29.3 Catabolism of Triacylglycerols: β-Oxidation
2 min read•may 7, 2024
Fatty acids are crucial energy sources for our bodies. β-oxidation breaks them down into smaller units, releasing energy. This process happens in mitochondria, the powerhouses of cells, and involves a series of chemical reactions that gradually shorten fatty acid chains.
The breakdown of fatty acids yields molecules, which enter the citric acid cycle for further energy production. Odd-chain fatty acids produce an extra molecule called , which gets converted to , another energy-rich compound used by cells.
Catabolism of Triacylglycerols: β-Oxidation
Sequence of β-oxidation reactions
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- Activation of fatty acids converts them to by the enzyme using ATP and coenzyme A (CoA)
- Transport of fatty acyl-CoA into the via the
- Oxidation of fatty acyl-CoA by forms a trans double bond between C-2 and C-3, reducing to
- Hydration of the double bond by adds water, forming with a hydroxyl group on C-3
- Oxidation of L-β-hydroxyacyl-CoA by forms , reducing to
- Thiolysis of β-ketoacyl-CoA by cleaves it using the thiol group of a new CoA molecule, releasing acetyl-CoA and a fatty acyl-CoA two carbons shorter than the original (, )
β-oxidation of odd vs even fatty acids
- Even-chain fatty acids (, palmitic acid) undergo complete β-oxidation, forming only acetyl-CoA molecules
- The final round of β-oxidation produces two acetyl-CoA molecules
- Odd-chain fatty acids (, ) undergo β-oxidation until a three-carbon propionyl-CoA remains
- Propionyl-CoA is converted to succinyl-CoA, a citric acid cycle intermediate, through a series of reactions:
- carboxylates propionyl-CoA to form
- isomerizes D-methylmalonyl-CoA to
- converts L-methylmalonyl-CoA to succinyl-CoA using as a cofactor
- Propionyl-CoA is converted to succinyl-CoA, a citric acid cycle intermediate, through a series of reactions:
Acetyl CoA yield from fatty acids
- For even-chain fatty acids with n carbon atoms:
- Number of acetyl-CoA produced =
- Palmitic acid (C16) will produce acetyl-CoA molecules
- For odd-chain fatty acids with n carbon atoms:
- Number of acetyl-CoA produced =
- One propionyl-CoA (3 carbons) is produced in addition to the acetyl-CoA molecules
- Heptadecanoic acid (C17) will produce acetyl-CoA molecules and 1 propionyl-CoA molecule
β-Oxidation Process and Energy Yield
- β-oxidation occurs in the mitochondrial matrix through a series of repeating reactions known as the
- Each cycle of the β-oxidation spiral produces:
- 1 FADH2
- 1 NADH
- 1 Acetyl-CoA
- The from β-oxidation is significant, with each acetyl-CoA entering the citric acid cycle for further oxidation
- Under conditions of low glucose availability, excess acetyl-CoA can be converted to , which serve as alternative fuel sources for tissues like the brain
Key Terms to Review (32)
Acetyl-CoA: Acetyl-CoA is a crucial metabolic intermediate that serves as a central hub in various cellular processes, including the citric acid cycle, fatty acid synthesis, and acetylation reactions. It is the primary entry point for the oxidation of carbohydrates, fats, and some amino acids, linking these catabolic pathways to the production of energy in the form of ATP.
Acyl-CoA Dehydrogenase: Acyl-CoA dehydrogenase is an enzyme that catalyzes the first step in the beta-oxidation pathway, which is the process of breaking down fatty acids to generate energy in the form of ATP. It is a crucial component in the catabolism of triacylglycerols, the main storage form of fat in the body.
Acyl-CoA Synthetase: Acyl-CoA synthetase is an enzyme that catalyzes the activation of fatty acids by converting them into acyl-CoA thioesters. This is a crucial step in the catabolism of triacylglycerols through the process of β-oxidation, where fatty acids are broken down to generate energy for the cell.
Carnitine Shuttle: The carnitine shuttle is a transport mechanism that facilitates the movement of long-chain fatty acids from the cytosol into the mitochondrial matrix, where they can undergo β-oxidation to generate energy for the cell.
