22 Voet , Chapter 26, pp. Purines and pyrimines are always required for DNA and RNA synthesis, and all organisms have the ability to synthesize them as needed.
Pyrimidine biosynthesis is based on combining preassembled parts.
Purine biosynthesis takes the atom by atom approach. Carbamoyl phosphate synthetase II acts in the cytoplasm , and uses glutamine as the N donor to produce carbamoyl phosphate for pyrimidine synthesis . A closely related enzyme is also found in prokaryotes. The complete enzyme hydrolyzes glutamine at site 1 in the small subunit, but the NH 3 product is not released to the medium. The crystal structure of the enzyme reveals a long solvent-protected tunnel linking the three catalytic sites.