Chapter 22 Biosynthesis of Amino Acids, Nucleotides, and Related Molecules Multiple Choice Questions 1. Overview of nitrogen metabolism Page: 835 Difficulty: 2 Ans: D Which of the following statements about the fixation of atmospheric nitrogen (N2) into NH3 by living cells is false? A) B) C) D) E) It involves the transfer of 8 electrons per mol of N2. It occurs in certain microorganisms, but not in humans. It requires a source of electrons, normally ferredoxin. It requires one ATP per mol of N2 fixed. It requires two key protein components, each containing iron. 2. Overview of nitrogen metabolism Pages: 835-838 Difficulty: 2 Ans: A Which of the following enzymes is not involved in the assimilation of inorganic nitrogen into an organic molecule? A) B) C) D) E) Arginase Glutamate dehydrogenase Glutamate synthase Glutamine synthetase Nitrogenase 3. Overview of nitrogen metabolism Page: 835 Difficulty: 2 Ans: B The enzymatic machinery to fix atmospheric N2 into NH4+ is: A) B) C) D) E) a means of producing ATP when excess N2 is available. composed of two key proteins, each containing iron. relatively stable when exposed to O2. specific to plant cells. unaffected by the supply of electrons. 4. Biosynthesis of amino acids Page: 841 Difficulty: 2 Ans: C Erythrose 4-phosphate is a precursor of: A) B) C) D) E) aspartate. cysteine. phenylalanine. serine. threonine. from α-ketoglutarate is: A) B) C) D) E) arginine. glutamine. the product(s) that would be readily formed is (are): A) B) C) D) E) serine labeled at the carboxyl carbon. none of the above. may all be derived from a Cori cycle intermediate. arginine. proline. may all be derived from a citric acid cycle intermediate. 7. and proline: A) B) C) D) E) do not have a common precursor. Biosynthesis of amino acids Page: 844 Difficulty: 2 Ans: D If glucose labeled with 14C at C-1 were the starting material for amino acid biosynthesis. serine labeled at the R-group carbon. . alanine. glutamate. glycine. and histidine Arginine. Biosynthesis of amino acids Page: 842 Difficulty: 3 Ans: B Glutamine. and glutamate 9. glutamine. serine labeled at alpha carbon. Biosynthesis of amino acids Page: 843 Difficulty: 2 Ans: E In which group are all the amino acids closely interrelated metabolically? A) B) C) D) E) Arginine. Biosynthesis of amino acids Page: 841 Difficulty: 2 Ans: D Nonessential amino acids: A) B) C) D) E) are amino acids other than those required for protein synthesis. arginine. and glutamate Glycine. 8. but not by humans. Nucleotides. are not utilized in mammalian proteins. all of the above.260 Chapter 22 Biosynthesis of Amino Acids. can be synthesized in humans as well as in bacteria. may all be derived from a glycolytic intermediate. are synthesized by plants and bacteria. at least in part. and aspartate Ornithine. tyrosine. may all be derived from a urea cycle intermediate. Biosynthesis of amino acids Page: 842 Difficulty: 3 Ans: E An amino acid that does not derive its carbon skeleton. proline. hydroxyproline. and Related Molecules 5. may be substituted with other amino acids in proteins. 6. valine. and valine Ornithine. threonine. A) B) C) D) E) acetyl-CoA. Biosynthesis of amino acids Page: 846 Difficulty: 2 Ans: A Homoserine is: A) B) C) D) E) a precursor of both methionine and threonine. Molecules derived from amino acids Page: 855 Difficulty: 2 Ans: B δ-Aminolevulinic acid is formed from succinyl-CoA and __________ and is an intermediate in the biosynthesis of _________. lactate. 14. heme serine. Nucleotides. a precursor of serine. leucine. derived from threonine. derived from serine. glutamate and proline . threonine. 13.Chapter 22 Biosynthesis of Amino Acids. 11. orotate. Biosynthesis of amino acids Page: 848 Difficulty: 2 Ans: D An important intermediate in the biosynthetic pathway to aromatic amino acids is: A) B) C) D) E) benzoic acid. serine. sphingosine α-ketoglutarate. Biosynthesis of amino acids Page: 846 Difficulty: 2 Ans: D If a cell were unable to synthesize or obtain tetrahydrofolic acid (H4 folate). and Related Molecules 261 10. lysine. Biosynthesis of amino acids Page: 845 Difficulty: 3 Ans: D An amino acid that does not derive its carbon skeleton. it would probably be deficient in the biosynthesis of: A) B) C) D) E) isoleucine. α-ketoglutarate. proline. 