Mobility of the Active Site Bound Paraoxon and Sarin in Zinc-Phosphotriesterase by Molecular Dynamics Simulation and Quantum Chemical Calculation

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Abstract
The kinetic data published on phosphotriesterase (PTE), with various complexed metals, clearly indicates that the PO and PS bonds of phosphotriester and thiophosphotriester substrates, respectively, are strongly polarized by one or both of the active site complexed metal ions. However, this observation is not consistent with the three-dimensional X-ray crystal structure of zinc-substituted PTE with active site bound substrate analogue diethyl 4-methylbenzylphosphonate. In this structure, the distance between the phosphoryl oxygen and the nearest zinc is 3.4 Å, a distance too large to afford strong polarization. In the present paper, the geometry and mobility of various PTE active site−substrate complexes are examined by performing both molecular dynamics (MD) simulations and quantum mechanical calculations. Two known substrates are considered, paraoxon and sarin, although their turnover rates vary about 100-fold. The results indicate that PTE forms a complex with either substrate in which the phosphoryl oxygen becomes strongly coordinated with the less buried zinc atom. It is shown that the geometry of the active site is changed when the protein is immersed in a water bath and relaxed by MD. The most substantial conformational change is the opening of the gateway in a pocket where the location of the leaving group is expected. The opening is observed for the pure enzyme as well as for the enzyme/substrate complexes and it ranges from 11 to 18 Å. It is also shown that the pockets, in which the substrate substituents are localized, exhibit different flexibility and interact with the substrate with coordinated conformational adjustments.