Midterms Doc Sange

• Despite a diet that may be rich in nucleoproteins, dietary purines & pyrimidines are not incorporated directly into tissue nuclei acids
• Abnormalities in Purine Metabolism


  • Purine Nucleoside Phosphorylase Deficiency
  • Lesch-Nyhan Syndrome
  • Adenosine Deaminase Deficiency
  • Gout
• Abnormalities in Pyrimidine Metabolism


  • Orotic Aciduria
  • B-hydroxybutyric aciduria (dihydropyrimidine dehydrogenase deficiency)
  • Uraciluria
  • Thyminuria (Disorder of β-amino biosynthesis)
• End Product of Pyrimidine Metabolism: Carbon dioxide, Ammonia, β-alanine, γ–aminoisobutyrate
• Purine & Pyrimidines are dietarily nonessential
• Normal Human Tissues can synthesize purine & pyrimidines from amphibolic intermediates
• Ingested nucleic acids & nucleotides -> degradation in the intestinal tract-> monucleotides-> absorbed or converted to purine & pyrimidines
• Biosynthesis of Purine Nucleotides


  • Synthesis from Amphibolic Intermediates (De Novo Synthesis)
  • Phosphoribosylation Purine
  • Phosphorylation of Purine Nucleosides
• The liver is the major site of Purine Nucleotide Biosynthesis
• Sources of Nitrogen & Carbon Atoms of Purine Ring


  • N5 N10 Methenyl Tetrahydrofolate
  • Amide Nitrogen of Glutamine
  • Glycine
  • Respiratory CO2
  • Aspartate
  • N10 -Formyl tetrahydrofolate
• De Novo Purine Nucleotide Synthesis


  1. 5 steps require ATP as Source of Energy

  2. 2 steps require N10 -Formyl-THF as one carbon donor

  3. 2 steps require Glutamine "GluTWOmine" as one-nitrogen donor

  4. 1 steps require Glycine as 2-carbon & 1-nitrogen donor

  5. 1 steps require Aspartate as one 1-nitrogen donor
• PRPP


  Initial reaction of purine biosynthesis transfer two phosphoryl group from ATP to Carbon 1 of ribose 5-phosphate to create PRPP catalyzed by PRPP synthetase
• 1. Phosphoribosylamine Synthesis


  Committed Step - Amide group of glutamine replaces the pyrophosphate group attached to carbon 1
• Inosine Monophosphate (IMP)


  Synthesized from amphibolic intermediates, End product of ten subsequent enzyme-catalyzed reaction
• Inhibitors of Purine Nucleotides Synthesis in Humans

  • Methotrexate (Folic Acid Analog)
  • Diazanorleucine (Glutamine Analog)
  • Azaserine (Glutamine-Dependent Enzymes)
  • Allopurinol (Purine Analog)
• Inhibitors of Purine Nucleotide Synthesis in Pathogens


  • Acyclovir (Purine Analog)
  • Sulfonamides (Para-aminobenzoic Acid (PABA) Analog)
• Adenosine and Guanosine Monophosphate Synthesis


  1. Conversion of IMP to either AMP or GMP uses a two-step, energy – and nitrogen -requiring pathway

  2. Requires Guanosine Triphosphate (GTP) as an energy source & aspartate as a nitrogen source for AMP synthesis

  3. Requires ATP and glutamine for GMP synthesis

  4. The first reaction in each pathway is inhibited by the end product of the pathway
• Mycophenolic acid


  Reversible inhibitor of IMP dehydrogenase (the enzyme used to generate GMP), Proliferating T & B lymphocytes are highly susceptible to low levels of this key purine nucleotide, Effective immunosuppressant agent
• Inhibitors of Adenosine and Guanosine Monophosphate Synthesis


  • Mercaptopurine (inhibits adenylosuccinase and IMP dehydrogenase)
• Purine Salvage Pathway


  • Purines that result from that normal turnover of cellular nucleic acids, or the small amount that is obtained from the diet and not degraded, can be converted to nucleoside triphosphates & used by the body
  • Requires far less energy than de novo synthesis
  • 2 "Salvage Reactions": Phosphoribosylation of purine, Phosphoribosylation of purine nucleosides
• The liver provides purines and purine nucleosides for salvage and for utilization tissues incapable of their biosynthesis
• Brain tissue has low levels of PRPP glutamyl amidotransferase, Erythrocytes and polymorphonuclear leukocytes cannot synthesize 5-phosphoribosylamine
• Phosphoribosylation of Purines


