pob5e_solutions_ch21

Pob5e_solutions_ch21 - 2608T_ch21sm_S247-S257 1:00 pm Page S-247 pinnacle 111:WHQY028:Solutions Manual:Ch-21 chapter Lipid Biosynthesis 21 1

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Lipid Biosynthesis chapter 21 S-247 1. Pathway of Carbon in Fatty Acid Synthesis Using your knowledge of fatty acid biosynthesis, pro- vide an explanation for the following experimental observations: (a) Addition of uniformly labeled [ 14 C]acetyl-CoA to a soluble liver fraction yields palmitate uni- formly labeled with 14 C. (b) However, addition of a trace of uniformly labeled [ 14 C]acetyl-CoA in the presence of an excess of unlabeled malonyl-CoA to a soluble liver fraction yields palmitate labeled with 14 C only in C-15 and C-16. Answer Recall that “loading” of the acyl carrier protein requires an initial addition of acetyl-CoA, followed by the addition of malonyl-CoA. Malonyl-CoA is normally produced by the addition of CO 2 to acetyl-CoA by acetyl-CoA carboxylase. (a) In the presence of an excess of [ 14 C]acetyl-CoA, the metabolic pool of malonyl-CoA be- comes labeled at C-1 and C-2. This results in the formation of uniformly labeled palmitate. (b) If a trace of [ 14 C]acetyl-CoA is introduced in the presence of a large excess of unlabeled malonyl-CoA, the metabolic pool of malonyl-CoA does not become labeled. The trace of [ 14 C]acetyl-CoA is loaded onto the acyl carrier protein (to become C-15 and C-16 of palmi- tate) rather than transformed into malonyl-CoA (a slow, rate-controlling process). In addi- tion, any labeled malonyl-CoA is diluted by the presence of excess unlabeled malonyl-CoA. 2. Synthesis of Fatty Acids from Glucose After a person has ingested large amounts of sucrose, the glucose and fructose that exceed caloric requirements are transformed to fatty acids for triacylglycerol synthesis. This fatty acid synthesis consumes acetyl-CoA, ATP, and NADPH. How are these substances produced from glucose? Answer Glucose and fructose are degraded to pyruvate via glycolysis in the cytosol. Pyruvate enters the mitochondrion and is oxidatively decarboxylated to acetyl-CoA, some of which en- ters the citric acid cycle. The reducing equivalents (NADH, FADH 2 ) produced in the citric acid cycle are used for ATP production by oxidative phosphorylation. The remainder of the acetyl-CoA is exported to the cytosol (via the acetyl group shuttle) for fatty acid synthesis (see Fig. 21–10). Some NADPH is produced in the pentose phosphate pathway (Fig. 14–21) in the cytosol, using glucose as a substrate, and some is produced by the action of malic enzyme on cytoplasmic malate. 3. Net Equation of Fatty Acid Synthesis Write the net equation for the biosynthesis of palmitate in rat liver, starting from mitochondrial acetyl-CoA and cytosolic NADPH, ATP, and CO 2 . Answer Most of the acetyl-CoA used in fatty acid synthesis is formed from the oxidation of pyruvate in mitochondria. The inner mitochondrial membrane is impermeable to acetyl-CoA, and transfer across the membrane occurs via the acetyl group shuttle (see Fig. 21–10). This process requires the input of 1 ATP per acetyl-CoA 2608T_ch21sm_S247-S257 02/22/2008 1:00 pm Page S-247 pinnacle 111:WHQY028:Solutions Manual:Ch-21:
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S-248 Chapter 21 Lipid Biosynthesis Acetyl-CoA (mit) ± ATP ± H 2 O 88n acetyl-CoA (cyt) ± ADP ±
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This note was uploaded on 09/24/2011 for the course CHEM 369 taught by Professor Wang during the Spring '11 term at University of Houston.

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Pob5e_solutions_ch21 - 2608T_ch21sm_S247-S257 1:00 pm Page S-247 pinnacle 111:WHQY028:Solutions Manual:Ch-21 chapter Lipid Biosynthesis 21 1

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