12. heme serine. lysine. at least in part. methionine. methionine. from oxaloacetate is: A) B) C) D) E) aspartate. derived from homocysteine. shikimate. long chain fatty acids glycine. Molecules derived from amino acids Page: 860 Difficulty: 2 Ans: C l-Dopa is an intermediate in the conversion of: A) B) C) D) E) phenylalanine to homogentisic acid. tyrosine to epinephrine. isomer of oxidized glutamic acid. generated by oxidation of sterols.262 Chapter 22 Biosynthesis of Amino Acids. tryptophan. tyrosine to phenylalanine. threonine. Molecules derived from amino acids Page: 857 Difficulty: 2 Ans: E Glutathione is a(n): A) B) C) D) E) enzyme essential in the synthesis of glutamate. responsible for light reception in the vertebrate eye. 19. tyrosine to phenylpyruvate. tripeptide of glycine. Molecules derived from amino acids Page: 859 Difficulty: 2 Ans: E The plant hormone indole-3-acetate (auxin) is formed from: A) B) C) D) E) arginine. . glutamate. 16. methyl-group donor in many biosynthetic pathways. phenylalanine. secreted from the pancreas the products of purine degradation. tryptophan. 18. Molecules derived from amino acids Page: 860 Difficulty: 2 Ans: E The hormones epinephrine and norepinephrine are derived biosynthetically from: A) B) C) D) E) arginine. 17. and cysteine. isoleucine. product of glutamate and methionine. histidine. and Related Molecules 15. tyrosine. Nucleotides. phenylalanine to tyrosine. histidine. Molecules derived from amino acids Pages: 854. 856 Difficulty: 2 Ans: A Bile pigments are: A) B) C) D) E) formed in the degradation of heme. ribulose 5-phosphate. ribose 5-phosphate. Nucleotides. participation of PRPP (phosphoribosyl pyrophosphate). glutamate. 263 . tryptophan. tryptophan 21. Biosynthesis and degradation of nucleotides Page: 864 Difficulty: 3 Ans: B 5-Phosphoribosyl-α-pyrophosphate (PRPP) is a synthetic precursor for all of the following except: A) B) C) D) E) AMP. inhibition by azaserine (a glutamine analog). inosinic acid (IMP).Chapter 22 Biosynthesis of Amino Acids. 23. arginine. Biosynthesis and degradation of nucleotides Pages: 864-865 Difficulty: 2 Ans: C Glutamine is a nitrogen donor in the synthesis of: A) B) C) D) E) CTP. 24. and guanylate is provided most directly by: A) B) C) D) E) 5-phosphoribosyl 1-pyrophosphate. inosinate. Biosynthesis and degradation of nucleotides Pages: 865. UMP. dTTP. 22. and Related Molecules 20. 867 Difficulty: 2 Ans: A The ribosyl phosphate moiety needed for the synthesis of orotidylate. orotate. histidine. aspartate. UMP. Biosynthesis and degradation of nucleotides Pages: 864-865 Difficulty: 3 Ans: B De novo purine biosynthesis is distinguished from de novo pyrimidine biosynthesis by: A) B) C) D) E) condensation of the completed purine ring with ribose phosphate incorporation of CO2. guanosine 5'-phosphate. Biosynthesis and degradation of nucleotides Page: 864 Difficulty: 2 Ans: B One amino acid directly involved in the purine biosynthetic pathway is: A) B) C) D) E) alanine. participation of aspartate. leucine. adenosine 5'-phosphate. pyrimidine breakdown. and CO2. whereas pyrimidine formation does not. Biosynthesis and degradation of nucleotides Pages: 865. 27. Inosinate is the purine nucleotide that is the precursor of both adenylate and guanylate. whereas the basic skeleton for purines is formed by two main precursors. The amino acid valine is one of the precursors contributing to purine nucleotides. and Related Molecules 25. glutamate. 28. Deoxyribonucleotides are formed from 5-phosphodeoxyribosyl 1-pyrophosphate. 