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Download Midterm Exam Advanced Biochemistry II (Answer) 1. At equilibrium
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Midterm Exam Advanced Biochemistry II (Answer) 1. At equilibrium in solution, D-glucose consists of a mixture of its anomers. Which statement most accurately describes the solution? A) The solution consists of approximately equal amounts of the - and -anomers. B) The straight-chain form is present in high concentration. C) The -anomer is more stable and is slightly preferred over the -anomer. D) The -anomer predominates over the -anomer by a ratio of approximately 2:1. E) None of the above. 2. A) B) C) D) E) Which of the following sugars is not a reducing sugar? glucose ribose sucrose starch galactose 3. A) B) C) D) E) How many stereoisomers are possible for an aldopentose such as ribose? 2 4 8 16 32 4. A) B) C) D) E) How many stereoisomers are possible for a ketopentose such as ribulose? 2 4 8 16 32 5. A) B) C) D) E) Which of the structures is a glucopyranose? A B C D None of the above 6. A) B) C) D) E) Which structure is a ketose? A B C D None of the above 7. A) B) C) D) E) Which structures are glucoses? A A, B A, C A, B, C None of the above 8. A) B) C) D) E) Which is deoxyribose? A B C D None of the above 9. Which of the following segments of the integral membrane protein glycophorin most likely contains the membrane-spanning sequence: A) LSTTEVAMHTTTSSSVSKSY B) ISSQTNDTHKRDTYAATPRA C) HEVSEISVRTVYPPEEETGE D) PEITLIIFGVMAGVIGTILL E) ISYGIRRLIKKSPSDVKPLP 10. A) B) C) D) E) Which would be most likely to be found in the cell membranes of arctic mammals? very high concentrations of cholesterol polyunsaturated lipids saturated lipids trans-fats elevated concentrations of hemoglobin 11. A) B) C) D) E) The mannose-6-phosphate marker is used to direct proteins to the _______________. lysosome rough endoplasmic reticulum outer leaflet of the cellular membrane nucleus cytoplasm 12. Glycosphingolipids with several attached carbohydrates (including at least one sialic acid) are called: A) lipid rafts B) ceramides C) sphingiosides D) gangliosides E) proteoglycans 13. What is true about the (Na+–K+)–ATPase transporter? A) three potassium ions bind to the cytostolic face of the transporter in the E1 state B) GTP binds to it and is hydrolyzed to form GDP and a high energy phosphorylated glutamyl residue C) formation of the high-energy phosphorylated glutamyl residue triggers a conformational shift from the E1 form to the E2 form D) bound Na+ ions are released from the E2 state to the extracellular medium, and two potassium ions are bound to the dephosphorylated E2 E) phosphate hydrolysis of the E2 state releases potassium ions to the cytosol and results in rearrangement to the E1 state 14. Ouabain inhibits the (Na+–K+)–ATPase pump, resulting in A) K+ ions moving into the cell along a concentration gradient B) K+ ions bringing water molecules into the cell in order to maintain an osmotic balance C) extra K+ ions and water in cells causing them to swell and lyse D) an increase the strength of heart muscle contractions when small amounts of Ouabain are used E) all of the above 15. Fermentation to Produce Soy Sauce Soy sauce is prepared by fermenting a salted mixture of soybeans and wheat with several microorganisms, including yeast, over a period of 8 to 12 months. The resulting sauce (after solids are removed) is rich in lactate and ethanol. How are these two compounds produced? To prevent the soy sauce from having a strong vinegar taste (vinegar is dilute acetic acid), oxygen must be kept out of the fermentation tank. Why? Answer Soybeans and wheat contain starch, a polymer of glucose, which is broken down to glucose by the microorganisms. The glucose is then degraded to pyruvate via glycolysis. Because the process is carried out in the absence of oxygen (i.e., it is a fermentation), pyruvate is reduced to lactic acid and ethanol. If oxygen were present, pyruvate would be oxidized to acetyl-CoA and then to CO2 and H2O. Some of the acetyl-CoA, however, would also be hydrolyzed to acetic acid (vinegar) in the presence of oxygen. 16. Explain the regulatory role of Phosphofructokinase (PFK) in glycolysis 17. Glycolysis Shortcut Suppose you discovered a mutant yeast whose glycolytic pathway was shorter because of the presence of a new enzyme catalyzing the reaction Would shortening the glycolytic pathway in this way benefit the cell? Explain. Answer Under anaerobic conditions, the phosphoglycerate kinase and pyruvate kinase reactions are essential. The shortcut in the mutant yeast would bypass the formation of an acyl phosphate by glyceraldehyde 3-phosphate dehydrogenase and therefore would not allow the formation of 1,3-bisphosphoglycerate. Without the formation of a substrate for 3phosphoglycerate kinase, no ATP would be formed. Under anaerobic conditions, the net reaction for glycolysis normally produces 2 ATP per glucose. In the mutant yeast, net production of ATP would be zero and growth could not occur. Under aerobic conditions, however, because the majority of ATP formation occurs via oxidative phosphorylation, the mutation would have no observable effect. 18. Role of Lactate Dehydrogenase During strenuous activity, the demand for ATP in muscle tissue is vastly increased. In rabbit leg muscle or turkey flight muscle, the ATP is produced almost exclusively by lactic acid fermentation. ATP is formed in the payoff phase of glycolysis by two reactions, promoted by phosphoglycerate kinase and pyruvate kinase. Suppose skeletal muscle were devoid of lactate dehydrogenase. Could it carry out strenuous physical activity; that is, could it generate ATP at a high rate by glycolysis? Explain. Answer The key point here is that NAD_ must be regenerated from NADH in order for glycolysis to continue. Some tissues, such as skeletal muscle, obtain almost all their ATP through the glycolytic pathway and are capable of short-term exercise only (see Box 14–2). In order to generate ATP at a high rate, the NADH formed during glycolysis must be oxidized. In the absence of significant amounts of O2 in the tissues, lactate dehydrogenase converts pyruvate and NADH to lactate and NAD_. In the absence of this enzyme, NAD_ could not be regenerated and glycolytic production of ATP would stop—and as a consequence, muscle activity could not be maintained.