Fatty Acid Oxidation Using Palmitic Acid as an Example
Introduction
Fatty acid oxidation, also known as β-oxidation, is the primary process by which fatty acids are broken down to produce energy. This process takes place in the mitochondria of eukaryotic cells. Palmitic acid (C16:0), a saturated long-chain fatty acid, is a common substrate used to illustrate the pathway.
Cellular Location
β-oxidation of fatty acids occurs in the mitochondrial matrix. Fatty acids must first be activated in the cytosol and then transported into the mitochondria via the carnitine shuttle system.
Steps of Fatty Acid Oxidation (β-Oxidation)
1. Activation of Palmitic Acid
- Palmitic acid is converted to palmitoyl-CoA by the enzyme acyl-CoA synthetase using ATP.
2. Transport into Mitochondria
- Palmitoyl-CoA is transported into the mitochondrial matrix by the carnitine shuttle system involving carnitine acyltransferase I and II.
3. β-Oxidation Cycle
Each cycle removes two carbon atoms from the fatty acid chain as acetyl-CoA. The steps in each cycle are:
- Dehydrogenation: Acyl-CoA is oxidized to trans-Δ²-enoyl-CoA by acyl-CoA dehydrogenase, producing FADH₂.
- Hydration: Trans-Δ²-enoyl-CoA is hydrated to L-3-hydroxyacyl-CoA by enoyl-CoA hydratase.
- Oxidation: L-3-hydroxyacyl-CoA is oxidized to 3-ketoacyl-CoA by 3-hydroxyacyl-CoA dehydrogenase, producing NADH.
- Thiolysis: 3-ketoacyl-CoA is cleaved by thiolase to form acetyl-CoA and a shortened acyl-CoA.
Total Energy Yield from Palmitic Acid
- Palmitic acid (16 carbons) undergoes 7 cycles of β-oxidation.
- This yields:
- 8 Acetyl-CoA
- 7 FADH₂
- 7 NADH
- Each Acetyl-CoA enters the TCA cycle producing ~10 ATP.
- Each NADH and FADH₂ produce ~2.5 and ~1.5 ATP respectively in oxidative phosphorylation.
- Total ATP: ~106 ATP (after subtracting 2 ATP for activation)
Conclusion
Fatty acid oxidation is a highly efficient process for energy production. Palmitic acid, when oxidized via the β-oxidation pathway in mitochondria, yields a significant amount of ATP, making it a major fuel source, especially during fasting or extended exercise.