CHAPTER 5. LIPID METABOLISM

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Nội dung Text: CHAPTER 5. LIPID METABOLISM

INTERNATIONAL UNIVERSITY SCHOOL OF BIOTECHNOLOGY CHAPTER 5. LIPID METABOLISM BIOCHEMISTRY 1
Learning objectives Describe the metabolism of fatty acids. 1. The two main components of fatty acid metabolism 2. are β oxidation and fatty acid synthesis. Understand that the fatty-acid breakdown reactions 3. of β oxidation result in the formation of reduced cofactors and acetyl-CoA molecules, which can be further catabolized to release free energy. Understand that the oxidation of unsaturated, odd- 4. chain, and very-long-chain fatty acids requires additional enzymes, some of them in peroxisomes. Understand how fatty acid synthesis resembles and 5. differs from β oxidation. 2
Content 1. Fatty acid activation 2. Steps of β oxidation 3. Energy yield of oxidation 4. Oxidation of palmitate 5. Unsaturated fatty acids 6. Odd-chain fatty acids 3
Content 7. Very long-chain fatty acids 8. Synthesis vs. oxidation 9. Steps of synthesis 10. Palmitate synthesis 11. Fatty acid synthases Conclusion 4
Fatty Acid Metabolism LEARNING OBJECTIVES Fatty acids are an important energy source, for they yield over twice as much energy as an equal mass of carbohydrate or protein. In humans, the primary dietary source of fatty acids is triacylglycerols. This lecture will describe the metabolism of fatty acids. The two main components of fatty acid metabolism are β oxidation and fatty acid synthesis. Upon completion of this lecture, you will understand that the fatty-acid breakdown reactions of β oxidation result in the formation of reduced cofactors and acetyl- CoA molecules, which can be further catabolized to release free energy. You will also understand that the oxidation of unsaturated, odd-chain, and very-long-chain fatty acids requires additional enzymes, some of them in peroxisomes. In addition, you will understand how fatty acid synthesis resembles and differs from β oxidation. 5
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FATTY ACID ACTIVATION Triacylglycerols are carried by lipoproteins to tissues, where hydrolysis releases their fatty acids from the glycerol backbone. Fatty acids enter the cell and are activated in the cytosol. This activation costs two ATP equivalents per fatty acid. Most of the activated fatty acids are then shuttled into the mitochondria for β oxidation, but a small percentage are carried to the peroxisomes. 8
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STEPS OF β OXIDATION The activated fatty acid is called a fatty acyl-coenzyme A, or fatty acyl-CoA. In the first step of β oxidation, an acyl-CoA dehydrogenase catalyzes the oxidation of the acyl group, resulting the formation of a double bond between carbons two and three. The two electrons removed from the acyl group are transferred to an FAD prosthetic group. These electrons are transferre to ubiquinone through a series of electron transfer reactions. In the second step of β oxidation, a hydratase adds a molecule of water across the double bond produced in the first step. In the third step of β oxidation, another dehydrogenase catalyzes the oxidation of the hydroxyacyl group. In this case, NAD+ is the cofactor. The fourth and final step of β oxidation is called thiolysis. In this step, a thiolase catalyzes the release of acetyl-CoA from the ketoacyl- CoA. 13
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ENERGY YIELD OF OXIDATION One round of β oxidation yields three products—one ubiquinol cofactor, one NADH cofactor, and one molecule of acetyl-CoA. During the citric acid cycle, the acetyl-CoA is used to produce three NADH cofactors, one ubiquinol cofactor, and one molecule of GTP. During oxidative phosphorylation, each ubiquinol cofactor is used to produce two ATP molecules, and each NADH cofactor is used to produce three ATP molecules. The GTP molecule is equivalent to one ATP molecule. In all, one round of β oxidation produces the equivalent of 17 molecules of ATP. Since two ATP equivalents were used for the activation step, the net yield is 15 molecules of ATP. 18
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