To make the deoxynucleotides, the enzyme ribonucleotide reductase is required. This is a very important enzyme since we could not make DNA without it. The fourth row shows deoxyuridine nucleotides, and the fifth row shows the thymidine nucleotides. There is no thymine base in RNA, so the synthesis of dTTP is a natural target for inhibition of DNA synthesis. We will begin with the de novo pathway for UMP synthesis.
Pyrimidines are smaller than purines, and their biosynthesis is correspondingly easier.
The pyrimidine ring is composed from only two compounds, carbamoyl phosphate and aspartate. CPS II is the one we are interested in. It is in the cytosol, while CPS I is in the mitochondria and is part of the urea cycle. We will come back to the urea cycle when we discuss amino acid metabolism. Since bacteria do not have these compartments, they have only one CPS enzyme that is used for both amino acid and pyrimidine metabolism.