867 Difficulty: 3 Ans: B The synthesis of purine and pyrimidine nucleotides differ in that: A) ATP is required in the synthesis of purines but not in the synthesis of pyrimidines. The metabolic pathway in which the enzyme defect occurs is: A) B) C) D) E) epinephrine synthesis. glycine and succinyl-CoA. glycine. purine synthesis. and urine. Biosynthesis and degradation of nucleotides Page: 867 Difficulty: 2 Ans: A Precursors for the biosynthesis of the pyrimidine ring system include: A) B) C) D) E) carbamoyl phosphate and aspartate. Nucleotides. E) pyrimidines go through many steps. Orotic acid is an essential precursor for purine nucleotides. glutamine. 26. blood. C) purine formation requires a THF derivative.264 Chapter 22 Biosynthesis of Amino Acids. Biosynthesis and degradation of nucleotides Page: 866 Difficulty: 2 Ans: C Which one of the following statements is true of the biosynthetic pathway for purine nucleotides? A) B) C) D) E) CO2 does not participate in any of the steps in this pathway. adding a single carbon or nitrogen each time. B) purine biosynthesis starts with the formation of PRPP. inosine and aspartate. whereas pyrimidines incorporate the PRPP near the end of the pathway. D) pyrimidine biosynthesis is tightly regulated in the cell. and aspartate. purine breakdown. whereas purine biosynthesis is not. CO2. pyrimidine synthesis. Biosynthesis and degradation of nucleotides Page: 867 Difficulty: 2 Ans: E Orotic aciduria is an inherited metabolic disease in which orotic acid (orotate) accumulates in the tissues. . NH3. Biosynthesis and degradation of nucleotides Page: 872 Difficulty: 2 Ans: A Which one of the following statements correctly describes the biosynthetic pathway for purine nucleotides? A) Purine deoxynucleotides are made by the same path as ribonucleotides. 30. There is a separate enzyme for each nucleotide (ADP. Nucleotides. not in animals. D) The pathway occurs only in plants and bacteria. and TMP all have ________________ as a common precursor. glutamate and carbamoyl phosphate. 32. glutamine and aspartate. glutamine and carbamoyl phosphate. C) The nitrogen in the purine base that is bonded to ribose in the nucleotide is derived originally from glycine. Thioredoxin acts as an essential electron carrier. CDP. and Related Molecules 265 29. It acts on nucleoside diphosphates. Biosynthesis and degradation of nucleotides Page: 867 Difficulty: 2 Ans: A The most direct precursors of the nitrogens of UMP are: A) B) C) D) E) aspartate and carbamoyl phosphate. glutamate and aspartate. E) The purine rings are first synthesized. then condensed with ribose phosphate. A) B) C) D) E) adenosine aspartate glutamine inosine S-adenosyl methionine 31. followed by reduction of the ribose moiety.Chapter 22 Biosynthesis of Amino Acids. . GDP. UMP. UDP). B) The first enzyme in the path is aspartate transcarbamoylase (ATCase). Biosynthesis and degradation of nucleotides Pages: 867-868 Difficulty: 2 Ans: B CMP. Biosynthesis and degradation of nucleotides Pages: 870-872 Difficulty: 2 Ans: D Which of the following is not true of the reaction catalyzed by ribonucleotide reductase? A) B) C) D) E) Glutathione is part of the path of electron transfer. Its mechanism involves formation of a free radical. Ans: α-Ketoglutarate + NH4+ + NADPH + ATP → glutamate + NADP+ + ADP + Pi. which is present in certain prokaryotes. 36. 34. UMP.266 Chapter 22 Biosynthesis of Amino Acids. succinate. urea. Biosynthesis and degradation of nucleotides Pages: 873-874 Difficulty: 2 Ans: E An intermediate of purine degradation in humans is: A) B) C) D) E) glutamate. Then ammonia is incorporated into glutamine in the reaction catalyzed by glutamine synthetase: Glutamate + NH4+ + ATP → glutamine + ADP + Pi + H+. NH4+. orotate. Short Answer Questions 35. Name the intermediates (no structures necessary) and enzymes. Finally. Ans: First. and Related Molecules 33. including some that live symbiotically with legumes: – N2 + 10H+ + 8e + 16ATP → 2NH4+ + 16ADP + 16Pi + H2. uric acid. glutamate synthase catalyzes formation of glutamate from glutamine: α-Ketoglutarate + glutamine + NADPH + H+ → 2 glutamate + NADP+. Nucleotides. . Overview of nitrogen metabolism Page: 838 Difficulty: 2 Give the overall reaction that results from the combined action of glutamate synthase and glutamine synthetase. molecular nitrogen (N2) is reduced to ammonia in the reaction