• Purines include adenine, guanine, hypoxanthine, xanthine, and uric acid
• Pyrimidines include cytosine, uracil, thymidine (5-methyl uracil), and orotic acid
• UÞ T is only non-SAM methylation (uses methylene from N⁵,N¹⁰-CH₂-THF)-hence methotrexate good to stop cell division
• Ribose-5-phosphate (from pentose pathway) reacted with ATPÞ AMP
• produces 5-phosphoribosyl-1-pyrophosphate (PRPP)
• catalyzed by ribose phosphate pyrophosphokinase
• regulation: P_i(+) – nucleotide breakdown; ADP/GDP(-) – don’t need
• bases are added to C-1; nucleosides (without C-5 PO₃) from breakdown of NT’s

• Orotate is the first pyrimidine; OMP is first nucleotide
• Purine synthesis begins with carbamoyl phosphate synthase (CPS II)
• similar to urea cycle CPS I, except in cytoplasm not mitochondira and less active
• CAD protein – performs first three reactions
• (CPS II, aspartate transcarbamoylase, dihydroorotase)
• major regulated step – UTP(-) – feedback control
• One enzyme condenses orotic acid with PRPP and decarboxylates orotidine-5P
• if deficient: orotic aciduria – lethal if untreated homozygote – no pyrimidines
• also excess PRPP, so sugar fermentation and overproducion of purines
• Nucleoside diphosphokinase – add pyrimidine to PRPP
• increased in metastatic cancer

• Hypoxanthine is the first purine; IMP is the first nucleotide
• Unlike pyrimidines, purines are manufactured directly on the PRPP
• begins with PRPP + glutamate Þ phosphoribosylamine (PRA)
• Amidophosphoribosyl transferase – primary regulated step
• PRPP(+) – substrate availability; AXP/GXP(-) – feedback
• Synthesis of 5-membered ring first, then 6-membered ring
• THF donates 2 formyl groups in the process
• IMP Þ GMP or AMP – presence of GTP or ATP initiates producion of the other
• salvage pathway – add purine to PRPP
• adenine or hypoxanthine/guanine phosphoribosyl transferase (APRT/HGPRT)
• Lesch-Nyham Syndrome – deficiency in HGPRT – self-mutilation

• (1) Use in RNA, ATP, etc.
• (2) Formation of deoxynucleotides – enzyme: ribonucleotide reductase; two regulatory regions:
• (1) overall activity - dATP(-)
• (2) specific activity – binding of purine or pyrimidine makes other
• (3) Degradation
• base oxidized to uric acid (too much uric acid Þ Gout, joint pain)
• allopurinol inhibits xanthine oxidase – hypoxanthine is more soluble
• adenosine deaminase deficiency Þ SCID (severe combined immunodefic.)

 Inhibition of Nucleotide Synthesis

• (1) Folic acid analog (methotrexate) – stop methylation (dTMP formation)
• (2) hypoxanthine analogs (6-mercaptopurine and azathioprine)
• react with PRPP to form nucleotides – inhibit synthesis of PRA, AMP, GMP
• used for organ transplantation
• (3) 5-fluorouracil – reacts with PRPP to form dUMP analogue
• irreversably inavtivates thymidylate synthase – similar to methotrexate