Molecular dynamics study of active-site interactions with tetracoordinate transients in acetylcholinesterase and its mutants

The role of active-site residues in the dealkylation reaction in the PSCS d iastereomer of 2-(3,3 -dimetfiylbutyl)methylphosphono-fluoridate (soman)-in hibited Torpedo californica acetylcholinesterase (AChE) was investigated by full-scale molecular dy namics simulations using CHARMM: > 400 ps equilibr ation was followed by 150-20Dps production runs with the fully solvated tet racoordinate phosphonate adduct of the wild-type, Trp84Ala and Gly 199Gln m utants of AChE, Parallel simulations were carried out with the tetrahedral intermediate formed between serine-200 Oy of AChE and acetylcholine, We fou nd that the N epsilonH in histidine H+-440 is positioned to protonate the o xygen in choline and thus promote its departure. In contrast, Nt-H in histi dine H+-440 is not aligned for a favourable proton transfer to the pinacoly l O to promote dealkylation. but electrostatic stabilization by histidine H +-440 of the developing anion on the phosphonate monoester occurs. Destabil izing interactions between residues and the alkyl fragment occurs. of the i nhibitor enforce methyl migration from C alpha to C beta concerted with C-O bond breaking in soman-inhibited AChE, Tryptophan-84, phrnyalanine-331 and glutamic acid-199 are within 3.7-3.9 Angstrom (1 Angstrom = 10(-10) m) fro m a methyl group in CP. 4.5-5.1 Angstrom from C beta and 4.8-5.8 Angstrom f rom C alpha. and can better stabilize the developing carbenium ion on C bet a than on C alpha. The Trp84Ala mutation eliminates interactions between th e incipient carbenium ion and the indole ring, but also reduces its interac tions with phenylalanine-331 and aspartic acid-72. Tyrosine-130 promotes de alkylation by interacting with the indole ring of tryptophan-84. Glutamic a cid-443 can influence the orientation of active-site residues through tyros ine-421, tyrosine-442 and histidine-440 in soman-inhibited AChE, and thus f acilitate of active-site histidine-440 dealkylation.