Which step in metabolism produces the greatest ATP yield per carbon atom?

Question

BlackTom AI · Accepted Answer

β-oxidation of fatty acids yields the most ATP per carbon because fatty acids are highly reduced (more H per C) and their stepwise cleavage generates more reduced cofactors (NADH, FADH2) per carbon than carbohydrate or amino-acid oxidation.

Relevant concepts:
- ATP equivalents from electron carriers: NADH ≈ 2.5 ATP, FADH2 ≈ 1.5 ATP, GTP = 1 ATP.
- β-oxidation repeatedly cleaves 2-carbon acetyl-CoA units, producing NADH and FADH2 plus acetyl‑CoA that enters the citric acid cycle (TCA), which yields additional NADH/FADH2/GTP.
- Compare complete oxidation yields on a per-carbon basis to decide which pathway is most ATP-dense.

Step-by-step calculation (using palmitate, C16, as a representative fatty acid):
1) β-oxidation of palmitate:
- Number of β-oxidation cycles = (n/2) − 1 = 8 − 1 = 7 cycles.
- Products from cycles: 7 FADH2 and 7 NADH.
- Acetyl-CoA produced = n/2 = 8 acetyl-CoA.
2) ATP from acetyl-CoA oxidation in TCA:
- Per acetyl-CoA: 3 NADH (3 × 2.5 = 7.5), 1 FADH2 (1 × 1.5 = 1.5), 1 GTP = 1 → total 10 ATP per acetyl-CoA.
- For 8 acetyl-CoA: 8 × 10 = 80 ATP.
3) ATP from β-oxidation cofactors:
- 7 NADH → 7 × 2.5 = 17.5 ATP.
- 7 FADH2 → 7 × 1.5 = 10.5 ATP.
4) Gross total = 80 + 17.5 + 10.5 = 108 ATP.
5) Subtract activation cost (palmitate → palmitoyl-CoA uses 2 ATP equivalents) → net = 106 ATP.
6) Per-carbon yield = 106 / 16 = 6.625 ATP per carbon.

Compare with glucose (complete oxidation of one glucose, C6):
- Standard yield ≈ 30 ATP per glucose → per-carbon = 30/6 = 5 ATP per carbon.

Citric acid cycle (per acetyl-CoA, 2 carbons) yields 10 ATP → per-carbon = 10/2 = 5 ATP per carbon.

Oxidation of amino acids is variable but generally gives similar or lower ATP per carbon than carbohydrates because of deamination costs and varying oxidation states of side chains; typical values are ≤ ~5 ATP/C depending on the amino acid.

Why the correct option is correct:
- β-oxidation (plus subsequent TCA/ETC oxidation of produced acetyl‑CoA) produces ≈6.625 ATP per carbon for palmitate, higher than the ≈5 ATP/C from carbohydrate oxidation or TCA alone, and higher than typical amino-acid oxidation values.

Why the other choices are incorrect:
- Glycolysis alone produces very little ATP per carbon (net 2 ATP per glucose = 0.33 ATP/C); even full glucose oxidation yields ≈5 ATP/C, less than β-oxidation.
- Citric acid cycle yields 10 ATP per acetyl-CoA (5 ATP/C), lower than β-oxidation’s per-carbon yield.
- Oxidation of amino acids varies and usually yields ≤ carbohydrate oxidation per carbon due to deamination and variable side-chain oxidation; it does not exceed fatty-acid oxidation.

Conclusion: β-oxidation of fatty acids produces the greatest ATP yield per carbon atom, which matches the provided answer.

Which step in metabolism produces the greatest ATP yield per carbon atom?单项选择题

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