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 phos. phate, and two are required for the synthesis of PRPP from ribose 5-phosphate and ATP (Figure 6- 9. The oxidation of dihydroorotate provides re- ducing 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.