Hydration change during the aging of phosphorylated human butyrylcholinesterase: importance of residues aspartate-70 and glutamate-197 in the water network as probed by hydrostatic and osmotic pressures

Wild-type human butyrylcholinesterase (BuChE) and Glu-197 → Asp and Asp-70 → Gly mutants (E197D and D70G respectively) were inhibited by di-isopropyl phosphorofluoridate under standard conditions of pH, temperature and pressure. The effect of hydrostatic and osmotic pressures on the aging process (dealkylation of an isopropyl chain) of phosphorylated enzymes [di-isopropylated (DIP)-BuChE] was investigated. Hydrostatic pressure markedly increased the rate of aging of wild-type enzyme. The average activation volume (δV≠) for the dealkylation reaction was -170 ml/mol for DIP wild-type BuChE. On the other hand, hydrostatic pressure had little effect on the aging of the DIP mutants (δV≠ = -2.6 ml/mol for E197D and -2 ml/mol for D70G), suggesting that the transition state of the aging process was associated with an extended hydration and conformational change in wild-type BuChE, but not in the mutants. The rate of aging of wild-type and mutant enzymes decreased with osmotic pressure, allowing very large positive osmotic activation volumes (δV≠osm) to be estimated, thus probing the participation of water in the aging process. Molecular dynamics simulations performed on the active-site gorge of the wild-type DIP adduct showed that the isopropyl chain involved in aging was highly solvated, supporting the idea that water is important for stabilizing the transition state of the dealkylation reaction. Wild-type BuChE was inhibited by soman (pinacolyl methylphosphonofluoridate). Electrophoresis performed under high pressure [up to 2.5 kbar (1 bar = 105 Pa)] showed that the soman-aged enzyme did not pass through a pressure-induced, molten-globule transition, unlike the native wild-type enzyme. Likewise, this transition was not seen for the native E197D and D70G mutants, indicating that these mutants are resistant to the penetration of water into their structure. The stability energetics of native and soman-aged wild-type BuChE were determined by differential scanning calorimetry. The pH-dependence of the midpoint transition temperature of endotherms indicated that the high difference in stabilization energy between aged and native BuChE (δδG = 23.7 kJ/mol at pH 8.0) is mainly due to the salt bridge between protonated His-438 and PO-, with pKHis-438 = 8.3. A molecular dynamics simulation on the MIP adduct showed that there is no water molecule around the ion pair. The ‘hydrostatic versus osmotic pressure’ approach probed the importance of water in aging, and also reveale