Model Answer
0 min readIntroduction
Fatty acid oxidation is a catabolic process that breaks down fatty acids to generate acetyl-CoA, which then enters the citric acid cycle, ultimately leading to ATP production. This process is a major energy source, particularly during periods of fasting or prolonged exercise. The complete oxidation of a fatty acid involves three distinct stages: activation of the fatty acid, its transport into the mitochondrial matrix, and the cyclical process of beta-oxidation itself. Each stage is crucial for efficient energy conservation in the form of ATP, either directly or through the generation of reducing equivalents like NADH and FADH2.
Stage 1: Activation of Fatty Acids
The first stage involves activating fatty acids in the cytosol. Long-chain fatty acids are first activated by attaching them to Coenzyme A (CoA) to form fatty acyl-CoA. This reaction is catalyzed by acyl-CoA synthetase and requires ATP hydrolysis.
- Reaction: Fatty acid + CoA + ATP → Fatty acyl-CoA + AMP + PPi
- ATP Consumption: This stage directly consumes 1 ATP molecule per fatty acid activated.
- Significance: Activation makes the fatty acid reactive for subsequent transport and oxidation.
Stage 2: Transport into the Mitochondria
Fatty acyl-CoA cannot directly cross the inner mitochondrial membrane. The carnitine shuttle system facilitates its transport. This involves three key steps:
- Step 1: Fatty acyl-CoA reacts with carnitine, catalyzed by carnitine palmitoyltransferase I (CPT I), located on the outer mitochondrial membrane, forming fatty acyl-carnitine.
- Step 2: Fatty acyl-carnitine is transported across the inner mitochondrial membrane by carnitine acylcarnitine translocase.
- Step 3: Inside the mitochondrial matrix, carnitine palmitoyltransferase II (CPT II) regenerates fatty acyl-CoA and carnitine.
This stage doesn't directly produce ATP, but it's essential for accessing the enzymes required for beta-oxidation within the mitochondrial matrix.
Stage 3: Beta-Oxidation Cycle
This is the core process of fatty acid breakdown, occurring in the mitochondrial matrix. It’s a repetitive four-step cycle that shortens the fatty acyl-CoA by two carbon atoms with each turn, releasing acetyl-CoA.
- Step 1: Oxidation: Fatty acyl-CoA is oxidized by acyl-CoA dehydrogenase, producing FADH2 and a trans-Δ2-enoyl-CoA.
- Step 2: Hydration: Enoyl-CoA is hydrated by enoyl-CoA hydratase, forming L-β-hydroxyacyl-CoA.
- Step 3: Oxidation: L-β-hydroxyacyl-CoA is oxidized by β-hydroxyacyl-CoA dehydrogenase, producing NADH and β-ketoacyl-CoA.
- Step 4: Cleavage: β-ketoacyl-CoA is cleaved by thiolase, releasing acetyl-CoA and a shortened fatty acyl-CoA.
The cycle repeats until the fatty acid is completely broken down into acetyl-CoA molecules.
ATP Conservation from Beta-Oxidation Products
The energy conserved during fatty acid oxidation comes primarily from the reducing equivalents (NADH and FADH2) generated during beta-oxidation and the acetyl-CoA produced.
- NADH: Each NADH molecule yields approximately 2.5 ATP molecules through oxidative phosphorylation in the electron transport chain.
- FADH2: Each FADH2 molecule yields approximately 1.5 ATP molecules through oxidative phosphorylation.
- Acetyl-CoA: Acetyl-CoA enters the citric acid cycle, generating more NADH, FADH2, and GTP (which is equivalent to ATP). Each acetyl-CoA yields approximately 10 ATP molecules when fully oxidized in the citric acid cycle and oxidative phosphorylation.
For example, the beta-oxidation of palmitic acid (a 16-carbon fatty acid) yields 8 acetyl-CoA molecules, 7 FADH2 molecules, and 7 NADH molecules. This translates to a significant ATP yield (approximately 106 ATP molecules) after accounting for the initial ATP investment in the activation stage.
| Product | ATP Yield per Molecule | Total ATP (Palmitic Acid) |
|---|---|---|
| NADH | 2.5 | 17.5 |
| FADH2 | 1.5 | 10.5 |
| Acetyl-CoA | 10 | 80 |
| ATP (Net) | - | 106 (approx.) |
Conclusion
In conclusion, the three stages of fatty acid oxidation – activation, transport, and beta-oxidation – are intricately linked to conserve energy as ATP. While the activation stage consumes ATP, the subsequent stages generate reducing equivalents (NADH and FADH2) and acetyl-CoA, which ultimately fuel the electron transport chain and citric acid cycle, leading to substantial ATP production. This process highlights the efficiency of fatty acids as a concentrated energy source, crucial for sustaining cellular functions.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.