Biochemistry Answer Key 35 Questions

I. MULTIPLE CHOICE. (51 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. If a cell were unable to synthesize or obtain tetrahydrofolic acid (H4 olate), it would probably be deficient in the biosynthesis of: lysine. thymidylate (TMP). malonate. leucine. The synthesis of both glycerophospholipids and triacylglycerols involves: CDP-diacylglycerol. phosphoethanolamine. phosphatidate. CDP-choline. phosphatidate phosphatase. Which of the following is not true of the reaction catalyzed by ribonucleotide reductase? It acts on nucleoside di phosphates. Thioredoxin acts as an essential electron carrier. Its mechanism involves formation of a free radical. There is a separate enzyme for each nucleotide (ADP, CDP, GDP, UDP). Glutathione is part of the path of electron transfer. 4. Cholesterol is synthesized from: acetyl-CoA. malate. oxalate. lipoic acid. choline. 5. Which of the following statements about the synthesis of isopentenyl pyrophosphate from acetate is not true? Biotin is required. Carbon dioxide is liberated. ATP is consumed. Mevalonate is an intermediate. NADPH is the electron donor. 6 What is the mnemonic for the essential amino acids used by Dr. Kornberg? Professor A.V. HILL was a Member of Parliament Oh My Such Good Apple Pie A HILL Makes People Think To Vomit Professor L.V. WIRTH Makes Klass Fun Page 2 7 The ribosyl phosphate moiety needed for the synthesis of orotidylate, inosinate, and guanylate is provided most directly by: adenosine-5'-phosphate. guanosine-5'-phosphate. 5-phosphoribosyl-1-pyrophosphate. ribulose-5-phosphate. ribose-5-phosphate. 8 Humans cannot convert dietary: carbohydrate into stored lipid. amino acids into stored lipid. glucose into amino acids. amino acids into glucose. fatty acids into glucose. 9 Which of these statements about the regulation of cholesterol synthesis is not true? Insulin stimulates HMG-CoA reductase. High intracellular cholesterol stimulates formation of cholesterol esters. Cholesterol acquired in the diet has essentially no effect on the synthesis of cholesterol in the liver. Oxysterol, a derivative of cholesterol, inhibits expression of HMGCoA reductase. Failure to regulate cholesterol synthesis predisposes humans to atherosclerosis. 0 In the synthesis of phosphatidylcholine from phosphatidylethanolamine methyl group donor is: the methanol. S -adenosylmethionine (adoMet or SAM). a tetrahydrofolate derivative. choline. serine. 1 Which of the following enzymes is not involved in the eventual assimilation of inorganic nitrogen molecules into an organic molecule? E 12 glutamine synthetase nitrogenase arginase carbamoyl phosphate synthetase GOGAT (glutamine amidotransferase) The carbon skeleton of several amino acids may be derived metabolically from glucose. If glucose labeled with 14C at C-1 were the starting material for amino acid biosynthesis, a product that would be readily formed is: serine labeled at C-2 (Ca). glycine labeled at C-1 (COO-). serine labeled at C-3 (CH2OH). glycine labeled at C-2 (Ca). [HINT: see question #21] 13. 14. Page 3 The biosynthesis of palmitoleate, a common unsaturated fatty acid with 9 a cis double-bond in the ∆ position, uses palmitate as a precursor. Palmitate is the precursor for either elongation reactions to produce other long-chain fatty acids, or a desaturation reaction to produce palmitoleate. Can palmitoleate synthesis be carried out under strictly anaerobic conditions? A. No, fatty acyl-CoA desaturase requires oxygen as a substrate in the reaction to introduce the double bond in palmitoleate. B. No, stearoyl-CoA desaturase (SCD) uses ferredoxin as the electron donor in an oxidation reaction to introduce the double bond in palmitoleate. C. Yes, acetyl-CoA carboxylase uses ATP and bicarbonate, not oxygen, to make the intermediate, malonyl-CoA, in palmitoleate synthesis. D. Yes, desaturase does not add oxygen to the molecule to form the double-bond in palmitoleate A researcher adds [14C]glucose to the balanced diet of adult rats. She finds no increase in the total amount of stored triacylglycerols, but the triacylglycerols become labeled with 14C. Which statement explains these results? A. Triacylglycerols in rat adipose tissue turn over due to a balance of degradation and biosynthesis. B. Adipocytes degrade previously stored triacylglycerols before storing any new molecules. C. The dietary change increases triacylglycerol biosynthesis and decreases triacylglycerol degradation. D. Rat adipose tissue stores glucose molecules, as well as triacylglycerol molecules, for later use. E. Adipocytes preferentially store triacylglycerols containing heavy isotopes. 