Biochemistry Exam 4 Answer Key 24 Questions

Biochemistry II I. MULTIPLE CHOICE. (45 points; 3 points each answer) Choose the BEST answer to the question by WRITING the appropriate letter in the BOX to the left of each question. 1. The glycogen-branching enzyme catalyzes: A. B. C. D. 2. The Cori cycle is: A. B. C. D. E. 3. B. C. D. pyridoxal phosphate. thiamine pyrophosphate. cobalamin (vitamin B12). tetrahydrofolic acid. Which of these chloroplast enzymes is not regulated by light (directly or indirectly)? A. B. C. D. E. 6. It catalyzes addition of glucose residues to the nonreducing end of a glycogen chain through (a1Æ4) bonds. The conversion of an active to an inactive form of the enzyme is controlled by the concentration of cAMP. Activation of the enzyme involves a phosphorylation. The enzyme has measurable activity only in liver. Transketolase requires the coenzyme: A. B. C. D. 5. the interconversion between glycogen and glucose-l-phosphate. the synthesis of alanine from pyruvate in skeletal muscle and the synthesis of pyruvate from alanine in liver. the synthesis of urea in liver and degradation of urea to carbon dioxide and ammonia by bacteria in the gut. the production of lactate from glucose in peripheral tissues with the resynthesis of glucose from lactate in liver. none of the above. Which of the following is true of glycogen synthase? A. 4. glycogen degradation in tree branches. formation of (a1Æ6) linkages during glycogen synthesis. formation of (a1Æ4) linkages in glycogen. removal of unneeded glucose residues at the ends of branches. rubisco fructose-1,6-bisphosphatase ribulose-5-phosphate kinase glyceraldehyde-phosphate dehydrogenase transketolase The decision to metabolize pyruvate through the TCA cycle or to the cytosol for gluconeogenesis is accomplished by: A. phosphorylation/dephosphorylation of pyruvate dehydrogenase (PDH) and pyruvate decarboxylase by protein kinase A/phosphoprotein phosphatase, respectively B. C. D. Reciprocal phosphorylation/dephosphorylation of pyruvate dehydrogenase and pyruvate carboxylase by PDH-kinase A/PDH-phosphatase, respectively Reciprocal allosteric inhibition/activation of pyruvate dehydrogenase and pyruvate carboxylase by acetyl-CoA, respectively Reciprocal allosteric activation/inhibition of pyruvate dehydrogenase and pyruvate carboxylase by acetyl-CoA, respectively Page 2 7. Glucagon (in liver) and epinephrine (in muscle): A. B. C. D. E. activate glycogen phosphorylase and inactivate glycogen synthase. inhibit glycogen synthesis and activate glycogenolysis. act by raising the concentration of cyclic AMP (cAMP). act through a protein kinase do all of the above. 8. Which of the following substrates cannot contribute to net gluconeogenesis in mammalian liver? A. B. C. D. E. alanine palmitate a-ketoglutarate glutamate pyruvate 9. In the light-independent ("dark") reactions of photosynthesis, the biosynthesis of 1 mole of hexose from 6 moles of carbon dioxide requires: A. B. C. D. E. 18 18 12 12 no moles moles moles moles NADPH of NADPH and of NADPH and of NADPH and of NADPH and and 12 moles 18 12 18 12 of moles moles moles moles ATP. of of of of ATP. ATP. ATP. ATP. 10. 11. In an organism growing on acetate (such as E. coli) that has both the citric acid cycle and the glyoxylate cycle, what determines which of these pathways isocitrate will enter? A. The enzyme metabolizing isocitrate in each pathway is under reciprocal allosteric regulation. B. A high [ATP]/[ADP] ratio favors the citric acid cycle. C. The accumulation of glycolytic intermediates favors the glyoxylate cycle. D. The pathway depends solely on the location of isocitrate in the cell. 12. Why is it important that gluconeogenesis is not the exact reversal of glycolysis? A. Gluconeogenesis would be highly endergonic, and it would be impossible to regulate the two processes separately. B. Gluconeogenesis would be highly exergonic, and it would be impossible to regulate the two processes separately. C. The continuous reversible conversion of glucose to pyruvate and back would deplete the brain’s energy supply. D. Alanine stimulation would inhibit pyruvate production. 13. Page 3 Malic enzyme, found in the bundle sheath cells of C4 plants and in liver cytosol, and 6-phosphogluconate dehydrogenase, as part of the pentose-phosphate pathway, carry out a reaction that has a counterpart in the citric acid cycle. What is the analogous reaction in the citric acid cycle? A. B. C. D. E. 14. the the the the the isocitrate dehydrogenase reaction succinate dehydrogenase reaction malate dehydrogenase reaction citrate synthase reaction fumarase reaction What impact would depletion of carbon dioxide have on photosynthesis? A. The photosystems would lose the ability to synthesize ATP and NADPH needed to synthesize carbohydrates. B. Water molecules would lose the ability to donate electrons to photosystem II, preventing the harvest of solar energy. C. Failure to regenerate ribulose 1,5-bisphosphate would prevent the regeneration stage of the Calvin cycle. D. Failure to synthesize 3-phosphoglycerate would prevent the carbon fixation stage of the Calvin cycle. The coenzyme NADP+ is the terminal electron acceptor in chloroplasts, according to the reaction 15. 