Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Respiration
Energy Balance

Previously (Table 12.1), it was shown that the complete Respiratory Chain features 3 oxidation stages that release sufficient energy to synthesize an ATP molecule. In other words, when two electrons are transferred from NADH to molecular oxygen, only three electron transitions can be coupled with ADP phosphorylation, meaning a maximum of 3 ATP molecules can be produced. This linkage between phosphorylation and O₂ reduction is conventionally denoted by the P/O ratio, which equals the number of ATP molecules formed per atom of oxygen consumed during Respiration.

Substrates transferring their reducing equivalents to NAD⁺ yield a P/O ratio of 3, whereas for succinate and other substrates donating electrons to FAD, the P/O ratio is 2 (in this case, electrons enter the chain at the quinone level and traverse a shorter path through the carrier system). Knowing these energetic coefficients makes it possible to calculate the energy yield of glucose oxidation.

Overall energy yield of glucose oxidation. If glucose is catabolized via The Glycolytic Pathway and the resulting Pyruvate is fully oxidized to CO₂ and H₂O via the TCA cycle, with all hydrogen being burned to Water in the respiratory chain, then for every 1 molecule of glucose consumed, the following are produced:

— in Glycolysis — 2 molecules of NADH;

— during pyruvate dehydrogenation — (1 × 2) = 2 molecules of NADH;

— in the TCA cycle — (3 × 2) = 6 molecules of NADH and (1 × 2) = 2 molecules of FADH₂;

Total: 10 molecules of NADH and 2 molecules of FADH₂.

Knowing that P/O = 3 for NADH and P/O = 2 for FADH₂, we can calculate the number of ATP molecules synthesized via Oxidative Phosphorylation: (10 × 3) + (2 × 2) = 34. To this amount, one should add 2 ATP molecules conserved via substrate-level phosphorylation in glycolysis, and 2 ATP molecules conserved during The oxidation of 2 molecules of a-ketoglutarate in the TCA cycle.

Thus, the grand total of ATP molecules capable of being generated through the Complete oxidation of glucose is 38. In reality, however, this figure is somewhat lower, amounting to 32–36 ATP molecules. This discrepancy is explained by the fact that The transport of metabolites (primarily ATP and ADP) across the mitochondrial membrane consumes energy from the proton gradient. Consequently, not all the energy stored in the proton gradient is channeled into ATP synthesis.

Energy yield of Fermentation. Comparing the energy efficiency of Respiration and Fermentation for Cells leads to the Conclusion that respiration is a far more advantageous process. A simple calculation shows that during the most common Types of fermentation, such as alcoholic and Lactic acid fermentation, The Cell can harvest only 2 ATP molecules per glucose molecule, since even the 2 NADH molecules generated in glycolysis are reoxidized to NAD⁺ during The formation of ethanol or lactate.