Metabolism of Purine and Pyrimidine

1. Metabolism of PurineandPyrimidine Prof.Orhan CANBOLAT ; MD , PhD

2. There are two major purines, adenine (A) and guanine (G), • and three major pyrimidines, cytosine (C), uracil (U), and thymine (T).

3. A nucleoside is formed from the linkage of a sugar with a nitrogen-containingbase., 1. The bases that make up the physiologically relevant nucleosides all have ringstructures. a. The purines adenine, guanine, and inosine have a double-ring system. b. The pyrimidines cytosine, thymine, and uracil have six-membered ringstructures. 2. Ribose and 2-deoxyribose are the main sugars found in nucleosides and nucleotides. Nucleotides ; one, two, or three phosphate groups havebeen added to the sugar. PurineandPyrimidine

4. Nucleosides - Nucleotides

5. BiomedicalImportance • DNA – RNA ( Nucleicacidmetabolism ) • ATP – GTP (EnergyMetabolism ) • NADH – FADH( Co- enzymes ) • S- Adenosyl – methionine(donorformethylgroup ) • UDP – glucronicacids( donorforconjugation ; bilirubinglucronid ) • cAMP – cGMP( HormoneSignal ) • AnticancerDrugs ( Mtx , 5Flu, 6-TG , 6-MG ) • Antiviraldrugs(Aciclovir , Lamivudine  ) • GoutDiseaseTreatment( Allopurinol ) Humandiseasthatinvolvepurineandpyrimidine (GoutDisease , LeshNyhanSynrome , ADA Deficiency , Oroticaciduria )

6. Purines & Pyrimidines Are Dietarily Nonessential Human tissues can synthesize purines and pyrimidines from amphibolic intermediates. Ingested nucleic acids and nucleotides, which therefore are dietarily nonessential, are degraded in the intestinal tract to mononucleotides, which may be absorbed or converted to purine and pyrimidine bases.

7. GLİSİN CO2 C N ASPARTAT N 6 C 1 5 7 8 C N5, N10 methenilH4folat 2 C 3 4 C 9 N N N10-Formiltetrahidrofolat GLUTAMAT Thesources of nitrogenandcarbonatoms of purine ring

8. ATP AMP Mg PRPP PRPP Synthetase • Ribose 5-phosphate derived from the pentose phosphate pathway or from dietarysources is the starting material that eventually gives rise to inosinemonophosphate (IMP) Biosynthesis of PurineNucleotides ; De- Novo Ribose 5-Phosphate keyregulatoryenzyme The first step creates the multi-purpose intermediate 5-phosphoribosyl-1-pyrophosphate (PRPP).

10. Inprokaryotes, is catalyzedby a differentpolypeptidebycontrast, in eukaryotes, theenzymesarepolypeptideswithmultiplecatalyticactivities

11. FOLIC ACID DEFICIENCY • Decreased levels of folate coenzymes needed for various reactions of de novo purine synthesis andthymine synthesis produce shortages of deoxyribonucleotides and consequent impaired DNA synthesisin manytissues. • Blood levels of folic acid may become inadequate due to dietary insufficiency or poor absorption dueto intestinal problems or alcoholism. • Folate coenzyme concentrations may also decline as a result of treatment with drugs that inhibit dihydrofolatereductase, eg, methotrexate . • Patients with folic acid deficiency may have diarrhea and nausea, but the principal symptoms areweakness and easy fatigability due to megaloblastic anemia arising from impaired cell division in thebone marrow. • Folate deficiency during pregnancy is a major contributor to neural tube defects because of the criticalrole of folate in neuronal development. • Folate supplementation of food in the United States is expected to reduce folate-associated birth defectsbyupto 70%.

12. Biosynthesis of PurineNucleotides ; Salvage SalvageReactions“ ConvertPurines & TheirNucleosidestoMononucleotides

13. Salvage of purine nucleosides Free purines may be joined with PRPP to produce mononucleotides by one oftwoenzymes. The reaction shown below is catalyzed by adenine phosphoribosyltransferase (APRT). Adenine + PRPP → AMP + Ppi 2. The following reactions are catalyzed by , hypoxanthine-guanine phosphoribosyltransferase (HGPRT). Hypoxanthine + PRPP → IMP + PPi Guanine + PRPP → GMP + Ppi Salvage of purine nucleosides is achieved by phosphorylation with ATP as thephosphatedonor.

14. LESCH-NYHAN SYNDROME • Lesch-Nyhan syndrome is an X-linked disorder arising from deficiency of HGPRT, which results infailure to salvage hypoxanthine and guanine to the corresponding nucleotides IMP and GMP. • Inability to utilize PRPP in the salvage pathway leads to PRPP accumulation, which, in conjunctionwith low levels of IMP and GMP, causes chronic allosteric activation of PRPP glutamylamidotransferaseandexcessivepurinesynthesis. • The excess purines are degraded to uric acid causing increased blood levels of this metabolite (hyperuricemia)and deposition of sodium urate crystals in the joints and kidneys. • Patients with Lesch-Nyhan syndrome experience gout-like episodes of joint pain and kidney stonesas well as severe neurologic problems, including self-mutilation, spastic movements, and mental retardation. •

