Category: biosynthesis
IMP (inosine monophosphate) is built up from PRPP through 10 enzymatic steps using formyl-THF (from folate_one_carbon) at two positions, glutamine amides at two more, glycine + aspartate + CO2 + bicarbonate at others. The most expensive biosynthesis in the cell. IMP → AMP (adenylosuccinate synthetase + lyase) or → GMP (IMPDH + GMP synthase). Drug targets: methotrexate depletes folate cofactors (slows steps 3 + 9 of purine synthesis + thymidylate synthesis); mycophenolate (MMF / Cellcept) inhibits IMPDH (selective T+B lymphocyte effect — these cells lack the salvage path and depend on de novo). Allopurinol's active metabolite oxypurinol also weakly inhibits PRPP synthetase upstream. Salvage path via HGPRT — deficient in Lesch-Nyhan (severe gout + self-mutilation behavior).