<Dl> <Dd> Orotate + 5 - Phospho - α - D - ribose 1 - diphosphate (PRPP) → Orotidine 5' - phosphate + Pyrophosphate </Dd> </Dl> <Dd> Orotate + 5 - Phospho - α - D - ribose 1 - diphosphate (PRPP) → Orotidine 5' - phosphate + Pyrophosphate </Dd> <P> Orotidine 5' - monophosphate is decarboxylated by orotidine - 5' - phosphate decarboxylase to form uridine monophosphate (UMP). PRPP transferase catalyzes both the ribosylation and decarboxylation reactions, forming UMP from orotic acid in the presence of PRPP . It is from UMP that other pyrimidine nucleotides are derived . UMP is phosphorylated by two kinases to uridine triphosphate (UTP) via two sequential reactions with ATP . First the diphosphate form UDP is produced, which in turn is phosphorylated to UTP . Both steps are fueled by ATP hydrolysis: </P> <Dl> <Dd> ATP + UMP → ADP + UDP </Dd> </Dl>

Atp is a nucleotide that is composed of