catalyzed by the nitrogenase complex. GMP. thymidylate (TMP). Overview of nitrogen metabolism Pages: 835-838 Difficulty: 2 Trace the path of nitrogen from atmospheric N2 into glutamate. Biosynthesis and degradation of nucleotides Page: 873 Difficulty 3 Ans: D A cell that is unable to synthesize or obtain tetrahydrofolic acid (H4 folate) would probably be deficient in the biosynthesis of: A) B) C) D) E) CMP. and show any coenzymes involved. An alternative minor route from ammonia to glutamate involves the reaction catalyzed by glutamate dehydrogenase: α-Ketoglutarate + NH4+ + NADPH → glutamate + NADP+ + H2O. carbamoyl phosphate. (c) aspartate. Biosynthesis of amino acids Page: 843 Difficulty: 2 Show the biosynthetic pathway for the conversion of a citric acid cycle intermediate into proline. 22-10. and Related Molecules 267 37. see Fig. For enzymes and cofactors. We need these amino acids for protein synthesis and for the production of a variety of products. .Chapter 22 Biosynthesis of Amino Acids. Ans: (1) The enzyme is subject to cumulative allosteric inhibition by six compounds: AMP. and inhibited by α-ketoglutarate and ATP. Adenylylation. is indirectly stimulated by glutamine and Pi. Overview of nitrogen metabolism Pages: 838-840 Difficulty: 3 Describe two types of regulation of the enzyme glutamine synthetase and explain why the regulation of this enzyme is so complex. Ans: (1) Glutamate + ATP → γ−glutamyl phosphate + ADP. Alanine and glycine are also allosteric inhibitors of the enzyme. Biosynthesis of amino acids Page: 841 Difficulty: 2 Why is it necessary to have protein in our (human) diets? Ans: Protein provides the 10 essential amino acids that humans cannot synthesize. derived from the essential amino acids. all of which are end products of pathways in which glutamine is a key precursor. 39. Overview of nitrogen metabolism Page: 838 Difficulty: 3 Give the equations for the two-step reaction sequence catalyzed by glutamine synthetase. p. Ans: (a) pyruvate (b) α-ketoglutarate (c) oxaloacetate CH3—CO—COO– –OOC—CH2—CH2—CO—COO– –OOC—CH2—CO—COO– 41. CTP. (2) γ-Glutamyl phosphate + NH4+ → glutamine + Pi + H+. (2) The enzyme is also subject to regulation by covalent alteration: adenylylation and deadenylylation. Nucleotides. Biosynthesis of amino acids Pages: 842. histidine. and glucosamine 6-phosphate. 40. 38. (b) glutamate. which inhibits the enzyme. and its level must be responsive to the concentrations of end-products and precursors of each pathway. Indicate where any cofactors participate. such as histamine and serotonin. The complexity of the regulation reflects the fact that glutamine is involved in many synthetic pathways. tryptophan. Ans: α-Ketoglutarate → glutamate → γ−glutamyl phosphate → glutamate γ-semialdehyde → pyrroline-5-carboxylate → proline. 843. 845 Difficulty: 2 Give the name and structure of the glycolytic or citric acid cycle intermediate that has the same carbon skeleton as (a) alanine. and Related Molecules 42. p. (3) Oxaloacetate + GTP → phosphoenolpyruvate + CO2 + GDP. p. What is the shortest pathway using known enzymes by which this conversion could be accomplished? Show intermediates and cofactors. 544. Biosynthesis of amino acids Page: 850 Difficulty: 3 Show the two-step reaction catalyzed by tryptophan synthetase. (2) Indole + serine → tryptophan + H2O. Biosynthesis of amino acids Page: 844 Difficulty: 3 Show the reaction catalyzed by glycine synthase. 14-16. Nucleotides. (Hint: The first step is removal of the nitrogen by transamination.) 46. Ans: Serine is derived from 3-phosphoglycerate by the pathway shown in Fig. alanine. Indicate below which of these “parent” compounds provides the carbon skeleton for each amino acid: Parent compound Asparagine Tryptophan Glycine __________________ __________________ __________________ .) Ans: (1) Alanine → pyruvate. p. Phosphoglycerate is oxidized by transamination from glutamate. 844. 3-phosphoglycerate + NAD+ → 3-phosphohydroxypyruvate + NADH. (See details in Fig. or chorismate. indicating the role of any cofactors that participate. 