| ENERGY YIELD FROM THE TCA CYCLE
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Table 13-1.
ATP yield from glucose during oxidative metabolism. |
| Body_ID: None |
Energetics of glucose oxidation |
| Body_ID: T013001.50 |
| Reaction | Mechanism | moles ATP/mol Glc |
| Body_ID: T013001.100 |
| hexokinase | phosphorylation | -1 |
| Body_ID: T013001.150 |
| phosphofructokinase | phosphorylation | -1 |
| Body_ID: T013001.200 |
| G3PDH | NADH, oxidative phosphorylation | +6 (+4)* |
| Body_ID: T013001.250 |
| phosphoglycerate kinase | substrate-level phosphorylation | +2 |
| Body_ID: T013001.300 |
| pyruvate kinase | substrate-level phosphorylation | +2 |
| Body_ID: T013001.350 |
| pyruvate dehydrogenase | NADH, oxidative phosphorylation | +6 |
| Body_ID: T013001.400 |
| isocitrate dehydrogenase | NADH, oxidative phosphorylation | +6 |
| Body_ID: T013001.450 |
| α-ketoglutarate dehydrogenase | NADH, oxidative phosphorylation | +6 |
| Body_ID: T013001.500 |
| succinyl CoA synthetase | substrate-level phosphorylation (GTP) | +2 |
| Body_ID: T013001.550 |
| succinate dehydrogenase | FADH2, oxidative phosphorylation | +4 |
| Body_ID: T013001.600 |
| malate dehydrogenase | NADH, oxidative phosphorylation | +6 |
| Body_ID: T013001.650 |
| TOTAL | | 38 (36)* |
| Body_ID: T013001.700 |
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| Body_ID: T013001.750 |
*Electrons from cytosolic NADH can result in the synthesis of 6 moles of ATP per mole of glucose via the malate-aspartate shuttle, but only 4 via the glycerol 3-phosphate shuttle (see Chapter 8).
The yields of ATP shown are approximate. Recent work suggests that the actual yields of ATP from NADH and FADH2 are closer to 2.5 and 1.5, respectively, yielding approximately 30 moles of ATP per mole of glucose. The oxidation of glucose in a bomb calorimeter yields 2870 kJ/mol (686 kcal/mol), while the synthesis of ATP requires 31 kJ/mol (7.3 kcal/mol). Aerobic metabolism of glucose is therefore about 40% efficient (2870 kJ/mol glucose / 31 kJ/mole ATP = 93 theoretical moles of ATP/mol glucose; 36/93 = 39%).
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| During the course of the TCA cycle, each mole of acetyl-CoA generates sufficient reduced nucleotide coenzymes for
synthesis of ∼11 moles ATP by oxidative phosphorylation. Note that the ATP yields used here are approximate and are actually slightly less according to recent measurements.
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| Together with the GTP synthesized by substrate-level phosphorylation in the succinyl-CoA synthetase (succinate thiokinase) reaction, a total of ∼12 ATP equivalents are available per mole of acetyl-CoA. Thus, complete metabolism of a mole of glucose through glycolysis and the TCA cycle yields ∼36-38 moles ATP (Table 13.1). [The actual ATP yield depends on the route of transport of redox equivalents to the mitochondrion, i.e. six ATP by the malate aspartate shuttle, and four ATP by the glycerol phosphate shuttle (Chapter 8).] In contrast, only 2 moles of ATP (net) are recovered by anaerobic glycolysis in which glucose is converted to lactate.
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