2 H2 0 + 2 NADP + � 2 NADPH + 2 H+ + 02 The equilibrium constant, K'eq, for this reaction at 25 °C is 1.35 xl0- 77 How can the chloroplast overcome this unfavorable equilibrium? A. B. C. D. II. Chloroplasts decrease the rate of CO2 assimilation via the Calvin cycle to overcome this barrier. The absorption of photons provides the energy needed for the chloroplasts to overcome this barrier. Chloroplasts decrease the Eo' of water to make water a strong electron donor and overcome this barrier. Phosphorylation of the P680 special pair in photosystem II allows the chloroplasts to overcome this barrier. SHORT ANSWER/FILL IN. (18 points) Give a brief answer or fill in the boxes as directed to each problem or question below. Put your answers WITHIN the boxes provided. 16. Some glucose produced by gluconeogenesis is stored in the body as glycogen. What is the correct order the steps of glycogen synthesis. (3 pts) −−> 17. −−> In muscle tissue, the rate of conversion of glycogen to glucose 6 phosphate is determined by the ratio of phosphorylase a (active) to phosphorylase b (less active). Determine what happens to the rate of glycogen breakdown, and place the correct choice in the adjacent box, if a muscle preparation containing glycogen phosphorylase is treated with: (6 pts) (a) phosphorylase kinase and ATP (b) PP1 (c) epinephrine. A. B. Choices: Increases glycogen breakdown Decreases glycogen breakdown Page 4 18. Choices: a. PSI, but not PSII b. PSII, but not PSI c. PSI and PSII d. PSI e. PSII f. P680 g. P700 (5 pts) 19. The pentose phosphate pathway provides for reducing oxidized glutathione (GSSG) biosynthesis and for nucleotide synthesis. (4 pts) III. FILL-IN/PATHWAYS/MECHANISM 20. (37 points) The mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the cytosol for fatty acid synthesis starts with Acetyl-CoA in the mitochondrial matrix, which condenses with a. to form b. in a reaction catalyzed by c. . b. moves out of the matrix via the tricarboxylate transporter. b. in the cytosol is cleaved by d. acetyl-CoA and a. . , yielding To complete the cycle, a. in the cytosol is reduced to e. pyruvate by f. fatty-acid synthase reactions). , which is converted to (thus creating g. for the Then pyruvate moves into the mitochondrial matrix through its own transporter, and is converted to a. in the matrix by the enzyme h. (8 pt) . 21. 22. Page 5 The energetics of photosynthesis is diagrammed below. Name each of the entities a.-i. in the boxes provided (abbreviations OK). Entities a. and b. are photosystems. Entities next to arrows are redox players, whereas entities e. and h. are enzymes/complexes. Entity c. is a cofactor. (5 pts) Fatty acid synthesis and fatty acid breakdown occur by similar pathways. The synthetic and breakdown pathways differ in six ways. Note these differences with 1-2 words or abbreviations in the boxes below. (6 pt) Fatty acid Oxidation Fatty Acid Synthesis Location Cofactor used for Reaction at b-carbonyl Stereochemistry of alcohol Cofactor used for Reaction at alkene Group Carrier C-2 unit Page 6 23. In the diagram of the Calvin Cycle below (with a stoichiometry of 6 CO2 molecules fixed to make 1 molecule of glucose), fill in the missing boxes using abbreviations from the table below. For those 9 boxes in dotted lines provide the names of an enzyme. For those 5 boxes in dashed lines provide the name of the sugar intermediates. For those 15 empty boxes with solid lines provide numbers pertaining to the correct stoichiometry. (11 pts) RUBISCO Mechanism (7 points) First, fill-in the missing arrows for electron movements for steps 1, 2, and 3. Second, Fill-in the names of the substrate, catalytic base, and catalytic intermediate in the 3 boxes (P=phosphoryl). Third, draw and arrow from the water molecule to the carbon atom in the last intermediate that is attacked in the next step leading to the cleavage of 3-phosphoglycerate. 24. Page 1 No. on Test Correct Answer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 B D A B E C E B C B A A A D B 16 A B C 17 B A A 18 f. a. g. b. 19 d. NADPH; ribose 5-phosphate 20 The mechanism for moving acetyl-CoA produced in the mitochondrial matrix into the cytosol for fatty acid synthesis starts with AcetylCoA in the mitochondrial matrix, which condenses with oxaloacetate to form citrate in a reaction catalyzed by citrate synthase. Citrate moves out of the matrix via the tricarboxylate transporter. Citrate in the cytosol is cleaved by citrate lyase, yielding acetyl-CoA and oxaloacetate. To complete the cycle, oxaloacetate in the cytosol is reduced to malate, which is converted to pyruvate by malic enzyme (thus creating NADPH for the fatty-acid synthase reactions). Then pyruvate moves into the mitochondrial matrix through its own transporter, and is converted to oxaloacetate in the matrix by the enzyme pyruvate carboxylase. Page 2 No. on Test Correct Answer 21 oxygen-evolving complex 22 Fatty acid Oxidation Fatty Acid Synthesis Mitochondria Location Cytosol NADH Reaction at b-carbonyl NADPH 3-L-hydroxyacyl Stereochemistry of alcohol 3-D-hydroxyacyl FADH2 Reaction at alkene NADPH Coenzyme A Group Carrier ACP acetyl-CoA C-2 unit malonyl-CoA 23 24