Biochemistry Exam 4 Answer Key Gluconeogenesis

EXAM IV Biochemistry II I. _________________/70 II. _________________/15 III. _________________/15 TOTAL _________________/100 I. MULTIPLE CHOICE. (70 points; 3 points each, except as noted) Choose the BEST answer to the question by WRITING the appropriate letter in the BOX to the left of each question. 1. Which of the following statements about gluconeogenesis is true? A. B. C. D. E. 2. Which of these chloroplast enzymes is not thioredoxin? A. B. C. D. E. 3. Gluconeogenesis uses the same glycolytic enzymes except for just two in the beginning which are used to circumvent the exergonic pyruvate kinase reaction, and to synthesize PEP. Regulation of gluconeogenesis uses the allosteric effector, Fru 1,6P2, whereas glycolysis is regulated by the effector, Fru 2,6P2. Pyruvate carboxylase fixes CO2 in much the same way as rubisco. Since both glycolysis and gluconeogenesis involve enzymes exclusively in the cytosol, they must be coordinately regulated. The exergonic nature of the fructose bisphosphatase step is important for helping make the whole gluconeogenesis pathway favorable. regulated by light through the use of rubisco fructose-1,6-bisphosphatase ribulose-5-phosphate kinase glyceraldehyde-3-phosphate dehydrogenase sedoheptulose-1,7-bisphosphatase Absorption of a photon in the chloroplast of a photosynthetic organism is not associated with which one of the following? A. B. C. D. E. release of most of the energy as fluorescence antenna molecules exciton transfer possible eventual transfer of energy to a photochemical reaction center transfer of energy from a chlorophyll molecule in a light-harvesting complex to another chlorophyll or other light-absorbing molecules 4. Glucagon (in liver) and epinephrine (in muscle): A. B. C. D. E. 5. 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 all of the above. The light reactions in photosynthetic higher plants: A. B. C. D. E. do not require chlorophyll. serve to produce light so that plants can see underground. result in the splitting of H2O, yielding O2. produce ATP and consume NADH. balance out the heavy reactions Page 2 The standard reduction potentials ( E ') for the following half reactions 6. o are given. Fumarate + 2H+ + 2e- Eo' = +0.031 V succinate FAD + 2H+ + 2e- Eo' = -0.219 V FADH2 If you mixed succinate, fumarate, FAD, and FADH2 together, all at l M concentrations and in the presence of succinate dehydrogenase, which of the following would happen initially ? A. B. C. D. E. 7. Both fumarate and succinate would become oxidized; both FAD and FADH2 would become reduced. Fumarate would become reduced, FADH2 would become oxidized. Succinate would become oxidized, FAD would become reduced. Succinate would become oxidized, FADH2 would be unchanged because it is a cofactor, not a substrate. No reaction would occur, because all reactants and products are already at their standard concentrations. 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. 8. The sequence of events in the response of hepatocytes to elevated glucagon is: 1. phosphorylated PFK-2/FBPase-2 rapidly degrades Fru2,6P2 shutting off glycolysis and activating gluconeogenesis. 2. adenylate cyclase is activated 3. glucagon binds to its receptor on the cell surface 4. PKA phosphorylates PFK-2/FBPase-2 5. cAMP-dependent protein kinase (PKA) is activated A. B. C. D. E. 9. 4, 2, 2, 5, 2, 1, 4, 5, 4, 3, 3, 5, 4, 2, 1, 5 1 1 1 4 Glucokinase, an isozyme of hexokinase, is _____________ A. B. C. D. E. 10. 2, 3, 3, 3, 5, more active in cytosol when blood glucose concentrations are low. has a lower Km value for glucose than the other hexokinase isozymes. is found in muscle and is important for partitioning glucose between glycolysis and gluconeogenesis. is inhibited by glucose 6‑phosphate. is sequestered in the nucleus and released when either glucose or Fru1P binds to the complex, allowing its migration to the cytosol. Which of the following statements about the light reactions in photosynthetic plants is false? A. B. C. D. E. There are two distinct photosystems, linked together by cytochrome b6f A membrane-bound ATPase couples ATP synthesis to a proton-motive force. The ultimate source of electrons for the process is H2O. The ultimate electron acceptor is O2. No CO2 is fixed during the light reactions. Page 3 11. The processes of oxidative phosphorylation coupled to electron transfer (in mitochondria) and photophosphorylation (in chloroplasts) resemble each other in certain respects. Which below is correct? A. B. C. D. each contain a chain of membrane-bound electron carriers and electron transfer leads to establishment of a proton-motive force. each uses light as a source of reducing power each produces NADPH as a product each uses O2 in the same way 12. All of the enzymes listed below involved in the flow of carbon from glucose to lactate (glycolysis), and are also involved in the reversal of this flow (gluconeogenesis) except : A. B. C. D. E. phosphoglucoisomerase. phosphofructokinase-1 (PFK-1). enolase. 