Beta-oxidation is the process by which fatty acids are broken down into acetyl-CoA molecules, which can then enter the citric acid cycle (also known as the Krebs cycle) to generate energy in the form of ATP. The process of beta-oxidation occurs in the mitochondria of cells and involves a series of enzymatic reactions. Let’s outline the cycles involved in beta-oxidation:
- Activation: Before beta-oxidation can begin, the fatty acid molecule must be activated. This involves the addition of a coenzyme A (CoA) molecule to the fatty acid, forming acyl-CoA. This step requires the input of ATP.
- First cycle: In the first cycle of beta-oxidation, a series of four enzymatic reactions occur: a. Oxidation: The fatty acyl-CoA molecule is oxidized by the enzyme acyl-CoA dehydrogenase, resulting in the formation of a trans double bond between the α and β carbons. b. Hydration: Water is added across the double bond by the enzyme enoyl-CoA hydratase, forming a hydroxyl group. c. Oxidation: The hydroxyl group is oxidized to a ketone by the enzyme hydroxyacyl-CoA dehydrogenase, regenerating NADH in the process. d. Cleavage: The ketone is cleaved by the enzyme thiolase, resulting in the formation of acetyl-CoA and a fatty acyl-CoA molecule shortened by two carbons.
- Subsequent cycles: The process continues iteratively, with each cycle shortening the fatty acyl-CoA chain by two carbons. The shortened fatty acyl-CoA molecule undergoes the same series of four enzymatic reactions until the entire fatty acid molecule is converted into acetyl-CoA units.
The number of cycles required depends on the length of the original fatty acid chain. For example, a fatty acid with 16 carbons would undergo seven cycles of beta-oxidation to produce eight acetyl-CoA molecules.
Overall, beta-oxidation is a crucial metabolic pathway for the catabolism of fatty acids, providing a major source of energy for many cells, particularly during periods of fasting or prolonged exercise.