The de novo synthesis of pyrimidines begins with the formation of orotic acid from carbamoyl phosphate and aspartate, catalyzed by the enzyme carbamoyl phosphate synthetase II.

Orotate phosphoribosyltransferase catalyzes the transfer of a phosphoribosyl group from 5-phosphoribosyl-1-pyrophosphate (PRPP) to orotic acid, forming orotidylate.

Cytidine kinase catalyzes the transfer of a phosphate group from ATP to UMP, forming CMP.

Cytidine deaminase catalyzes the removal of the amino group from cytosine, converting it to uracil.

The de novo synthesis of thymidine involves the methylation of deoxyuridine by thymidylate synthase, using 5,10-methylenetetrahydrofolate as the methyl donor.

The degradation of pyrimidines primarily occurs through the pentose phosphate pathway, where they are converted to ribose-5-phosphate and further metabolized.

Uracil dehydrogenase catalyzes the oxidation of uracil to dihydrouracil, initiating the degradation pathway.

Cytidine deaminase catalyzes the deamination of cytosine to uracil, which is a key step in the degradation of cytosine.

The final product of pyrimidine degradation is urea, which is excreted from the body as a waste product.

The regulation of pyrimidine metabolism is primarily controlled by feedback inhibition, where the end products of the pathway inhibit the activity of the enzymes involved in their synthesis.

CTP synthetase is responsible for the feedback inhibition of carbamoyl phosphate synthetase II, the first enzyme in the de novo pyrimidine biosynthesis pathway.

The degradation of pyrimidines can be regulated by the availability of PRPP, which is a key substrate for the initial steps of the degradation pathway.

Uracil phosphoribosyltransferase is involved in the salvage pathway for pyrimidine nucleotides, where it catalyzes the transfer of a phosphoribosyl group from PRPP to uracil, forming uridine monophosphate (UMP).

UTP-CMP kinase catalyzes the interconversion of UTP and CTP, transferring a phosphate group from UTP to CMP.

Cytidine deaminase is responsible for the deamination of deoxycytidine to deoxyuridine, which is a key step in the degradation of deoxycytidine.