Two adenosinetriphosphate (ATP) molecules are required for the synthesis of carbamoyl phosphate intricate reaction catalyzed by carbamoyl phosphate synthetase ll (Figure 14-3).
The first ATP activates bicarbonate by forming an energyrich carbonyl phosphate.
The high-energy carbonyl phosphate reacts with glutamine, yielding carbamic acid; this reaction, which occurs with the loss of a high-energy bond, is exergonic.
Reaction with a second molecule of ATP yields carbamoyl phosphate.
This second reaction is functionally isoergonic because carbamoyl phosphate is an energyrich compound with the mixed-acid anhydride linkage.
The carbamoyl group is activated with a high group transfer potential (Tables 6-2 and 6-3) and is suitable for biosynthetic reactions.
The overall reaction proceeds with the loss of a high energy bond and is exergonic.
Carbamoyphosphate synthetase ll is the rate-limiting enzyme of pyrimidine biosynthesis in humans (Table 25-3).