22-18. 45. 22-12. 22-12. (See Fig. yielding 3-phosphoserine. Biosynthesis of amino acids Page: 844 Difficulty: 3 Show the steps by which an intermediate of glycolysis can be converted into serine. Ans: CO2 + NH4+ + NADH + H+ + N5. aspartate. and Fig. p. the amino acids listed below can be derived directly or indirectly from serine.) 44. – (2) Pyruvate + HCO3 + ATP → oxaloacetate + ADP + Pi. 844. Biosynthesis of amino acids Pages: 842-849 Difficulty: 3 In bacteria. Removal of the phosphate yields serine. (4) Phosphoenolpyruvate → 2-phosphoglycerate → 3-phosphoglycerate.N10-methylenetetrahydrofolate → glycine + NAD+ + tetrahydrofolate 43. (6) 3-Phosphoserine → serine + Pi. Biosynthesis of amino acids Page: 844 Difficulty: 3 An animal cell is capable of converting alanine into serine. (5) 3-Phosphohydroxypyruvate + glutamate → 3-phosphoserine + α-ketoglutarate. no enzyme names are required. glutamate.268 Chapter 22 Biosynthesis of Amino Acids. 850. Ans: (1) Indole-3-glycerol phosphate → indole + glyceraldehyde 3-phosphate. glutamate (proline). Ans: The structure of AMP is in Fig. circle the atom(s) derived from glycine. concerted (cumulative) feedback regulation and sequential feedback inhibition. 22-34. and Related Molecules Methionine Threonine Cysteine Proline Isoleucine Phenylalanine 269 __________________ __________________ __________________ __________________ __________________ __________________ Ans: Aspartate (asparagine). 864. Biosynthesis and degradation of nucleotides Page: 865 Difficulty: 3 Draw the structure of inosinic acid (IMP). aspartate (threonine). Biosynthesis and degradation of nucleotides Pages: 864-866 Difficulty: 3 Draw the structure of any purine nucleotide. The origin of each of its atoms is shown in Fig. with diagrams. Two molecules of δ-aminolevulinate condense to form the five-membered ring of porphobilinogen. and circle the atoms derived from glycine. Nucleotides. In cumulative feedback regulation. Molecules derived from amino acids Page: 855 Difficulty: 3 Show the biosynthetic pathway from succinyl-CoA and glycine to porphobilinogen. 22-24. name it correctly. chorismate (phenylalanine) 47. The synthesis of AMP and GMP (Fig. p. glycine and succinyl-CoA condense to form α-amino-β-ketoadipate that is decarboxylated to form δ-aminolevulinate. alanine via pyruate (isoleucine). serine (cysteine). serine (glycine). Indicate the source of each N in this structure. Ans: Both are allosteric mechanisms to reduce the flux through a pathway when the end product(s) are present in sufficient quantities. (See Fig. 22-32. Glutamine synthetase exhibits this kind of regulation (Fig. 48. 866. p. chorismate (tryptophan). Biosynthesis of amino acids Pages: 839. 50. 866) exemplifies this mechanism. 839). 866. 855. each of several end products partially inhibits the first enzyme in the pathway. (See also Fig. p. p. AMP. Sequential feedback inhibition occurs in a branched pathway. What is the . Indicate with arrows those carbon atoms donated by derivatives of tetrahydrofolate. aspartate (methionine).) 49. 864.Chapter 22 Biosynthesis of Amino Acids. 22-6. p. 22-34. and GMP is shown in Fig. and it feeds back on (inhibits) the first enzyme in the main path.) 51. Ans: The origin of each atom in the purine ring of IMP. 22-34. p. Indicate with an arrow the atom(s) derived from glutamine's amide group(s). 22-32. Ans: Briefly. 866 Difficulty: 3 Describe and contrast. When both end products are present. the last intermediate before the branch accumulates. Biosynthesis and degradation of nucleotides Pages: 864-866 Difficulty: 3 Draw the structure of 5'-AMP. p. the product of each branch inhibits the first step in that branch. as seen in Fig. Indicate the source of each N and each C in the thymine ring. Biosynthesis and degradation of nucleotides Pages: 869. 22-36. 52. 22-43. Nucleotides. Biosynthesis and degradation of nucleotides Page: 867 Difficulty: 3 Draw the structure of 5'-UMP (uridylic acid). 