Unlike purines, pyrimidines are assembled before being attached to 5-phosphoribosyl-1-pyrophosphate (PRPP).
|carbamoyl phosphate synthetase II||carbamoyl phosphate||This is the regulated step in the pyrimidine biosynthesis.|
|aspartic transcarbamolyase (aspartate carbamoyl transferase||carbamoyl aspartic acid||-|
|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.|
|nucleoside monophosphate kinase||UDP||Phosphorylation. ATP is used.|
|nucleoside diphosphate kinase||UTP||Phosphorylation. ATP is used.|
|CTP synthase||CTP||Glutamine and ATP are used.|
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.