pyrimidine

Pyrimidines (cytosine, thymine, uracil) are single-ring nitrogenous bases, contrasted with purines (adenine, guanine) which have fused double rings. Pyrimidine synthesis begins with formation of the ring structure first (from carbamoyl phosphate and aspartate, forming orotic acid), followed by attachment to ribose-5-phosphate (PRPP) to form the nucleotide—opposite order compared to purine synthesis. Key enzyme: carbamoyl phosphate synthetase II (CPS II) in the cytosol, distinct from CPS I in the urea cycle. Orotic aciduria results from a deficiency in UMP synthase (orotidine 5'-phosphate decarboxylase), causing megaloblastic anemia unresponsive to B12/folate, failure to thrive, and increased orotic acid in urine, without hyperammonemia (distinguishing it from ornithine transcarbamylase deficiency, which also causes elevated orotic acid but with hyperammonemia). Pyrimidine catabolism produces water-soluble products (unlike purines, which produce uric acid), making pyrimidine metabolism disorders generally less prone to crystal deposition. Clinically important pyrimidine analogs used in chemotherapy include 5-fluorouracil (inhibits thymidylate synthase, blocking dTMP synthesis), cytarabine (inhibits DNA polymerase), and gemcitabine. Antiviral pyrimidine analogs include zidovudine and lamivudine. Leflunomide, used in rheumatoid arthritis, inhibits dihydroorotate dehydrogenase, blocking de novo pyrimidine synthesis and impairing lymphocyte proliferation. Understanding pyrimidine structure and synthesis is essential for grasping mechanisms of antimetabolite chemotherapy, antiviral therapy, and inherited metabolic disorders affecting nucleotide synthesis.