3-phosphoglycerate kinase. aldolase. 13. Which of the following substrates cannot contribute to net gluconeogenesis in mammalian liver? In other words, it is not anaplerotic and/or glucogenic. A. B. C. D. E. alanine palmitate a-ketoglutarate glutamate odd-chain fatty acids 14. Which one of the following statements about gluconeogenesis in the liver is not true? A. For starting materials, it can use carbon skeletons derived from certain amino acids. B. It requires metabolic energy (ATP or GTP). C. It is one of the ways that mammals maintain normal blood glucose levels between meals. D. It employs the enzyme glucose-6-phosphatase. E. It consists entirely of the reactions of glycolysis, operating in the reverse direction. 15. In the hormonal regulation of carbohydrate metabolism, ___________ is released when sources of energy are abundant, and __________ is released when there is a need for energy. The former leads to storage of glucose as _____________, and the later leads to the breakdown of glucose polymers. A. B. C. D. E. 16. insulin; glucagon; insulin; glucagon; glucagon; insulin; glucagon; insulin; insulin; cortisol; starch glycogen starch glygogen glycogen The evolution of two distinct photosystems was necessary _______. A. due to the energy difference in the reduction potentials from oxidation of water to reduction of NADP+, which is larger than the energy of a single photon of light. B. due to the complexity of the process, which needed two mechanisms C. because making water into NADPH, and making oxygen into gas, are two separate reactions D. because the energy in a photon can only excite one chlorophyll and two are needed to make dioxygen E. because two is always better than one. Page 4 17. Which of the following is true of glycogen synthase? A. 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 a high concentration of citrate. Activation of the enzyme involves a phosphorylation. The enzyme has measurable activity only in liver. B. C. D. 18. The synthesis of glycogen, as well as starch and sucrose: A. B. C. D. 19. Glycogenin: A. B. C. D. E. 20. D. E. transketolase aldolase glyceraldehyde-3-phosphate dehydrogenase phosphofructokinase-1 3-phosphoglycerate kinase When transketolase acts on fructose 6-phosphate and glyceraldehyde 3phosphate, the products are: A. B. C. D. 24. anaplerotase. pyruvate dehydrogenase pyruvate decarboxylase pyruvate carboxylase PEP carboxylase Which of these enzymes is not part of the Calvin cycle (2 pts)? A. B. C. D. E. 23. cannot occur in the light. are driven ultimately by the energy of sunlight. are important to plants, but ultimately of little significance for any bacteria or animals. include reactions catalyzed by transaldolase and transketolase. yield ATP, which is required for the light reactions. Which of the enzymes listed below catalyzes the major anaplerotic reaction to replenish the TCA cycle in animals. A. B. C. D. E. 22. is the gene that encodes glycogen synthase. is the enzyme that creates the primer on which new glycogen particles are initiated. is the enzyme responsible for forming branches in glycogen. regulates the synthesis of glycogen. catalyzes the conversion of starch into glycogen. The light-independent ("dark") reactions of photosynthetic plants (2 pts): A. B. C. 21. uses glucose 1-phosphate as the direct substrate for synthesis. uses a sugar nucleotide as the direct substrate for synthesis. takes place in liver and muscle of mammals. involves addition of a sugar residue at the reducing end of the growing polymer. In humans, A. B. C. D. E. xylulose 5-phosphate and ribose 5-phosphate. xylulose 5-phosphate and erythrose 4-phosphate. 3-phosphoglycerate and glyceraldehyde 3-phosphate. dihydroxyacetone phosphate and glucose 6-phosphate. gluconeogenesis: helps to reduce blood glucose after a fat-rich meal. is essential in the conversion of fatty acids to glucose. can result in the conversion of protein into blood glucose. requires the enzyme hexokinase. requires ATP, but not NADH. II. MATCHING. (15 points) WRITE the best choice as a LETTER in the BOX(s) to the left of each number. 