Pyrimidine metabolism

Pyrimidine biosynthesis occurs both in the body and through organic synthesis.

## De novo biosynthesis of pyrimidine

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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

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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]

## External links

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* Overview at Queen Mary, University of London

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Metabolism map

Template:Metabolic pathways

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Protein metabolism: protein metabolism

Protein synthesis \- Amino acid synthesis \- Catabolism
Nucleotide metabolism: Purine metabolism \- Nucleotide salvage \- Pyrimidine metabolism

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* Metabolism: amino acid metabolism
* nucleotide enzymes

Purine metabolism|

| Anabolism|

| R5P→IMP:|

* Ribose-phosphate diphosphokinase
* Amidophosphoribosyltransferase
* Phosphoribosylglycinamide formyltransferase
* AIR synthetase (FGAM cyclase)
* Phosphoribosylaminoimidazole carboxylase
* Phosphoribosylaminoimidazolesuccinocarboxamide synthase
* IMP synthase

|

IMP→AMP:|

* Adenylosuccinate synthase
* Adenylosuccinate lyase
* reverse
* AMP deaminase

IMP→GMP:|

* IMP dehydrogenase
* GMP synthase
* reverse
* GMP reductase

Nucleotide salvage|

* Hypoxanthine-guanine phosphoribosyltransferase
* Adenine phosphoribosyltransferase

Catabolism|

* Adenosine deaminase
* Purine nucleoside phosphorylase
* Guanine deaminase
* Xanthine oxidase
* Urate oxidase

Pyrimidine metabolism|

| Anabolism|

* CAD
* Carbamoyl phosphate synthase II
* Aspartate carbamoyltransferase
* Dihydroorotase

* Dihydroorotate dehydrogenase
* Orotidine 5'-phosphate decarboxylase/Uridine monophosphate synthetase

* CTP synthetase

Catabolism|

* Dihydropyrimidine dehydrogenase
* Dihydropyrimidinase/DPYS
* Beta-ureidopropionase/UPB1

Deoxyribonucleotides|

* Ribonucleotide reductase
* Nucleoside-diphosphate kinase
* DCMP deaminase
* Thymidylate synthase
* Dihydrofolate reductase

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