Pyrimidine metabolism
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Pyrimidine biosynthesis occurs both in the body and through organic synthesis.
De novo biosynthesis of pyrimidine
Unlike purines, pyrimidines are assembled before being attached to 5-phosphoribosyl-1-pyrophosphate (PRPP).
The synthesis of UMP.
The color scheme is as follows: enzymes, coenzymes, substrate names, inorganic molecules
| Enzyme | Product | Description |
| carbamoyl phosphate synthetase II | carbamoyl phosphate | This is the regulated step in the pyrimidine biosynthesis. |
| aspartic transcarbamolyase (aspartate carbamoyl transferase | carbamoyl aspartic acid | - |
| dihhydroorotase | dihydroorotate | Dehydration |
| dihydroorotate dehydrogenase (the only mitochondrial enzyme) | orotate | Dihydroorotate then enters the mitochondria where it is oxidised through removal of hydrogens. This is the only mitochondrial step in nucleotide rings biosynthesis. |
| orotate phosphoribosyltransferase | OMP | PRPP is used. |
| OMP decarboxylase | UMP | Decarboxylation |
| nucleoside monophosphate kinase | UDP | Phosphorylation. ATP is used. |
| nucleoside diphosphate kinase | UTP | Phosphorylation. ATP is used. |
| CTP synthase | CTP | Glutamine and ATP are used. |
The first three enzymes are all coded by the same gene (CAD).
Pyrimidine catabolism
Pyrimidines are ultimately catabolized (degraded) to CO2, H2O, and urea. Cytosine can be broken down to uracil which can be further broken down to N-carbamoyl-β-alanine. Thymine is broken down into β-aminoisobutyrate which can be further broken down into intermediates eventually leading into the citric acid cycle.
β-aminoisobutyrate acts as a rough indicator for rate of DNA turnover.[citation needed]