153CProblemSetIVAnswers

- CHEMISTRY 153C Answers to Study Questions and Problems IV Winter 2009 1 List some regulatory factors which might tend to favor the storage of

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Revised 1/5/09 - 1 - CHEMISTRY 153C Answers to Study Questions and Problems IV Winter 2009 1. List some regulatory factors which might tend to favor the storage of carbon derived from glucose as fatty acids under circumstances where cellular adenylate energy charge is somewhat higher than normal. 2. Regulation of fatty acid biosynthesis. The probable rate determining step for fatty acid biosynthesis in mammalian liver is the reaction catalyzed by acetylCoA carboxylase. (a) Indicate briefly how each of the following conditions would influence the physical and chemical properties of this enzyme and its catalytic activity. (1) The presence of elevated levels of circulating insulin. (1) Acetyl CoA carboxylase would be in a non-phosphorylated polymeric form and highly active. (2) The presence of elevated levels of circulating glucagon. (2) Acetyl CoA carboxylase would be in a phosphorylated protomeric (dissociated monomer) form with little catalytic activity. (3) The presence of elevated levels of citrate and/or isocitrate in the cytosol. (3) High levels of citrate or isocitrate cause the protomeric form with little catalytic activity to associate into a highly active polymeric form. 1. (a) High energy charge would tend to limit the activity of the R-type enzyme isocitrate dehydrogenase in the tricarboxylic acid cycle and thus lead to increased steady state concentrations of citrate and isocitrate (which are in virtual equilibrium). This should make citrate more available for transport to the cytosol to serve as a carbon source for fatty acid synthesis. (b) High energy charge would tend to stimulate the U-type enzyme citrate lyase and thus increase the supply of cytosolic acetylCoA. (c) Citrate (and isocitrate) is a positive modifier of acetylCoA carboxylase, which is believed to be the rate-limiting step for fatty acid synthesis under many circumstances. Thus, increased levels of the tricarboxylic acids would stimulate the overall process.
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Revised 1/5/09 - 2 - 2. (b) Explain briefly how a product of the reaction catalyzed by acetylCoA carboxylase can help direct the long chain fatty acyl products of fatty acid biosynthesis to anabolic rather than catabolic fates.
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This note was uploaded on 08/26/2011 for the course CHEMISTRY Chem 153C taught by Professor Cathyclarke during the Spring '11 term at UCLA.

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- CHEMISTRY 153C Answers to Study Questions and Problems IV Winter 2009 1 List some regulatory factors which might tend to favor the storage of

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