Overview Phosphoribosyl pyrophosphate (PRPP) sugar building block formed in nucleotide synthesis added to nitrogenous base to form nucleoside formed from ribose-5-phosphate product of pentose phosphate shunt ATP + ribose-5-phosphate = PRPP + AMP catalyzed by PRPP synthetase once PRPP is made it can add to either a de novo or salvaged base Pyrimidine pathway diagram important enzymes carbamoyl phosphate synthetase-2 rate limiting step not the same carbamoyl phosphate as in urea cycle ribonucleotide reductase inhibited by hydroxyurea also reduces UDP, CDP, ADP, GDP dADP and dATP negatively feedback and inhibit enzyme result = ↓ dTMP, dUDP, dCDP, dADP, dGDP note: good to target thymidine synthesis because it is not involved in RNA thymidylate synthase inhibited by 5-fluorouracil (5-FU) result = ↓ dTMP dihydrofolate reductase inhibited by methotrexate (MTX) in eukaryotes inhibited by trimethoprim (TMP) in prokaryotes sulfamethoxazole (SMX) interferes with DHF synthesis in prokaryotes co-trimoxazole = TMP + SMX inhibited by pyrimethamine in protozoa result = ↓ dTMP deficiency orotic aciduria inability to convert orotic acid to UMP defect in uridine monophosphate (UMP) synthase AR presentation ↑ orotic acid crystals in urine megaloblastic anemia does not improve with administration of vitamin B12 or folic acid not enough thymidine to sustain normal erythropoiesis failure to thrive no hyperammonemia distinguishes between ornithine transcarbamylase (OTC) deficiency with high [orotic acid] in urine with hyperammonemia treatment oral uridine administration bypasses defect in de novo pyrimidine pathway purine pathway diagram insufficient capacity in most cells important enzymes PRPP amidotransferase inhibited by AMP, GMP, IMP rate-limiting step indirectly inhibited by allopurinol indirectly inhibited by 6-mercaptopurine note: base of inosine = hypoxanthine