UMP synthase

UMP synthase is a cytosolic bifunctional enzyme located on chromosome 3q13 that catalyzes the last two steps of the de novo pyrimidine biosynthetic pathway: (1) orotate phosphoribosyltransferase (OPRT) transfers a ribose-5-phosphate group from PRPP to orotic acid forming orotidine monophosphate (OMP), and (2) OMP decarboxylase converts OMP to uridine monophosphate (UMP), the precursor for all pyrimidine nucleotides (UTP, CTP, dTTP).

Hereditary orotic aciduria results from an autosomal recessive deficiency in UMP synthase, blocking pyrimidine synthesis and causing accumulation of orotic acid. Clinical features include: megaloblastic anemia that does NOT respond to vitamin B12 or folate supplementation (distinguishing it from other megaloblastic anemias), failure to thrive, developmental delay, and orotic acid crystals in urine. Importantly, there is NO hyperammonemia, which distinguishes it from ornithine transcarbamylase (OTC) deficiency—another cause of orotic aciduria—where orotic acid accumulates due to a urea cycle defect causing excess carbamoyl phosphate shunting into pyrimidine synthesis, accompanied by hyperammonemia.

Treatment involves oral administration of uridine (or uridine triacetate), which bypasses the enzymatic block by providing UMP directly, allowing downstream synthesis of pyrimidine nucleotides and also exerting feedback inhibition on carbamoyl phosphate synthetase II (CPS II), reducing orotic acid overproduction.

This topic is high-yield for distinguishing pyrimidine synthesis disorders from urea cycle disorders (both can present with orotic aciduria) and for testing understanding of nucleotide synthesis pathways relevant to chemotherapy drug mechanisms (e.g., 5-FU inhibits thymidylate synthase, methotrexate inhibits dihydrofolate reductase, both downstream of pyrimidine metabolism).