867. Ans: UMP + ATP → UDP + ADP (nucleoside monophosphate kinase) UDP + NADPH → dUDP + NADP+ (ribonucleotide reductase and thioredoxin) dUDP + ATP → dUTP + ADP (nucleoside diphosphate kinase) dUTP → dUMP + PPi (dUTPase) dUMP + N5. Use abbreviations (e. not complete structures.) . and Related Molecules first “committed” step in the biosynthetic sequence that leads to IMP? How is this step regulated? Ans: The structure of IMP is in Fig.N10-methylene tetrahydrofolate. 54. 867. all but the terminal —NH2 and the attached carbon in carbamoylaspartate come from aspartate. Biosynthesis and degradation of nucleotides Page: 869 Difficulty: 2 Describe the pathway by which GMP is converted into GTP. The methyl substituent on the ring is derived from N5. The reaction that converts a base into a nucleotide is the condensation of PRPP with orotate. 865. Draw the reaction (with structures) in which a nitrogenous base is converted to a nucleotide on the pathway to 5'-UMP. p.g. The origin of each atom in IMP is shown in Fig. as shown in Fig. (See Fig. 22-36. 873. show coenzymes that are involved and name the enzymes. 22-33 (bottom right). 53. The first committed step in the pathway to IMP is the formation of 5phosphoribosylamine from PRPP. p. 865.270 Chapter 22 Biosynthesis of Amino Acids. 864. p. p. The other atoms in the ring are derived from carbamoyl phosphate and aspartate. 2235. 872. Ans: The structure of dTMP is shown in Fig. p. Biosynthesis and degradation of nucleotides Pages: 867. Ans: The structure of UMP is shown in Fig.N10-methylene tetrahydrofolate → dTMP + dihydrofolate (thymidylate synthase) dTMP + ATP → dTDP + ADP (nucleoside monophosphate kinase) dTDP + ATP → dTTP + ADP (nucleoside diphosphate kinase) (See Fig. p. 22-33.) This step is regulated by feedback inhibition. 873 Difficulty: 3 Diagram the biosynthetic pathway from UMP to dTTP. including its substituents. 867. UMP). Ans: GMP + ATP → GDP + ADP (nucleoside monophosphate kinase) GDP + ATP → GTP + ADP (nucleoside diphosphate kinase) 55. 872. accumulated IMP allosterically inhibits the first enzyme in the path. which also shows the atoms contributed by aspartate in the first step of the pathway to UMP. p.. 22-44. p. and indicate where any cofactors participate. 22-32. 873 Difficulty: 3 Draw the structure of deoxythymidylic acid (dTMP). Circle those carbon atoms donated by atoms derived from aspartate. Glutamine amido transferases are described on p. and tryptophan. It is a competitive inhibitor of many enzymes that use glutamine as substrates. 873. Name (no structures necessary) three biosynthetic products. Eating azaserine would not be immediately fatal because it would take some time for the deficiencies of CTP. Methotrexate inhibits the regeneration of tetrahydrofolate. Nucleotides. 22-49 and 22-50. Biosynthesis and degradation of nucleotides Pages: 873. a potent inhibitor of thymidylate synthase.Chapter 22 Biosynthesis of Amino Acids. 877. 840. 57. including those leading to CTP. 877 Difficulty: 3 Show the reaction catalyzed by thymidylate synthase and explain with a simple diagram how the chemotherapeutic agents fluorouracil and methotrexate inhibit the synthesis of dTMP. p. acting as a potent competitive inhibitor of dihydrofolate reductase. AMP. . Biosynthesis and degradation of nucleotides Page: 876 Difficulty: 3 Azaserine is a structural analog of glutamine. p. to become severe enough to cause death. p. GMP. 22-48. 22-8. and dihydrofolate as the other product. etc.N10-methylenetetrahydrofolate as methyl donor. 22-44. The modes of action of fluorouracil and methotrexate are shown in Fig. Fluorouracil is metabolically converted to fluoro-dUMP. AMP. GMP. Do you think that eating azaserine would be immediately fatal? Why or why not? Ans: Azaserine (shown in Fig. with N5. and Related Molecules 271 56. Ans: The reaction catalyzed by thymidylate synthase is shown in Fig. including Fig. tryptophan. 876) inhibits enzymes involved in the transfer of the amido nitrogen of glutamine in many biosynthetic pathways. the synthesis of which you would expect to be inhibited by azaserine. dUMP is methylated on the thymine ring.
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