The ratio of the number of moles of ATP synthesized per mole of O atoms reduced to water (OR, equivalently, per mole of cofactor re-oxidized OR per 2e^– passed along the ETC) is called the P/O ratio.

x (ADP + P_i) + NADH + H⁺ + ½ O₂ → x ATP + NAD⁺ + H₂O

x = P/O ratio

To measure the overall efficiency of oxidative phosphorylation, we have to make a preparation of mitochondria and measure simultaneously

(a) the amount of ATP they produced and

(b) the amount of oxygen they consume (and, hence, the amount of NADH they oxidize).

The presently accepted values of the mitochondrial P/O ratios are about 2.5 for NADH and 1.5 for FADH₂.

The P/O ratios are not intergers. This is because the connection between electron transport and ATP synthesis is indirect – chemiosmotic – not stoichiometric: the coupling occurs only via the proton-motive force. So the P/O ratio depends on the detailed mechanism by which the proton gradient energy is used to power ATP synthesis.

Note that the ATP yield for FADH₂ is about 60% of the value for NADH. This is consistent with the observation mentioned earlier that the yield of protons translocated across the IMM is 6 per FADH₂ vs. 10 per NADH.

Now that we know the P/O ratios, we can finalize our calculation of the energy yield from lipid catabolism.

The oxidation of fatty acids begins with the pathway of beta oxidation (Figure 26.1), which breakdowns the fatty acid chain into units of acetyl CoA. Acetyl CoA then will enter the TCA cycle to be completely oxidized to CO₂. There is one “substrate level phosphorylation” step in the TCA cycle and none in the beta oxidation pathway. The majority of energy of oxidation of the fatty acid to CO₂ is conserved in the formation of reduced cofactors. These cofactors will be gathered and oxidized through the ETC leading to the synthesis of ATP through the process of “oxidative phosphorylation”.

In order to calculate the number of ATP generated through the oxidation of a mole of palmitate (or any other fatty acid), we need to calculate the total number of reduced cofactors that would be generated when it is completely converted to CO₂. We will also need to account for any ATP synthesized through substrate level phosphorylation.

Here’s how to carry out this calculation:

1. Determine the number of rounds of beta oxidation that are required to remove all carbon atoms of the fatty acid as acetyl CoA.
2. Determine the number of Acetyl CoA’s that would be generated through the beta oxidation of the fatty acid.

For the fatty acid palmitate, the answer to question 1 is 7 and for question 2 is 8. We need these numbers to determine the total number of reduced cofactors formed (and any ATP through substrate level phosphorylation).

 

Determining the total yield of ATP from oxidation of a mole of palmitate to CO₂ and H₂O

Each FADH₂ yields 1.5 ATPs and each NADH yields 2.5 ATPs

Beta oxidation (7 turns )

• 1 NADH per turn x 7 turns x 2.5 (P/O ratio) = 17.5
• 1 FADH₂ per turn x 7 turns x 1.5 (P/O ratio) = 10.5

TCA Cycle (8 turns)

• 3 NADH per turn x 8 turns x 2.5 = 60
• 1 FADH₂ per turn x 8 turns x 1.5 = 12
• 1 GTP/ATP per turn x 8 turns = 8

TOTAL = 108

Table 26.1 shows a summary of this calculation.

Note that if we were calculating the net yield of ATP for the oxidation of a mole of palmitate (instead of palmitoyl CoA), then we have to subtract the ATPs used in the activation of palmitate to palmitoyl CoA. Because this reaction hydrolysed both phosphoanhydride bonds of ATP, the energetic cost is considered equivalent to two ATPs (ie. Similar to two moles of ATP being hydrolysed into two moles of ADP and 2 moles of Pi), and the net gain per molecule of palmitate is 106 ATP.

 

Thermodynamic efficiency of fatty acid oxidation

CH₃(CH₂)₁₄COOH  +  23 O₂   →   16 O₂  +  16 H₂O

The ΔG^0′ for the oxidation of palmitate to CO₂ and H₂O is about 9800 kJ/mol. Under standard conditions the energy recovered as the phosphate bond energy of ATP = 106 x 30.5  kJ/mol = 3230 kJ/mol. This is ~33% of the theoretical maximum. However, under intracellular conditions, the free energy recovery is more than 60%! This is almost twice as efficient as a modern combustion engine in an automobile (35%)! This is a remarkable conservation of energy!

 

Calculating the ATP yield for glucose oxidation

Here is a balance sheet for aerobic oxidation of glucose. Glucose will proceed through the pathways/reactions of glycolysis, pyruvate dehydrogenase and the TCA cycle to be completely oxidized to CO₂. As you study it, think about the enzymes which contribute to each line item. The stoichiometry is given relative to one mole of glucose. The energy yield of the cytosolic NADH is given as 3 or 5 ATP, depending on which of the two shuttle systems is used to move the reducing equivalents into the mitochondrion.

Glycolysis (2 turns pay-off phase)

1 NADH x 2 turns x 1.5^*(or 2.5^**)= 3^* or 5^**

2 ATP x 2 turns = 4

2 ATPs used = -2

Pyruvate dehydrogenase (2 turns)

1 NADH x 2 turns x 2.5 = 5

TCA Cycle (2 turns)

3 NADH per turn x 2 turns x 2.5 = 15

1 FADH₂ per turn x 2 turns x 1.5 = 3

1 GTP/ATP per turn x 2 turns = 2

Total = 30 or 32

* Glycerol 3-phosphate shuttle ; ** Malate aspartate shuttle

Table 26.2 below shows a summary of this calculation:

 

Thermodynamic efficiency of Glucose oxidation

Glucose: C₆H₁₂O₆  +  6 O₂   →   6 CO₂  +  6 H₂O

ΔG˚ = -2,840 kJ/mol

32 ATP ~ 32 x 50 kJ/mol

= 1,600 kJ/mol

Efficiency ~ 1,600 / 2,840 ~ 56%

Now we can calculate the thermodynamic efficiency of carbohydrate metabolism: what fraction of the free energy tied up in glucose is recovered as ATP energy, under aerobic conditions? The answer is … somewhat more than 50%. Under anaerobic (fermentative) conditions, the efficiency of glycolysis is similar, but the ATP yield is much less – the free energy liberated by merely cleaving glucose into trioses is much less than that liberated by complete oxidation to CO₂.

The energy yield (108 ATP per palmitoyl CoA) from fatty acids is much higher than for glucose – but of course palmitic acid has 16 carbon atoms, whereas glucose has only 6. Even so, the energy yield per carbon atom is also higher for the fatty acid – and you should understand why this is so.