Kinetic mechanism of Escherichia coli carbamoyl-phosphate synthetase.

The kinetic mechanism of Escherichia coli carbamoyl-phosphate synthetase has been determined at pH 7.5, 25 degrees C. With ammonia as the nitrogen source, the initial velocity and product inhibition patterns are consistent with the ordered addition of MgATP, HCO3-, and NH3. Phosphate is then released and the second MgATP adds to the enzyme, which is followed by the ordered release of MgADP, carbamoyl phosphate, and MgADP. With glutamine as the ammonia donor, the patterns are consistent with a two-site mechanism in which glutamine binds randomly to the small molecular weight subunit producing glutamate and ammonia. Glutamate is released and the ammonia is transferred to the larger subunit. Carbamoyl-phosphate synthetase has also been shown to require a free divalent cation for full activity.