D-methylmalonyl-CoA: D-methylmalonyl-CoA is an important intermediate in the catabolism of certain amino acids and odd-chain fatty acids. It is a key metabolite involved in the process of β-oxidation, which is the breakdown of fatty acids to produce energy for the body.
Energy Yield: Energy yield refers to the amount of usable energy released or generated during a metabolic process, such as the breakdown or oxidation of organic compounds. It is a crucial concept in understanding the efficiency and energy dynamics of various catabolic pathways, including the β-oxidation of triacylglycerols.
Enoyl-CoA hydratase: Enoyl-CoA hydratase is an enzyme that catalyzes a key step in the beta-oxidation of fatty acids, where it adds water to an enoyl-CoA intermediate to form a 3-hydroxyacyl-CoA product. This enzyme plays a crucial role in the catabolic breakdown of triacylglycerols and the generation of energy from fatty acid sources.
FAD: FAD, or Flavin Adenine Dinucleotide, is a coenzyme that plays a crucial role in various metabolic processes within the body. It serves as an essential cofactor for numerous enzymes involved in energy production, oxidation-reduction reactions, and other vital biochemical pathways.
FADH2: FADH2 is a reduced coenzyme that plays a crucial role in various metabolic pathways, including the biosynthesis of steroids, the catabolism of triacylglycerols, and the citric acid cycle. It is the reduced form of flavin adenine dinucleotide (FAD), a cofactor essential for many oxidation-reduction reactions in the body.
Fatty Acyl-CoA: Fatty acyl-CoA refers to the coenzyme A (CoA) thioester derivative of a fatty acid, which is the activated form of a fatty acid that can undergo β-oxidation to generate energy for the cell. This term is crucial in understanding the catabolism of triacylglycerols through the process of β-oxidation.
Heptadecanoic Acid: Heptadecanoic acid, also known as margaric acid, is a saturated fatty acid with a 17-carbon chain. It is a minor component found in some animal fats and vegetable oils, and is particularly relevant in the context of the catabolism of triacylglycerols through β-oxidation.
Ketone Bodies: Ketone bodies are a group of three water-soluble molecules (acetoacetate, beta-hydroxybutyrate, and acetone) that are produced by the liver as a byproduct of the breakdown of fatty acids through a process called beta-oxidation. They serve as an alternative energy source when glucose is scarce, such as during fasting or starvation.
L-methylmalonyl-CoA: L-methylmalonyl-CoA is an intermediate metabolite that plays a crucial role in the catabolism of triacylglycerols through the process of β-oxidation. It is formed during the breakdown of certain amino acids and odd-chain fatty acids.
L-β-hydroxyacyl-CoA: L-β-hydroxyacyl-CoA is an important intermediate in the process of fatty acid β-oxidation, which is the catabolic pathway for breaking down triacylglycerols to generate energy for the body. This compound is a key step in the stepwise degradation of fatty acid chains within the mitochondria.
L-β-hydroxyacyl-CoA dehydrogenase: L-β-hydroxyacyl-CoA dehydrogenase is an enzyme that catalyzes a key step in the β-oxidation pathway, which is the process of breaking down fatty acids to generate energy for the body. This enzyme is responsible for the oxidation of L-β-hydroxyacyl-CoA intermediates, an essential part of the catabolism of triacylglycerols.
Methylmalonyl-CoA Epimerase: Methylmalonyl-CoA epimerase is an enzyme that catalyzes the interconversion of the two stereoisomers of methylmalonyl-CoA, a key intermediate in the catabolism of odd-chain fatty acids and certain amino acids. This enzyme plays a crucial role in the β-oxidation pathway, ensuring the proper processing of these substrates.
Methylmalonyl-CoA Mutase: Methylmalonyl-CoA mutase is an enzyme that catalyzes a key step in the catabolism of certain amino acids and odd-chain fatty acids. It is an essential enzyme involved in the breakdown and utilization of these molecules for energy production within the body.