15. 16. Cholesterol in humans can be obtained from the diet or synthesized de novo. An adult human on a low-cholesterol diet typically synthesizes 600 mg of cholesterol per day in the liver. If the amount of cholesterol in the diet is large, de novo synthesis of cholesterol is drastically reduced. How is this regulation brought about? A. Cholesterol in the diet induces the ubiquination of VLVD, which degrades cholesterol back to acetyl-CoA. B. Mevalonate, a key intermediate in cholesterol biosynthesis, is ubiquitinated and degraded by the hormone glucagon to inhibit the pathway. C. The catabolism of excess cholesterol produces metabolites that act in a negative feedback loop to inhibit the first two condensation reactions in the cholesterol biosynthesis pathway. D. The enzyme that catalyzes the rate-determining reaction in cholesterol biosynthesis, β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) reductase, is subject to negative feedback regulation by downstream modulators and to covalent and transcriptional regulation. E. Cholesterol itself acts as a transcriptional repressor of the HMG-CoA reductase enzyme by entering the nucleus and inhibiting the transcription of this enzyme. What is the function of nitrogenase in nitrogen fixation? A. B. C. D. E. 17. converts nitrogen gas to oxygen transforms ammonia to nitrites changes nitrates to nitrogen gas converts nitrogen gas to ammonia incorporates nitrogen into animal proteins Mammalian asparagine synthetase is a glutamine-dependent amidotransferase. Efforts to identify an effective inhibitor of human asparagine synthetase for use in chemotherapy for patients with leukemia have focused not on the amino-terminal glutaminase domain but on the carboxyl-terminal synthetase active site. Why is the glutaminase domain not a promising target for a useful drug? A. B. C. D. E. The glutaminase domain is mutated in patients with leukemia. The glutaminase domain is not unique to only asparagine synthetase. The tunnel in the enzyme for the released ammonia does not bind drugs. No chemical reactions occur at the glutaminase domain. The enzyme buries the glutaminase domain, preventing access by drugs. Page 4 II. SHORT ANSWER/FILL-IN (22 points) Fill in answers in the BOXES provided as directed. 18. 19. What w Prostaglandins, thromoxanes, and leukotrienes are synthesized from PUFAs. What is the enzyme that initiates the production of the first two, and the enzyme that initiates the leukotrienes (abbreviations OK)? (2 pts) PG/TB 20 low Conditionally essential Essential 21 Leukotrienes Page 5 22 8 23 Page 6 III. I. MATCHING 24. (5 points) Regulation Match items on the left with the best answer from items on the right (Column B). All items on the right (Column B) are used Column A Column B Thr and Ile biosynthesis ____1. inactivation of acetyl-CoA carboxylase ____2. ____3. inhibition of pyrimidine biosythesis by CTP Allosteric effector for inhibition of ribonucleotide reductase ____4. phosphorylation by AMPK causes ______. ____5. de-adenylation causes ______. HMGCoA reductase action of Lys and Ile on Asp-kinase A1 first step in de novo purine biosynthesis activation of glutamine synthetase dATP de novo purine biosynthesis ____6. regulation of glutamine synthetase ____7. ____8. B ____9. ____10. IV. EXAMS 1-4 QUESTIONS 25. (12 points) Malic enzyme, found in the bundle sheath cells of C4 plants and in liver cytosol, and 6-phosphogluconate dehydrogenase, as part of the pentosephosphate 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. the fumarase reaction 26. 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? Page 7 27. 