15. Degradation of purine nucleotides to uric acid

16. GMP GUANOZİN GUANİN . O2 Ksantin Oksidaz ADP R-1P PNP H2O2 5’NT Pi ADA Ksantin Oksidaz NH3

17. FeedbackRegulation AMP & GMP Feedback-RegulateTheirFormationfrom IMP GMP , ADP feedback-inhibitsPRPP glutamylamidotransferase ADP ,GDP feedback-inhibits PRPP synthetase AMP and GMP alsoinhibit hypoxanthine-guaninephosphoribosyltransferase,

18. ? Hyperuricemia

19. GOUT •Hyperuricemiaand chronic or episodic joint pain due to deposition of sodium urate crystals and consequent inflammation (gouty arthritis) are the hallmarks of gout. • Uric acid is minimally water-soluble and most cases of gout arise from inadequate excretion by the kidneys,leading to build-up of uric acid and precipitation of urate stones inthekidneysandextremities. • The joints of the hands and feet are prone to accumulation of crystals because of reduced solubility of sodium urate at the slightly cooler temperature of the extremities.

20. AMP ATP Riboz 5-P • 1.PRPP synthetase PRPP PRPP sentetaz ÜRİK ASİT

21. H2O2 . O2 Ischemia - Reperfusion ATP YIKIMI ARTIŞI ADP HİPOKSANTİN KO KO ÜRİK ASİT

22. 3. VonGierke’sDisease G-6-Paz G-6-P GLUKOZ glukoneogenez HMŞ R-5-P üretiminde artış • ÜRİK ASİT ARTIŞI

23. 5’NT ADA PNP 5’AMP AMP dATP ADENOZİN ADENOZİN RNdiP Ribo- nükleotid redüktaz İNOZİN İNOZİN dRNdiP DNA sentezinin bozulması HİPOKSANTİN T ve B hücre Immunsystemdeficiency

24. KSANTİN OKSİDAZ KSANTİN OKSİDAZ HİPOKSANTİN KSANTİN KSANTİNÜRİ Ve KSANTİN TAŞI ÜRİK ASİT HİPOÜRİSEMİ

25. RibonucleotideReductase

26. ASPARTAT GLUTAMİN C 4 N C 5 3 2 C 6 1 C N HCO3 Biosynthesis of PyrimidineNucleotides

27. The pyrimidine ring is synthesized first and is then attached to ribose 5-phosphateto eventually produce the nucleotide uridine 5′-monophosphate (UMP). Biosynthesis of pyrimidinenucleotides

28. An ammonium ion contributed by glutamine is combined with bicarbonate(derived from dissolved CO2) in a two-step reaction that requireshydrolysis of two molecules of ATP. • b. This complex reaction is catalyzed by carbamoyl phosphate synthetase II(CPS-II). • c. CPS-II, the critical enzyme regulating the pyrimidine synthetic pathway, isactivated by ATP and PRPP and feedback-inhibited by the end product UTP. CPS-II, a cytosolic enzyme, is different from the mitochondrial enzyme oftheureacycle CPS-I. Carbamoyl phosphate synthetase II ; CPS-II

29. Synthesis of dTTP

30. Mechanism of Tymidylatesynthesis Deoxythymidylate (dTMP) is formedfrom 2′-deoxyuridylate (dUMP) in a onecarbontransfer by thymidylatesynthetase 1. The donor coenzyme for the one-carbon transfer is N5,N10-methylenetetrahydrofolate (N5,N10-methylene THF); simultaneous reduction to amethyl group leaves dihydrofolate (DHF) as byproduct. 2. N5, N10-methylene THF is regenerated from DHF by a series of reactions, oneof which involves dihydrofolatereductase.

31. INHIBITORS OF dTMP SYNTHESIS AS ANTICANCER AGENTS • • Several drugs that interfere with production of dTMP by blocking the reaction catalyzed by thymidylatesynthetaseare inhibitors of DNA synthesis and cell proliferation. • The thymine analog 5-fluorouracil (5-FU) is converted to 5-fluoro-dUMP, which acts as a suicide inhibitor of thymidylatesynthetase. • • Methotrexate is a folate analog that acts as a potent competitive inhibitor of dihydrofolatereductase,causing a decreased supply of THF coenzymes needed by thymidylatesynthetase.

32. İnhibition of CPSII by UMP andUTP • Activation of PRPP and ATP CO2 + Glutamin + ATP UMP ,UTP Karbomoilfosfat Aspartat CTP N-karbomoilaspartat Control of PyrimidineNucleotide UMP UTP CTP

33. Catabolism of pyrimidine

34. Oroticaciduria • Mutation of one of the two enzyme activities of UMP synthase leads to oroticaciduria, characterizedby accumulation of its first substrate orotic acid and insufficient levels of the product UMP, • which reducesavailability of uridinetriphosphate (UTP) and cytidinetriphosphate (CTP) for use in nucleic acidsynthesis. • • Patients with oroticaciduriaexcrete large amounts of orotic acid in their urine, and they exhibitlethargy, weakness, severe anemia, and growth retardation. • • This autosomal recessive disorder can be treated by feeding a diet rich in uridine, which is salvaged toUMP and finally to UTP.

35. İnhereteddisorders of pyrimidinemetabolism