NADH is re-oxidised using pyruvic acid as the hydrogen acceptor. This enables glucose breakdown to work anaerobically. 

Phosphorylation of ftuctose-6-phosphate Dephosphorylation of 2 mols of phosphoenolpyruvate (2 mols of NADH are formed in the oxidation of 2 mols of glyceraldehyde-3-phosphate and 2 mols NADH in the conversion of pyruvate into acetyl Co-A). 

Formation of 2 mols of GTP from 2 mols of succinyl CoA (6NADH are formed in the oxidation of 2 mols of isocitrate, a-ketoglutarate and malate 2FADH2 are formed in the oxidation of 2 mols of succinate) 

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As an example of the energy yield from p-oxidation, the balance sheet for ATP production when the sixteen-carbon-fatty acid palmitic acid is degraded by p-oxidation is summarized in Figure 23.8. Complete oxidation of palmitate results in production of 129 molecules of ATP, three and one half times more energy than results from the complete oxidation of an equivalent amount of glucose. 

Recall Compare the energy yields from the oxidative metabolism of glucose and of stearic acid. To be fair, calculate it on the basis of ATP equivalents per carbon and also ATP equivalents per gram. 

By combining the glycolytic pathway, the Krebs cycle, and oxidative phosphorylation, the energy yield from the aerobic degradation of glucose will be... 

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... 

Knowing this information and keeping in mind that two turns of the citric acid cycle are required, we can sum up the total energy yield from the complete oxidation of one glucose molecule. 

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. 

It is instructive to compare the energy yield from the complete oxidation of fatty acids with that obtained from an equivalent amount of glncose becanse both are important constituents of the diet. In Section 13.8, we saw that the oxidation of a single glucose molecule produces 32 ATP molecules. The complete oxidation of three 6-carbon glucose molecules... 

In order for the cell to carry out a controlled oxidation of D-glucose and conserve some of the energy derived from the process, it is lira necessary to add phosphate to the hexose with the expenditure of energy. The necessary energy and the phosphate per se is supplied by ATP in two separate reactions of the system. Since each molecule of glucose can yield two molecules of irioxe phosphate for oxidation, the conversion of glucose in pyruvic acid nets two molecules of ATP per molecule of hexose utilized. 

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