1 28 (1 pt) 9 Flux [X] 30. (C18:0) 3 3 3 3 γ V. LABORATORY QUESTIONS (10 points) Page 8 35. Lately you have been feeling unusually fatigued and thirsty, and your friend mentioned that these could be symptoms of high blood glucose levels. Worried, you decided to test your blood glucose levels after fasting. Remembering what you did in the 422 lab, you have to first deproteinize your blood to remove the , which Dr. Low said were there and could interfere with measuring the free sugars in the sample. To check if Dr. Low was telling the truth, you took the pellet from the blood deproteinization centrifugation step and ran an SDS polyacrylamide gel along with a positive control, GLUT1. You stained the gel by and obtained the result shown on the right: Was Dr. Low telling the truth? Still feeling thirsty and tired, you go back to your deproteinized blood sample to specifically measure the glucose concentration present. Thus, of the two methods learnt, you decided to use the method. So, you prepared tubes with varying glucose concentration and an aliquot of your own blood sample, and performed the assay using 20 µL of your deproteinized blood sample, which was from a 100-fold dilution from the original blood sample, and measured absorbance at 540 nm with a spectro-photometer. You generated a glucose standard curve shown on the right, and the result from your own deproteinized blood sample is as follows: Fold dilution after deproteinization Sample volume in assay (µL) Absorbance at 540 nm 100X 20 0.240 How many micrograms of glucose are in the 20 μL deproteinized sample? What is the concentration of glucose in the deproteinized sample in units of mg/dL? (Hint: 1 dL = 100 mL) What is the concentration of glucose in the original blood sample? Based on the reference chart, should you still be worried? Here is a reference glucose levels: Glucose range (mg/dL) < 40 mg/dL 40 – 110 mg/dL > 110 mg/dL chart of various Result Hypoglycemic Normal Hyperglycemic If you were to have measured your deproteinized sample in triplicate and measured an absorbance of 0.25 ±0.025, and Dr. Low told you that your pipetting error in making your 100-fold dilution was 10%, what would be the error in the calculated [Glc] in your blood? A. It would be the same as your dilution error: 10% B. It would be the same as your absorbance error C. It would be less that either error, absorbance or dilution D. It would be greater than either error, absorbance or dilution E. The error cannot be calculated. No. on Test Correct Answer 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 B C D A A A C E C B C C A A D D B 18 bilirubin 19 COX (cyclooxygenase), LOX (lipooxygenase) 20 Any two of these pairs: Proline Tyrosine Arginine Phenylalanine Cysteine Methionine Glycine Serine Serine Threonine Serine is derived from 21 3-phosphoglycerate by the pathway shown in Fig. Phosphoglycerate is oxidized by transamination from glutamate, yielding 3phosphoserine. Removal of the phosphate yields serine. 22 The origin of each atom in the purine ring of IMP, AMP, and GMP is shown in Fig. The Circle should be on the carbon at 7 o'clock of 6membered ring for G, and the double circle should be on the carbon at the top of 6membered ring for A. The label (*) should appear only in the formate-derived carbons. A G * * No. on Test Correct Answer 23 24 The origin of each atom in the pyrimidine ring of CMP: I A H B G C J E D F 25 E 26 A 27 Answer: Phosphorolysis does not waste ATP energy Explanation: Phosphorolysis yields glucose-1-phosphate, which can be converted into glucose-6-phosphate without the investment of energy from ATP. Hydrolysis of glycogen yields free glucose, which must be converted into glucose-6-phosphate (at the expense of ATP) before it can enter glycolysis. 28 Answer: Yes Explanation: Electrons transferred from 1 to 2 require going from a 2-electron carrier (NADH) and a 1-electron carrier (cytochrome c). This is one major role of the "Q-cycle." 29 Answer: Add more "X" and measure flux Explanation: Even a hyperbolic plot has a ;inear portion aldolase low [X]. By adding more, the fulx will either go up linearly or saturate. 30 Answer: 6.5 Explanation: the C18 ∆6,9,12 has two even-numbered double bonds and one odd-numbered. In class, we discovered that each even-numbered results in the use of 1 NADPH and each odd-numbered loses one FADH2. This is a loss of 2.5x2 + 1x1.5 = 6.5. 31 32 33 34 T F T T 35 1. glycoproteins +1 pt 2. PAS stain/ PAS staining +1 pt 3. Yes +1 pt 4. Glucose Oxidase +1 pt 5. 0.240 corresponds to 0.12 µg of glucose +2 pts 6. 0.120 µg/ 20 µL = 0.006 mg/mL x (100 mL/1 dL) = 0.6 mg/dL +0.5 pts 7. Original blood concentration = 100 x 0.6 mg/dL= 60 mg/dL +0.5 pts 8. No +1 pts 9. D +2 pts Page 2