THE CITRIC ACID CYCLE

THE CITRIC ACID - THE CITRIC ACID CYCLE CHAPTER 16 1 Yeast can grow both aerobically and anaerobically on glucose Explain why the rate of glucose

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THE CITRIC ACID CYCLE, CHAPTER 16. 1. Yeast can grow both aerobically and anaerobically on glucose. Explain why the rate of glucose consumption decreases when yeast that has been obtained under anaerobic conditions is exposed to Oxygen? 2. Trace the fate of 14CH3 –labeled acetyl CoA through two rounds of the citric acid cycle. Assume that the labeled acetyl CoA enters only in the first round. 3. Trace the fate of the isotopic label of [2-18 O]-pyruvate through the pyruvate dehydrogenase reaction complex and one round of the TCA cycle. 4. The following labeled glucose molecules are metabolized to 14CO2 via glycolysis and the TCA cycle. Which labeled molecule yields 14CO2 fastest and which takes longest to yield 14CO2 ? Assume pyruvate only enters the TCA cycle only as acetyl-CoA. A) 1- 14C-glucose B) 2-14C-glucose C) 3-14C-glucose. 5. Which statements are true or false. a. Dihydrolipoamide dehydrogenase catalyzes the only redox reaction in the pyruvate dehydrogenase complex. b. Hydrolysis of the thioester bond of acetyl CoA yields insufficient energy to drive phosphorylation of ADP. c. Even if aconitase were unable to discriminate between the two ends of the citrate molecule, the CO2 released would still come from oxaloacetate rather than the acetyl CoA substrate of the citrate synthase reaction. d. Malate cannot be converted to fumarate because the TCA cycle is unidirectional. 6. Lipoic acid is bound to enzymes that catalyze oxidative decarboxylation of a-keto acids. a. What is the chemical mode of attachment of lipoic acid to enzymes? b. Using chemical structures, show how the lipoic acid participates in the oxidative decarboxylation of a-keto acids. 7. How is the pyruvate dehydrogenase complex regulated? What are the allosteric effects and the covalent modification effects? 8. What is meant by the statement that the TCA cycle is amphibolic? Give examples of processes affected by the TCA cycle intermediates. 9. How are ATP, ADP and Ca+2 involved in the regulation of the citrate cycle? 10. What are the anaplerotic reactions? Give two examples. What is the rationale for activation of pyruvate carboxylase by acetyl CoA?
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ANSWERS TO QUESTIONS ON THE CITRIC ACID CYCLE, CHAPTER 16. 1. The decrease in glucose consumption when oxygen is present is known as the Pasteur Effect. Under anaerobic conditions , there is a net production of only two molecules of ATP for each molecule of glucose converted to ethanol and CO2 by the anaerobic glycolytic pathway. When Oxygen is introduced, the cell can utilize glucose much more efficiently, producing 30-32 molecules of ATP for each glucose molecule oxidized completely to CO2 and water by glycolysis, the citric acid cycle and the respiratory electron transport chain. Thus the amount of glucose consumption required to maintain the proper ATP levels in the cell is greatly reduced in aerobically growing cells. In short, glucose is oxidized completely under aerobic conditions and fewer molecules of glucose must be consumed to produce an amount of ATP equivalent to the amount produced
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This note was uploaded on 04/02/2008 for the course BMB 461 taught by Professor Stoltzfus during the Spring '07 term at Michigan State University.

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THE CITRIC ACID - THE CITRIC ACID CYCLE CHAPTER 16 1 Yeast can grow both aerobically and anaerobically on glucose Explain why the rate of glucose

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