Beta Oxidation
Beta-Oxidation of Fatty Acids
Beta-oxidation is a crucial metabolic process in which fatty acid molecules are broken down to generate energy. This process predominantly occurs in the mitochondria of eukaryotic cells and the cytosol of prokaryotes. The primary function of beta-oxidation is to convert fatty acids into acetyl-CoA, which then enters the citric acid cycle to produce ATP.
Mechanism of Beta-Oxidation
The process of beta-oxidation involves several steps:
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Activation of Fatty Acids: Fatty acids are activated in the cytosol by conjugation with Coenzyme A to form acyl-CoA through the action of acyl-CoA synthetase. This step requires ATP.
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Transport into Mitochondria: In eukaryotes, the acyl-CoA must be transported into the mitochondria. This occurs via the carnitine shuttle, which involves the conversion of acyl-CoA to acyl-carnitine by carnitine acyltransferase, its transport across the mitochondrial inner membrane, and reconversion back to acyl-CoA.
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Beta-Oxidation Cycle: Once inside the mitochondria, the acyl-CoA undergoes a series of reactions catalyzed by the mitochondrial trifunctional protein, which includes:
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Entry into the Citric Acid Cycle: The acetyl-CoA generated enters the citric acid cycle, contributing to further production of ATP, NADH, and FADH2.
Role of Peroxisomes
In addition to mitochondria, beta-oxidation also occurs in peroxisomes, particularly for very long-chain fatty acids. The peroxisomal beta-oxidation differs slightly, as it does not produce ATP directly. Instead, the electrons generated are transferred to oxygen to form hydrogen peroxide.
Importance in Energy Production
Beta-oxidation is a critical process in energy metabolism, especially during fasting or low-carbohydrate conditions, when the body relies on lipids as a major energy source. The complete oxidation of fatty acids yields more energy compared to carbohydrates. For instance, palmitic acid, a common fatty acid, yields about 129 ATP molecules from its complete oxidation.