Mitochondrial Matrix: The mitochondrial matrix is the dense, liquid-filled space within the inner membrane of a mitochondrion, the powerhouse of the cell. It is the site where key metabolic processes like the citric acid cycle and fatty acid oxidation take place, providing the cell with the energy it needs to function.
Myristic Acid: Myristic acid is a saturated fatty acid with a 14-carbon chain. It is a common component of many animal and vegetable fats and oils, and it plays a role in the catabolism of triacylglycerols through the process of β-oxidation.
NAD+: NAD+ (Nicotinamide Adenine Dinucleotide) is an essential coenzyme involved in numerous metabolic processes within the body. It plays a crucial role in the oxidation of organic compounds, serving as an electron acceptor in various redox reactions.
NADH: NADH, or nicotinamide adenine dinucleotide, is a coenzyme that plays a crucial role in numerous metabolic processes within the body. It is the reduced form of NAD+, an important electron carrier that is involved in oxidation-reduction reactions throughout the cell's energy-producing pathways.
Palmitic Acid: Palmitic acid is a saturated fatty acid that is commonly found in various lipids, including waxes, fats, and oils. It plays important roles in the formation of soap, the structure of phospholipids, and the metabolic processes of fatty acid catabolism and biosynthesis.
Pentadecanoic Acid: Pentadecanoic acid is a saturated fatty acid with 15 carbon atoms. It is a naturally occurring compound found in various food sources and is an important intermediate in the catabolism of triacylglycerols through the process of β-oxidation.
Propionyl-CoA: Propionyl-CoA is a key metabolic intermediate that is produced during the catabolism of certain amino acids and the breakdown of odd-chain fatty acids. It plays a crucial role in the context of 29.3 Catabolism of Triacylglycerols: β-Oxidation, as it is an important product of this process.
Propionyl-CoA Carboxylase: Propionyl-CoA carboxylase is an enzyme complex that catalyzes the carboxylation of propionyl-CoA to form D-methylmalonyl-CoA, an important step in the catabolism of certain amino acids, odd-chain fatty acids, and cholesterol. This enzyme plays a crucial role in the breakdown of triacylglycerols through the process of β-oxidation.
Stearic Acid: Stearic acid is a long-chain saturated fatty acid that is commonly found in various fats and oils. It plays important roles in the context of functional groups, waxes, fats, and oils, soap, as well as the catabolism of triacylglycerols through β-oxidation.
Succinyl-CoA: Succinyl-CoA is a key intermediate in cellular metabolism, serving as an important link between the catabolism of fatty acids, amino acids, and carbohydrates, and the citric acid cycle. It is a coenzyme A thioester that is produced during the breakdown of various macromolecules and plays a crucial role in energy production within the cell.
Vitamin B12: Vitamin B12, also known as cobalamin, is an essential nutrient that plays a crucial role in various metabolic processes, including the catabolism of triacylglycerols through β-oxidation. It is a water-soluble vitamin that is required for the proper functioning of the nervous system and the production of red blood cells.
β-ketoacyl-CoA: β-ketoacyl-CoA is an important intermediate in the process of fatty acid β-oxidation, which is the catabolic pathway that breaks down fatty acids to generate acetyl-CoA for energy production.
β-ketothiolase: β-ketothiolase is an enzyme that plays a crucial role in the catabolism of triacylglycerols through the process of β-oxidation. It catalyzes the cleavage of the carbon-carbon bond between the α and β carbons of a β-keto acyl-CoA molecule, releasing acetyl-CoA and a shorter acyl-CoA chain.
β-Oxidation pathway: The β-Oxidation pathway is a metabolic process that breaks down fatty acids into acetyl-CoA units, which can then enter the citric acid cycle to produce energy. This pathway occurs in the mitochondria of cells and is a major source of ATP from fat metabolism.
β-Oxidation Spiral: The β-oxidation spiral is a cyclic process that breaks down fatty acids to generate acetyl-CoA, which can then enter the citric acid cycle to produce energy in the form of ATP. This process is a key component of the catabolism of triacylglycerols, the main storage form of fat in the body.