Beta oxidation is a metabolic process in which fatty acid molecules are broken down to generate energy. This process plays a central role in cellular metabolism and supports ATP production.
Mechanism:
Fatty acids are first converted to acyl-CoA chains, which are then progressively shortened through a series of enzymatic reactions. Each cycle releases acetyl-CoA, FADH2, and NADH, which enter the citric acid cycle (Krebs cycle) to produce ATP.
Location of Beta Oxidation
- Eukaryotic Cells: Beta oxidation primarily occurs in the mitochondria.
- Prokaryotic Cells: Beta oxidation takes place in the cytosol.
- Peroxisomal Beta Oxidation: In peroxisomes, the process produces H2O2 and heat instead of FADH2 and NADH, highlighting an important difference in energy handling.
Step 1: Entry of Fatty Acids into Cells
- Fatty acids cross the cell membrane and bind to coenzyme A, forming fatty acyl-CoA.
- In eukaryotes, long-chain acyl-CoA molecules use the carnitine shuttle to enter the mitochondria.
- CPT-1 converts acyl-CoA to acyl-carnitine on the outer mitochondrial membrane.
- Carnitine translocase (CAT) transports acyl-carnitine across the mitochondrial membrane.
- Short-chain acyl-CoA molecules can freely diffuse into the mitochondria.
Step 2: Beta Oxidation Inside the Mitochondria
Beta oxidation occurs through four repeating steps:
1. Dehydrogenation
- Enzyme: Acyl-CoA dehydrogenase
- Reaction: Introduces a double bond between C2 and C3, forming trans-Δ2-enoyl-CoA.
- Produces FADH2, which contributes to ATP production via the electron transport chain.
2. Hydration
- Enzyme: Enoyl-CoA hydratase
- Reaction: Adds a hydroxyl group at C3, forming L-3-hydroxyacyl-CoA.
- Water is required in this step.
3. Oxidation
- Enzyme: 3-hydroxyacyl-CoA dehydrogenase
- Reaction: Oxidizes the hydroxyl group to form β-ketoacyl-CoA, producing NADH.
- NADH enters the citric acid cycle to generate ATP.
4. Thiolysis
- Enzyme: β-ketothiolase
- Reaction: Cleaves β-ketoacyl-CoA using CoA, releasing acetyl-CoA.
Special Cases: Odd-Numbered Fatty Acids
- The last cycle produces propionyl-CoA (3 carbons) and acetyl-CoA (2 carbons).
- Propionyl-CoA converts to succinyl-CoA, which enters the citric acid cycle for ATP production.
Energy Yield per Beta Oxidation Cycle
- 1 FADH2 → 2 ATP
- 1 NADH → 3 ATP
- 1 Acetyl-CoA → 12 ATP
- Total ATP per cycle ≈ 17 ATP
- Actual yield: ~12–16 ATP per cycle depending on chain length and cellular conditions.
- Beta oxidation provides a high-energy source from fatty acids.
- It occurs in both mitochondria and peroxisomes, with slight differences in energy products.
- The process is regulated by transport mechanisms, chain length, and enzyme activity
