Pyrimidines

The synthesis of pyrimidines also involves glutamine and aspartate (Fig. 4.7). However, in contrast to the synthesis of purines, the

Urea

Canaline

Guanidinoacetate

Glycine

Guanidinoacetate

Carbamoylphosphate

Canavanine

O-Ureidohomoserine

Fumarate

Fumarate

Canavaninosuccinate

Aspartate

Canavaninosuccinate

Guanidinosuccinate + Homoserine Fig. 4.4. Mammalian metabolism of canavanine. Enzymes: analogous to those involved in the urea cycle; thus 1, arginase. (Adapted from D'Mello, 1991.)

pyrimidine ring is assembled first and subsequently linked to ribose phosphate. It will be noted that pyrimidine synthesis begins with the production of carbamoyl phosphate. The enzyme, carbamoyl phosphate synthetase is distinct from that involved in the formation of urea, thus enabling all species, including uricotelic animals to generate the pyrimidines necessary for nucleotide production. The committed reaction in the pyrimidine pathway is formation of N-carbamoylaspartate.

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