Orotidylate formation from orotate

Even though orotidylic acid or orotidine was implicated in pyrimidine formation, the precise role of orotic acid per se remained to be evaluated. On the other hand, it was posable that orotic acid was a normal intermediate that condensed with a ribose compound to yield orotidine or orotidylic acid during the biosynthetic process. In support of this thesis, it was found that 5-phosphoribosyl-l-pyrophosphate was utilized for nucleotide formation from orotic acid (83). On the other hand, it was equally posable that an aliphatic compound, such as aminofumaric acid (335) or carbamylaspartic acid (339), could have coupled with a ribose compound and formed orotidine or orotidylic acid directly without the existence or participation of orotic acid per se. In this latter instance, orotic acid would not be conadered a true intermediate in pyrimidine biosynthesis but merely an accidental cleavage product of hi ly labile orotidine or orotidylic acid. At this time research in the area of purine biosynthesis indicated that a series of acyclic intermediates attached to ribose 6-phosphate were biosynthetic intermediates and that free purines per se were not (Section II, B.). 

The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. CTP is then formed by the amination of UTP. 

As early as 1949, it was demonstrated that injected or " C-labeled orotic acid was readily incorporated into DNA and RNA of mammalian tissue, indicating that orotic acid is a precursor of nucleic acid pyrimidine. The next step in pyrimidine biosynthesis is the formation of the first nucleotide in the sequence. It involves the reaction between ribosyl pyrophosphate and orotic acid to yield 5 -orotidylic acid the reaction is catalyzed by orotidylic pyrophosphorylase. Thus, the first steps of pyrimidine biosynthesis differ from the early steps of purine biosynthesis in at least two ways. Orotic acid, instead of being synthesized atom by atom as is the case for the purine ring, is made from the condensation of rather large molecules, namely, carbamyl phosphate and aspartic acid. Furthermore, all the steps of purine biosynthesis occur at the level of the nucleotide, but the the pyrimidine ring is closed at the level of the base. 

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