Fat Synthesis and Degradation

Fat Synthesis and Degradation - C H A P T E R 13 FAT...

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155 C H A P T E R 13 FAT SYNTHESIS AND DEGRADATION Fatty Acid Synthesis Function Fatty Acid Synthesis Location Fatty Acid Synthesis Connections Fatty Acid Synthesis Regulation Fatty Acid Synthesis ATP Costs (for C 16 ) Fatty Acid Synthesis Equation Elongation and Desaturation Triglyceride and Phospholipid Synthesis b -Oxidation Function b -Oxidation Location Carnitine Shuttle b -Oxidation Connections b -Oxidation Regulation b -Oxidation ATP Yield b -Oxidation Equation b -Oxidation of Unsaturated Fatty Acids b -Oxidation of Odd-Chain-Length Fatty Acids
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156 Basic Concepts in Biochemistry FATTY ACID SYNTHESIS LOCATION Liver and adipose cytoplasm FATTY ACID SYNTHESIS FUNCTION To synthesize fatty acids from acetyl-CoA (Fig. 13-1) NADPH NADPH O O 2 C H 2 O H 2 O O - ATP CO 2 Malonyl-CoA S-Enz S-Enz S-Enz S-Enz S-Enz SCoA Acetyl CoA Acetyl-CoA Carboxylase (biotin) FATTY ACID SYNTHASE FATTY ACID In the first turn of the cycle, AcetylCoA serves as a primer to get things started In the last turn of the cycle, hydrolysis releases a C 16 fatty acid O O O O O O O OH Figure 13-1 Fatty Acid Synthesis The first formation of a carbon–carbon bond occurs between malonyl and acetyl units bound to fatty acid synthase. After reduction, dehydration, and further reduction, the acyl enzyme is condensed with more malonyl-CoA and the cycle is repeated until the acyl chain grows to C 16 . When the growing fatty acid reaches a chain length of 16 carbons, the acyl group is hydrolyzed to give the free fatty acid.
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13 Fat Synthesis and Degradation 157 The major control point for fatty acid synthesis is acetyl-CoA car- boxylase. The enzyme is inactivated by phosphorylation and activated by high concentrations of citrate. Calculating energy costs for the synthesis of a C 16 fatty acid from acetyl-CoA is not as simple as you might first think. The major compli- cation is that acetyl-CoA is made in the mitochondria, but fatty acid syn- thesis occurs in the cytosol—acetyl-CoA can’t cross the mitochondrial membrane. Acetyl-CoA gets out of the mitochondria disguised as citrate. The acetyl-CoA is condensed with oxaloacetate to give citrate, and the citrate leaves the mitochondria. In the cytosol, the citrate is cleaved by an ATP-dependent citrate lyase into acetyl-CoA and oxaloacetate: Citrate 1 ATP 1 CoA ¡ acetyl-CoA 1 ADP 1 P i 1 oxaloacetate FATTY ACID SYNTHESIS ATP COSTS (FOR C 16 ) 8 acetyl-CoA mito ¡ 8 acetyl-CoA cyto 2 16 ATP 7 acetyl-CoA ¡ 7 malonyl-CoA 2 7 ATP 14NADPH ¡ 14NADP 1 2 42 ATP Total cost: 2 65 ATP FATTY ACID SYNTHESIS REGULATION Primary signals: Insulin turns on . Glucagon turns off . Epinephrine turns off . Phosphorylation
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Fat Synthesis and Degradation - C H A P T E R 13 FAT...

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