Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy storage processes
Respiration
Energy balance

As previously shown (Table 12.1), the complete Respiratory Chain contains 3 oxidation steps at which sufficient energy is released to synthesize an ATP molecule. In other words, during The transfer of two electrons from NADH to molecular oxygen, only three electron transitions can be coupled with ADP phosphorylation, meaning a maximum of 3 ATP molecules can be formed. This Coupling of phosphorylation with O₂ reduction is conventionally expressed by the P/O ratio, which equals the number of ATP molecules synthesized per oxygen atom consumed during Respiration.

Substrates transferring their reducing equivalents to NAD⁺ yield a P/O ratio of 3, whereas for succinate and other substrates transferring electrons to FAD, the P/O ratio is 2 (in this case, electrons enter the chain at the quinone level and travel a shorter path through the carrier system). Knowing these energy yields, we can calculate the Energy Balance of glucose oxidation.

Overall energy balance of glucose oxidation. If glucose is catabolized via The Glycolytic Pathway, and the resulting Pyruvate is completely oxidized to CO₂ and H₂O via the TCA cycle, and all hydrogen is oxidized to Water in the respiratory chain, then, per 1 molecule of glucose consumed, the following are formed:

— in Glycolysis — 2 NADH molecules;

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

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

Total: 10 NADH molecules and 2 FADH₂ molecules.

Knowing that the P/O ratio is 3 for NADH and 2 for FADH₂, we can calculate the number of ATP molecules synthesized during Oxidative Phosphorylation: (10 x 3) + (2 x 2) = 34. To this amount, we must add 2 ATP molecules generated by substrate-level phosphorylation in glycolysis, and 2 ATP molecules generated during The oxidation of 2 molecules of alpha-ketoglutarate in the TCA cycle.

Thus, the total yield of ATP that can potentially be formed during the Complete oxidation of glucose is 38 molecules. In reality, however, this number is somewhat lower, at 32—26 molecules of ATP. This is because The transport of metabolites (primarily ATP and ADP) across the mitochondrial membrane consumes the energy of the proton gradient. Consequently, not all the energy stored in the proton gradient is utilized for ATP synthesis.

Energy balance of Fermentation. Comparing the cellular energy yield of Respiration and Fermentation reveals that respiration is a far more efficient 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 oxidized back to NAD⁺ during The formation of ethanol or lactate.