Question

Why can mitochondria make only 34 molecules (not 61 molecules) of ATP from the energy obtained from the oxidation in the electron transfer system of NADH and FADH_2 generated by oxidation of glucose in glycolysis and the tricarboxylic acid cycle? The electron transfer system oxidizes 10 mol of NADH and 2 mol of FADH_2obtained from glycolyses and the tricarboxylic acid cycle, and up to 34 mol of ATP can be obtained by the electron transfer system alone. Complexes I and II take energy from NADH and FADH_2, and complexes I, III, and IV are also proton pumps that pump hydrogen ions against a concentration gradient. The reactions carried out by the four enzymes, known as complexes I–IV, and their Gibbs free energies are as follows; Complex I: (NADH + H^+ + CoQ → NAD^+ + CoQH_2 — ΔG˚′(1) = -71 kJ/mol)×10

Complex Ⅱ： (FADH_2 + succinic acid + CoQ → FAD + CoQH_2 + Fumaric acid — ΔG˚′(2) =-2.9 kJ/mol)×2
Complex Ⅲ：(CoQH_2 + 2 cyt c(Fe3+) → CoQ + 2 cyt c(Fe^2+) + 2H^+ — ΔG˚′(3) = -41 kJ/moll)×12
Complex Ⅳ：(4 cyt c(Fe^2+) + O_2 + 4H^+ → 4 cyt c(Fe^3+) + 2H_2O — ΔG˚′(4) = -110 kJ/mol)×6
Therefore, the ‘net’ reaction, from complex I–IV, is as follows:
10 NADH + 2 FADH_2 + 6 O_2+ 2 Succinic acid → 12 H_2O + 10 NAD + 2 FAD + 2 Fumaric acid — ΔG˚′ (total)

Here,

ΔGo' (total)= 10ΔGo'(1) + 2Go'(2) + 12ΔGo'(3) + 6ΔGo'(4)
= 10*(-71) + 2*(-2.9) + 12*(-41) + 6*(-110) kJ/mol
= -1867.8 kJ/mol

The following reactions then occurs in complex V using the above ΔG˚′ (total);
ADP + Pi → ATP , ΔGo'(5)= -30.5 kJ/mol Simply considering abovementioned only, the amount of ATP that can be generated from the energy derived from a single molecule of glucose entering the electron transfer system via NADH or FADH_2 is as follows;
-1867.8/-30.5 =61.2 ATPs So, my question is as follows; Why can mitochondria make only 34 molecules (not 61 molecules) of ATP from the energy obtained from a single molecule of glucose in the electron transfer system? I would like to ask this question especially from the perspective of Gibbs energy balance (in terms of energy loss).

Answer 1

The number of ATP molecules generated from oxidation of glucose in the electron transfer system varies due to factors such as the number of hydrogen ions pumped and the starting point of FADH₂. The theoretical maximum yield is 38 ATP molecules, with 4 from substrate-level phosphorylation and 34 from oxidative phosphorylation. However, the actual ATP yield can be lower due to factors like intermediates transportation into the mitochondria.

Step-by-step explanation:

The number of ATP molecules generated from the oxidation of glucose in the electron transfer system is not always the same due to several factors.

One major factor is that the electron transport chain complexes can pump different numbers of hydrogen ions through the membrane, resulting in varying ATP yields. Additionally, FADH₂ generates less ATP compared to NADH due to its starting point in the electron transport chain.

In the case of glucose, 10 NADH molecules generated during glycolysis and the tricarboxylic acid cycle can produce 30 ATP molecules through oxidative phosphorylation. The 2 FADH₂ molecules can generate 4 ATP molecules.

When combined with the 4 ATP molecules produced through substrate-level phosphorylation, the theoretical maximum yield is 38 ATP molecules. However, factors such as the transportation of intermediates into the mitochondria affect the actual ATP yield, which can range from 1 to 34 ATP molecules.