Although both pyrimidines and purines are components in nucleic acids, they are made in different ways. Likewise, the products of pyrimidine degradation are more water‐soluble than are the products of purine degradation.
Pyrimidine biosynthesis
Unlike in purine biosynthesis, the pyrimidine ring is synthesized before it is conjugated to PRPP. The first reaction is the conjugation of carbamoyl phosphate and aspartate to make N‐carbamoylaspartate. The carbamoyl phosphate synthetase used in pyrimidine biosynthesis is located in the cytoplasm, in contrast to the carbamoyl phosphate used in urea synthesis, which is made in the mitochondrion. The enzyme that carries out the reaction is aspartate transcarbamoylase, an enzyme that is closely regulated.
Figure 1
The second reaction is ring closure to form dihydroorotic acid by the enzyme dihydroorotase. This circular product contains a 6‐ membered ring with nitrogen and carbons located in the same positions as in the mature pyrimidine ring.
The third reaction is the oxidation of the ring to form a carbon‐ carbon bond. The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. The product is orotic acid.
Fourth, the orotate ring is transferred to phosphoribosyl pyrophosphate (PRPP) to form a 5′ ribose‐phosphate, orotidylic acid.
Finally orotidylate is decarboxylated to yield UMP, which of course contains one of the bases of RNA. Cellular kinases convert UMP to UTP. Transfer of an amido nitrogen from glutamine by CTP synthetase converts UTP to CTP; this reaction uses an ATP high‐energy phosphate.
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Control
Pyrimidine synthesis is controlled at the first committed step. ATP stimulates the aspartate transcarbamoylase reaction, while CTP inhibits it. CTP is a feedback inhibitor of the pathway, and ATP is a feed‐forward activator. This regulation ensures that a balanced supply of purines and pyrimidines exists for RNA and synthesis.
Eukaryotic organisms contain a multifunctional enzyme with carbamoylphosphate synthetase, aspartate transcarbamoylase, and dihydroorotase activities. Two mechanisms control this enzyme. First, control at the level of enzyme synthesis exists; the transcription of the gene for the enzyme is reduced if an excess of pyrimidines is present. Secondly, control exists at the level of feedback inhibition by pyrimidine nucleotides. This enzyme is also an example of the phenomenon of metabolic channeling: aspartate, ammonia, and carbon dioxide enter the enzyme and come out as orotic acid.