Biosynthesis of Pyrimidine Nucleotides
المؤلف:
Peter J. Kennelly, Kathleen M. Botham, Owen P. McGuinness, Victor W. Rodwell, P. Anthony Weil
المصدر:
Harpers Illustrated Biochemistry
الجزء والصفحة:
32nd edition.p342
2025-09-01
430
Figure 1 illustrates the intermediates and enzymes of pyrimidine nucleotide biosynthesis. The catalyst for the initial reaction is cytosolic carbamoyl phosphate synthetase II (EC 6.3.5.5), a different enzyme from the mitochondrial carbamoyl phosphate synthetase I of urea synthesis. Compartmentation thus provides an independent pool of carbamoyl phosphate for each process. Unlike in purine biosynthesis where PRPP serves as a scaffold for assembly of the purine ring (Figure2), PRPP participates in pyrimidine biosynthesis only subsequent to assembly of the pyrimidine ring. As for the biosynthesis of pyrimidines, purine nucleoside biosynthesis is energetically costly.
Fig1. The biosynthetic pathway for pyrimidine nucleotides.
Fig5. Purine biosynthesis from ribose 5-phosphate and ATP. See the text for explanations. ( P , PO3 2– or PO2–.)
Multifunctional Proteins Catalyze the Early Reactions of Pyrimidine Biosynthesis
Five of the first six enzyme activities of pyrimidine biosynthesis reside on multifunctional polypeptides. CAD, a single polypeptide named for the the first letters of its enzyme activities, catalyzes the first three reactions of Figure 1. A second bifunctional enzyme catalyzes reactions ➄ and ➅ of Figure 1. The close proximity of multiple active sites on a multifunctional polypeptide facilitates efficient channeling of the intermediates of pyrimidine biosynthesis.
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