25. □ □ □ □ □ □ Page 5 For each item on the left, match it to its function from the right-hand column. Items on the right will be used only once or not at all. (6 pts) 1. Glyoxylate/Kornberg Cycle A. hormonally controlled phosphorylation/ de-phosphorylation 2. Gluconeogenesis B. the committal step for this pathway is catalyzed by phosphoenolpyruvate carboxykinase 3. Glycogen synthesis C. the committal step for this pathway is catalyzed by glycogen synthase 4. Pentose phosphate pathway D. the committal step for this pathway is catalyzed by pyruvate carboxylase 5. Regulation of glycolysis versus gluconeogenesis E. the committal step for this pathway is glucose-6-phosphate dehydrogenase 6. Regulation of FBPase-2 and PFK-2 F. concentration of 3-phosphoglycerate G. concentration of malate H. citrate synthase is the key enzyme discovered by Hans Krebs I. concentration of Fru-2,6P2 J. concentration of Fru-1,6P2 K. malate synthase is the key enzyme discovered by Hans Kornberg 26. □ □ □ □□ □ Match the intermediates or cofactor from the list in the right-hand column that are/is used by each enzyme on the left. Put the corresponding letter in the BOX to the left of each numbered enzyme. Items on the right may be used more than once. (5 pts) 1. transaldolase A. has FeS cluster B. TPP (thiamine pyrophosphate) cofactor 2. Ribose 5-phosphate isomerase 3. transketolase D. free-radical intemediate 4. Rubisco E. divalent-metal stabilized enolate/ carbanion intermediate C. uses only acid/base catalysis to activate a water molecule F. enzyme-stabilized endiol intermediate 5. 6-phosphogluconate dehydrogenase G. base-catalyzed hydride elimination followed by β-ketoacid decarboxylation H. Schiff-base stabilized enolate/ carbanion I. phosphorolysis of thioester Page 6 26. □ □ □ □ Experiments: For each scientist on the left, match to the key experiment or result for which they are known. (4 pts) 1. Robert (Robin) Hill 2. Severo Ochoa 3. Melvin Calvin 4. Samuel Ruben and Martin Kamen A. using radioactive water (18O), showed that dioxygen from photosynthesis comes from water. B. using radioactive carbon (14C), showed that the carbon is initially assimilated and appears in 3-phosphoglycerate C. using redox indicators, showed that light causes evolution of dioxygen in photosynthesis D. showed that the electron acceptor in the light-induced production of dioxygen in photosynthesis was NADP+ EXTRA CREDIT. Glucose-6-phosphate (Glc6P) can be utilized for FOUR possible fates in the liver. Each fate is given as a molecule and/or place next to a box. In each box, indicate the product of the first enzymatic step from Glc6P leading to each fate (abbreviations OK). Then, in the boxes near each arrow, name the enzyme that catalyzes this step. Finally, put a check mark in the small box next to each enzyme if the enzyme is allosterically regulated in its activity. (10 pts) Glycogen Blood Glucose 6-phosphate Mitochondria/Pyruvate Rib 5-P Page 7 III. MECHANISMS & PATHWAYS. (15 points) Fill in boxes as directed for each question. 28. RUBISCO: provide arrows for the reaction mechanism to convert one of the key intermediates shown to another key intermediate in the mechanism, then draw the structure of that intermediate bound to the Mg2+ on the bottom. Name each intermediate in the appropriate box. (5 pts) 29. CALVIN CYCLE: This is for the end of stage 3 of the cycle. It involves starting with 10 moles of glyceraldehyde 3-phosphate (GA3P) to generate 6 moles of ribulose 1,5-bisphosphate. Fill in the blanks for the missing names (abbreviations OK) of intermediates, enzymes, and/or stoichiometry. (10 pts) 2 DHAP 2 10 GA3P 4 2 Fru6P (a) + (b) (c) (e) (d) + 2 Xyl5P Aldolase 2 Sed1,7P2 Sed1,7P2 phosphatase 2 Sed7P + (b) (a) (c) 2 Rib5P (f) + 2 Xyl5P (g) 6 Rub5P 6 ATP 6 ADP 6 Ribulose 1,5-bisphosphate (Rub1,5P2) (h) Page 1 No. on Test Correct Answer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 EC. E A A E C B C C E D A B B E B A A B B B D D B C 25 Glc 1-P 1. 2. 3. 4. 5. 6. K B C E I A 26 1. 2. 3. 4. 5. Fru 6-P or just here. G6Pase its only indirect 27 1. 2. 3. 4. Glucose Phosphoglucomutase X phosphohexose isomerase or phosphoglucoisomerase H F B E, F G X Glc-6-Pase Glc-6-P dehydrogenase 6-phospho gluconolactone C D B A Page 2 No. on Test Correct Answer 28 29 Rub5P Epimerase