A combined theoretical and experimental study of the ammonia tunnel in carbamoyl phosphate synthetase.
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The transfer of ammonia in carbamoyl phosphate synthetase (CPS) was investigated by molecular dynamics simulations and experimental characterization of mutations within the ammonia tunnel. In CPS, ammonia is derived from the hydrolysis of glutamine and this intermediate must travel approximately 45 A from the site of formation in the small subunit to the site of utilization in the large subunit. In this investigation, the migration of ammonia was analyzed from the exit of the small subunit through the large subunit where it ultimately reacts with the carboxy phosphate intermediate. Potential of mean force calculations along the transfer pathway for ammonia indicate a relatively low free-energy barrier for the translocation of ammonia. The highest barrier of 7.2 kcal/mol is found at a narrow turning gate surrounded by the side chains of Cys-232, Ala-251, and Ala-314 in the large subunit. The environment of the ammonia tunnel from the exit of the small subunit to the turning gate in the tunnel is filled with clusters of water molecules and the ammonia is able to travel through this area easily. After ammonia passes through the turning gate, it enters a hydrophobic passage. A hydrogen bond then forms between the ammonia and Thr-249, which facilitates the delivery to a more hydrophilic environment near the active site for the reaction with the carboxy phosphate intermediate. The transport process from the turning gate to the end of the tunnel is favored by an overall downhill free-energy potential and no free-energy barrier higher than 3 kcal/mol. A conformational change of the turning gate, caused by formation of the carboxy phosphate intermediate, is consistent with a mechanism in which the reaction between ATP and bicarbonate triggers the transport of ammonia and consequently accelerates the rate of glutamine hydrolysis in the small subunit. A blockage in the turning gate passageway was introduced by the triple mutant C232V/A251V/A314V. This mutant is unable to synthesize carbamoyl phosphate using glutamine as a nitrogen source.