437Set6_Ans2 - METABOLIC BIOCHEMISTRY Immo E Scheffler...

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METABOLIC BIOCHEMISTRY Summer 2005 Immo E. Scheffler FINAL EXAM All answers are to be written into the Blue Book. Leave the first inside page blank for scoring. There are 16 questions. Make sure that each answer is clearly identified with the question number at the top or left side of the page. QUESTION 1 (10 points) Draw a portion of the Krebs cycle (tricarboxylic acid cycle) from oxaloacetate to α -ketoglutarate with structural formulae and names for each of the cycle intermediates (but not the co-factors). Highlight (or color) the carbons of isocitrate which are derived from acetyl-CoA. QUESTION 2 (2 points for each part)) a) Oxaloacetate uniformly labeled with 14 C (i.e., with equal amounts of 14 C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate? A) all
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B) 1/2 C) 1/3 D) 1/4 E) 3/4 b) The reaction of the citric acid cycle that is most similar to the pyruvate dehydrogenase complex-catalyzed conversion of pyruvate to acetyl-CoA is the conversion of: A) citrate to isocitrate. B) fumarate to malate. C) malate to oxaloacetate. D) succinyl-CoA to succinate. E) α -ketoglutarate to succinyl-CoA . c) The reaction of the citric acid cycle that produces an ATP equivalent (in the form of GTP) by substrate level phosphorylation is the conversion of: A) citrate to isocitrate. B) fumarate to malate. C) malate to oxaloacetate. D) succinate to fumarate. E) succinyl-CoA to succinate . QUESTION 3 (4 points each part) a) The citric acid cycle is frequently described as the major pathway of aerobic catabolism, which means that it is an oxygen-dependent degradative process. However, none of the reactions of the cycle directly involves oxygen as a reactant. Why is the pathway oxygen-dependent? Ans: The citric acid cycle produces NADH. If this NADH is not reoxidized via the electron transport chain, the citric acid cycle will come to a stop, by feedback inhibition (or because NAD+ is a required substrate for several reactions). Oxygen is consumed by the re-oxidation of NADH. b) A function of the glyoxylate cycle, in conjunction with the citric acid cycle, is to accomplish the: A) complete oxidation of acetyl-CoA to CO 2 plus reduced coenzymes. B) net conversion of lipid to carbohydrate. C) net synthesis of four-carbon dicarboxylic acids from acetyl-CoA. D) net synthesis of long-chain fatty acids from citric acid cycle intermediates. E) both B and C are correct
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QUESTION 4 (8 points) a) [5 points] The pentose phosphate pathway starts with an oxidation of glucose-6-phosphate instead of an isomerization to fructose-6-phosphate. Describe this reaction with the help of structural formulae. What is the required cofactor and its reaction (no structure needed).
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