  Involves the reaction between PRPP and a free purine (Pu) to form a purine 5' – mononucleotide (Pu-RP)
• Phosphorylation of Purine Nucleosides


  Involves phosphoryl transfer from ATP to a purine ribonucleoside (Pu-R)
• Regulation of the Purine Nucleotide Biosynthesis


  • Concentration of PRPP is the overall determinant of the rate of de novo purine nucleotide biosynthesis
  • PRPP synthetase is feedback inhibited by AMP, ADP, GMP and GDP
  • AMP & GMP feedback regulate their formation from IMP
  • Cross-Regulation serves as balance the biosynthesis of purine nucleoside triphosphate by decreasing the synthesis of one purine nucleotide when there is deficiency of the other nucleotide
• Reduction of Ribonucleoside Diphosphates


  • Provides the deoxyribonucleoside diphosphates (dNDPs) needed for both the synthesis and repair of DNA
  • Catalysis by the complex that includes ribonucleotide reductase
  • Reduction requires reduced Thioredoxin, Thioredoxin reductase, NADPH
• Steps in the Pyrimidine Nucleotides Synthesis


  1. Carbamoyl Phosphate Synthesis (Regulate step, Substrates: GLUTAMINE, CO2, ATP, Catalyzed by Carbamoyl Phosphate Synthetase (CPS) II)

  2. Formation of Carbamoylaspartate (requires ASPARTATE, Catalyzed by aspartate transcarbamoylase)

  3. Formation of the Close Ring Dihydroorotate (Pyrimidine ring is formed)
• Nucleoside triphosphate


  Decreases the synthesis of one purine nucleotide when there is deficiency of the other nucleotide
• Conversion of IMP to ADENYLOSUCCINATE


  Requires GTP
• Conversion of Xanthinylate (XMP) to GMP


  Requires ATP
• Reduction of ribonucleoside diphosphates


  • Provides the deoxyribonucleoside diphosphates (dNDPs) needed for both the synthesis and repair of DNA
  • Catalysis by the complex that includes ribonucleotide reductase
• Reduction of ribonucleoside diphosphates


  Requires reduced thioredoxin, thioredoxin reductase, and NADPH
• Pyrimidine nucleotides biosynthesis


  1. Carbamoyl phosphate synthesis (regulated step, substrates: glutamine, CO2, ATP, catalyzed by carbamoyl phosphate synthetase (CPS) II)

  2. Formation of carbamoylaspartate (requires aspartate, catalyzed by aspartate transcarbamoylase)

  3. Formation of the closed ring dihydroorotate (pyrimidine ring is closed by dihydroorotase)

  4. Oxidation of dihydroorotate to orotic acid

  5. Conversion of orotic acid to orotidine monophosphate (OMP) (OMP is the parent pyrimidine nucleotide for CTP and GTP, PRPP is the ribose 5-phosphate donor)

  6. Decarboxylation of OMP to uridine monophosphate (UMP) (catalyzed by orotidylate decarboxylase, UMP is sequentially phosphorylated to UDP and UTP)

  7. Synthesis of cytidine triphosphate (CTP is produced by amination of UTP by CTP synthetase, with glutamine providing nitrogen, some CTP is dephosphorylated to CDP)

  8. Synthesis of thymidine triphosphate
• CPS I vs CPS II


  CPS I is in the mitochondria and involved in the urea cycle, using ammonia as the nitrogen source and regulated by N-acetyl glutamate

  CPS II is in the cytosol and involved in pyrimidine synthesis, using the gamma-amide group of glutamine as the nitrogen source and regulated by PRPP and inhibited by UTP
• Multifunctional proteins catalyze the early reactions of pyrimidine biosynthesis


  • The CAD polypeptide catalyzes the first 3 steps (CPS II, aspartate transcarbamoylase, dihydroorotase)

  Orotate phosphoribosyl-transferase and orotidylic acid decarboxylase catalyze the 5th and 6th steps
• The deoxyribonucleosides of uracil and cytosine are salvaged
• Adenine, guanine, and hypoxanthine released during nucleic acid turnover are reconverted to nucleoside triphosphates via salvage pathways
• Pyrimidine salvage pathways


  • Pyrimidine ribonucleosides (uridine, cytidine)

  Pyrimidine deoxyribonucleosides (thymidine, deoxycytidine)
• Methotrexate


  Inhibits dihydrofolate reductase, indirectly affecting the methylation of dUMP, affecting both pyrimidine and purine biosynthesis
• 1. fluorouracil


  Inhibits thymidylate synthase, used as an antitumor agent