Computational Modeling of the Catalytic Mechanism of Human Placental Alkaline Phosphatase (PLAP)

Alkaline phosphatases (APs) catalyze the hydrolysis and transphosphorylation of phosphate monoesters. Quantum mechanical, molecular dynamics, and molecular docking techniques were applied to computationally model the catalytic mechanism of human placental AP (PLAP). Kinetic and thermodynamic evaluations were performed for each reaction step. The functional significances of the more important residues within the active site were analyzed. The role of the metal ion at the metal binding site M3 was also examined. The calculated activation and reaction energy and free energy values obtained suggested the nucleophilic attack of the Ser92 alkoxide on the phosphorus atom of the substrate would be the rate-limiting step of the catalytic hydrolysis of alkyl phosphate monoesters by PLAP. The reactivities of the wild-type M3-Mg enzyme and the M3-Zn protein were compared, and the main difference observed was a change in the coordination number of the M3 metal for the M3-Zn enzyme. This modification in the active site structure lowered the free energy profile for the second chemical step of the catalytic mechanism (hydrolysis of the covalent phosphoserine intermediate). Consequently, a greater stabilization of the phosphoseryl moiety resulted in a small increment in the activation free energy of the phosphoserine hydrolysis reaction. These computational results suggest that the activation of APs by magnesium at the M3 site is caused by the preference of Mg(2+) for octahedral coordination, which structurally stabilizes the active site into a catalytically most active conformation. The present theoretical results are in good agreement with previously reported experimental studies.