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Lecture 30 Pyrimidine Metabolism/Disease

Lecture 30 Pyrimidine Metabolism/Disease. Raymond B. Birge, PhD. Pyrimidine metabolism (Overview) 1. Nomenclature/nucleotide structure 2. S ynthesis pathways 3. Synthesis of deoxy-ribonucleotides 4. Salvage & degradation pathways 5. Metabolic disease of pyrimidine

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Lecture 30 Pyrimidine Metabolism/Disease

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  1. Lecture 30 Pyrimidine Metabolism/Disease Raymond B. Birge, PhD

  2. Pyrimidine metabolism (Overview) 1. Nomenclature/nucleotide structure 2. Synthesis pathways 3. Synthesis of deoxy-ribonucleotides 4. Salvage & degradation pathways 5. Metabolic disease of pyrimidine metabolism (orotic aciduria) Suggested reading: Lippencott’s Chapter 22

  3. Structure of Pyrimidines O NH2 O O Uracil Cytosine Uracil O O CH3 O O Thymine Orotic Acid Orotic acid C= 2 oxy, 4 amino T= 2,4 dioxy 5-methyl U= 2,4 dioxy O= 2,4 dioxy 6 carboxy

  4. Nomenclature of Pyrimidines Cytosine Cytidine Cytidine Monophosphate Base Nucleoside* Base + ribose Nucleotide Base + ribose + P04 ester * when the base is purine, then the nucleoside ends in OSINE (AdenOSINE, GuanOSINE, InOSINE) when the base is pyrimidine, then the nucleoside ends in IDINE (UrIDINE, CytIDINE, ThymIDINE) PO4 is an acid: cytidylic acid/cytidylate; note thymidine only deoxyribose

  5. 1st Step is Regulated (occurs in cytosol) Inhibited by UTP; Activated by ATP & PRPP 5-phosphoribosylpyrophosphate

  6. From Figure 22.21 in Lippincott Multifunctional enzyme synthesis: CAD Carbamoyl phosphate synthase II Aspartate transcarbamylase Dihyroorotase Carbamoyl phosphate synthetase II: by UTP; byATP & PRPP; A spartate transcarbamylase D ihydroorotase 1 polypeptide, 3 domains, 3 activities Contrast to purines: Pyrimidines synthesized as free ring

  7. Making a pyrimidine Carbamoyl phosphate synthase II Aspartate transcarbamylase Dihyroorotase Desaturating the ring gives the pyrimidine, OROTATE. Dihyroorotate dehydrogenase From Figure 22.21 in Lippincott

  8. Precursors

  9. Which of the following contributes nitrogen atoms DIRECTLY to both purine and pyrimidine rings? • Aspartate • Carbamoyl phosphate • Bicarbonate • Glutamate • Tetrahydrofolate 10

  10. Making UMP: another multifunctional enzyme UMP synthase: Orotate phosphoribosyl transferase OMP decarboxylase 1 polypeptide, 2 domains, 2 activities Orotate phosphoribosyl transferase OMP decarboxylase Low UMP activity Orotic aciduria (abnormal growth; megaloblastic anemia; treat with uridine-rich diet) From Figure 22.21 in Lippincott

  11. Ribose-Tri-P04 Ribose-Tri-P04 Synthesis of CTP (Uracil) (Cytosine) ADP, Pi, Glu ATP, Gln CTP Synthetase UTP CTP

  12. Clinical Significance-pyrimidine metabolism ID: A 2 year old female referred to a pediatric clinic Chief Complaint: My baby doesn’t play, sleeps all the time and is weak. History Present Illness: Baby was treated for anemia by family doctor but did not respond to vitamin B12, folic acid, iron or vitamin C. She is the third-born child of a healthy white couple; her mother had an uneventful pregnancy and a eutopic delivery. Both brothers are healthy. Physical Exam: Low weight and height for age, marked pallor; flacidity & lethargy; sleepiness Pathology: CBC: megaloblastic anemia; UA: increased orotic acid excretion with formation of orotic acid crystals.

  13. AMP Purine biosynthesis PRPP PRPP _ _ _ + Pyrimidine Biosynthesis-IV UMP ODC CPS II OPRT ATC DHO OA DHOD UTP Eukaryote Prokaryote Committed Steps

  14. ATP CTP Rate [Aspartate] ATCase is feedback inhibited by the end-products of pyrimidine biosynthesis C02 + Glutamine + ATP Carbamoyl Phosphate Inhibited by CTP Carbamoyl Asparate UMP UTP CTP

  15. Ribonucleotides to Deoxyribonucleotides Ribonucleotide Reductase Thymidylate Synthase: (prevent incorporation into RNA)

  16. Ribonucleotides to Deoxyribonucleotides Ribonucleotide reductase Thioredoxin reductase Inhibited by dATP; Activated by ATP

  17. ATP ATP ATP ADP ADP ADP ATP ATP ATP ADP ADP ADP UDP GDP CDP ADP Ribonucleotide Reductase dUDP dGDP dADP dCDP dUTP H20 PPi dUMP 5,10 THF DHF dTDP dTTP dGTP dATP dCTP

  18. Thymidine biosynthesis Thymidylate synthase dUMP TDP reduced oxidized N5,N10-methylene- tetrahydrofolate Dihydrofolate NADPH Dihydrofolate reductase Serine transhydroxymethylase Tetrahydrofolate NADP+

  19. Salvage & degradation of pyrimidines uridine-cytidine kinase: nucleoside to nucleotide Salvage: (deoxycytidine kinase) (thymidine kinase) Degradation: pyrimidine rings cleaved and degraded to soluble structures (contrast to purines)

  20. F 5FU is a simple derivative of Uracil Uracil 5-Fluoro-Uracil (5FU)

  21. Targets of drug therapy Fluorodeoxyuradylate (5-FU) Thymidylate synthase dUMP dTMP reduced oxidized N5,N10-methylene- tetrahydrofolate Dihydrofolate NADPH Dihydrofolate reductase Methotrexate Aminopterin Tetrahydrofolate NADP+

  22. Conversion of Serine to Glycine Dihydrofolate reductase Folate Serine Tetrahydrofolate (FH4) Serine hydroxymethyl transferase (PLP-dep.) Key intermediate in biosynthesis of purines and formation of thymine Glycine Important in biosynthesis of heme, porphyrins, and purines N5, N10-Methylene FH4

  23. Using nucleotides for selecting hybrid cells

  24. AZT inhibits HIV reverse transcriptase (RNA-dependent DNA polymerase) 3’ AZido-2’3’ dideoxyThymine (AZT) This class of compounds (chemotherapeutics, viral inhibitors, etc.) are called nucleoside analogs.

  25. Recognize names and structures of pyrimidines; NMPs/dNTPs Orotate, Uracil, Cytosine, Thymine; CTP/dCTP, TTP Name the sources of atoms in the pyrimidine ring: carbamoyl phosphate (C,N: from Gln, CO2 ); Aspartate (C,N) Recognize the regulated reaction: Carbamoyl phosphate synthase II: UTP; ATP, PRPP Contrast the synthesis of purines & pyrimidines Explain the cause of Orotic aciduria; Contrast with hyperuricemia Explain mechanisms of the following treatments: sulfonamides, methotrexate, 5-Fluorouracil Bottom Line

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