Mechanical aspects of the phosphotriesterase activity of human butyrylcholinesterase G11 7H mutant

The G117H mutant of human butyrylcholinesterase (BuChE) is able to catalyze the hydrolysis of organophosphate paraoxon and echothiophate. In order to understand this property, a molecular mechanic study of the mutant adduct was carried out. When the imidazole ring was protonated on the N delta 1 site, the N delta 1 hydrogen was found to bridge the two alkoxy oxygens of the phosphorylated serine in the minimum energy conformation. This conformation was shown to be very stable during molecular dynamics. In particular, the phosphoryl oxygen was found to make strong hydrogen bonds with the oxyanion hole, resulting in the weakening of the O gamma-P bond to be broken. The positive electrostatic field generated by H117 and the adjacent protonated H438 could attract and direct a water molecule for nucleophilic attack and subsequent dephosphorylation. The double hydrogen-bonding of alkoxy oxygens may account for the faster aging of G117H mutant when compared to the wild-type enzyme. When the imidazole ring was protonated on the N epsilon 2 site, the position of H117 ring in the minimum energy conformation was similar to that found for the other protonation site. In this case, the specific position of the H117 ring could allow a direct attack of nitrogen N delta 1 on the phosphorus atom, the resulting phosphoenzyme intermediate being cleaved by a water molecule in a second reaction to regenerate the active enzyme.