Biophysical and Mechanistic Characterization of Carbamoyl Phosphate Synthetase from Escherichia coli

Carbamoyl phosphate synthetase (CPS) from E. coli catalyzes the formation of carbamoyl phosphate, an intermediate in the biosynthesis of pyrimidine nucleotides and arginine, from glutamine, bicarbonate and two molecules of MgATP. This reaction is catalyzed by three separate active sites that are separated in space by ~100 ?. The transfer of ammonia and carbamate through the two intramolecular tunnels was investigated by molecular dynamics simulations and experimental characterization of mutations within. The presence of an unstable reaction intermediate, carboxyphosphate, was established. A method for studying the synchronization of the two active sites on the large subunit of CPS was developed. The potential of mean force (PMF) calculations along the ammonia and carbamate transfer pathways indicate a low free-energy path for the translocation of ammonia. The highest barrier for ammonia is 7.2 kcal/mol which corresponds to a narrow turning gate surrounded by the side chains of Cys-232, Ala-251, and Ala-314 in the large subunit. A blockage in the passageway was introduced by the triple mutant C232V/A251V/A314V, which was unable to synthesize carbamoyl phosphate. The release of phosphate is necessary for the injection of carbamate into the carbamate tunnel. Two mutants, A23F and G575F, were designed to block the migration of carbamate through carbamate tunnel. The mutants retained only 1.7 percent and 3.8 percent of the catalytic activity for the synthesis of carbamoyl phosphate relative to the wild-type CPS, respectively. Formate can be utilized by CPS in the absence of bicarbonate to form formyl phosphate. This intermediate was observed by 31P, 13C, and 1H NMR. For the three NMR methods a peak corresponding to formyl phosphate was observed at 2.15 ppm (31P) , 162.4 ppm (13C), and 8.39 and 7.94 ppm (1H). The rate of formation of formyl phosphate is 0.025 ? 0.005 s-1. Formamide was not detected in the presence of an ammonia source. Fluorescence anisotropy measurements on the C551A/S171C and C551A/S717C mutants provided insight into a possible mechanism of synchronization between the two active sites on the large subunit. The biggest fluorescence anisotropy change was observed at the N-terminal domain in the presence of AMPPNP and ATP.