Unsaturated fatty acids are degraded through a process that essentially mirrors the beta-oxidation of saturated fatty acids, with a few key differences to handle the double bonds. The molecule's carbon chain is systematically shortened by removing acetyl-CoA units until the entire carbon skeleton is broken down.
The Process of Unsaturated Fatty Acid Degradation
The degradation of unsaturated fatty acids involves a modified version of beta-oxidation. This process breaks down fatty acids into acetyl-CoA molecules, which can then enter the citric acid cycle to produce energy. Here's a breakdown:
- Initial Steps: The initial steps are the same as those for saturated fatty acids, involving activation and transport into the mitochondria.
- Beta-Oxidation: This involves a repeating sequence of four reactions: oxidation, hydration, oxidation, and thiolysis. These steps shorten the fatty acid chain by two carbons, releasing one molecule of acetyl-CoA.
- Handling Double Bonds: The presence of double bonds requires additional enzymatic steps. These enzymes rearrange or reduce the double bonds to a form that can be processed by the standard beta-oxidation enzymes. Two key enzymes involved are:
- Enoyl-CoA isomerase: Converts cis double bonds to trans double bonds and repositions them so that beta-oxidation can continue.
- 2,4-Dienoyl-CoA reductase: Reduces conjugated double bonds, requiring NADPH.
- Repetition: The beta-oxidation cycle continues, removing two-carbon units as acetyl-CoA, until the fatty acid is completely degraded.
- Final Products: The final products are acetyl-CoA, FADH2, and NADH. Acetyl-CoA enters the citric acid cycle, while FADH2 and NADH donate electrons to the electron transport chain for ATP production.
Enzymes Involved
| Enzyme | Function |
|---|---|
| Acyl-CoA dehydrogenase | Catalyzes the formation of a double bond between the alpha and beta carbons. |
| Enoyl-CoA hydratase | Catalyzes the addition of water across the double bond. |
| Hydroxyacyl-CoA dehydrogenase | Catalyzes the oxidation of the hydroxyl group. |
| Thiolase | Catalyzes the cleavage of the carbon-carbon bond, releasing acetyl-CoA. |
| Enoyl-CoA isomerase | Converts cis double bonds to trans double bonds for further processing in beta-oxidation. |
| 2,4-Dienoyl-CoA reductase | Reduces conjugated double bonds, requiring NADPH. |
Key Considerations
- Isomerization: Unsaturated fatty acids often have cis double bonds, which are not suitable substrates for the enzymes of beta-oxidation. Isomerases convert these cis bonds into trans bonds.
- Reduction: Some unsaturated fatty acids have double bonds in positions that interfere with beta-oxidation, necessitating reduction reactions to prepare them for further degradation.
- Energy Yield: While the overall process is similar to saturated fatty acid oxidation, the presence of double bonds can slightly affect the overall energy yield. However, degradation proceeds by splitting off acetyl-CoA until the entire carbon skeleton is degraded.
