The overall pathway for pyrimidine biosynthesis is exergonic.
Four high-energy bonds are required for the biosynthesis of UMP from carbon dioxide, glutamine, aspartate, and ribose 5-phosphate.
Two are required for the synthesis of carbamoyl phosphate, and two are required for the synthesis of PRPP from ribose 5-phosphate and ATP (Figure 6- 9).
The oxidation of dihydroorotate provides reducing equivalents to coenzyme Q that lead to the formation of 1.5 moles of ATP per mole oxidized.
This oxidative pathway from dihydroorotatc to oxygen, which involves the mitochondrial electron transport chain, is exergonic.