Compensational phenomena in reactivation of dimethyl- and diethylphosphoryl butyrylcholinesterases

The thermodynamic and kinetic parameters for spontaneous and oxime reactivation of dimethyl- and diethylphosphoryl butyrylcholinesterases (acylcholine acyl-hydrolase, EC 3.1.1.8) are reported. The enthalpy and entropy changes in both the binding (deltaH0 and deltaS0) and the dephosphorylation steps (deltaH* and deltaS*) were found to be coupled, resulting in a minor variation in free energy changes (deltaG0 and deltaG*). While neither enthalpies nor entropies alone bore any relationship with the kinetic parameters KD and kR, the changes of free energies (deltaG0 and deltaG*) correlated linearly with the logarithmic values of the dissociation constants (KD) and bimolecular rate constants (kR/KD), respectively. Compensation plots of entropies versus enthalpies gave straight lines with compensation temperatures of 275 K for the binding 260 K for the dephosphorylation. Spontaneous reactivation of dimethyl phosphoryl butyrylcholinesterase was investigated at various pH values and three temperatures. It implicated two catalytic sites with values of pKi of 9.4 and 7.5, and heats of ionisation of 5.3 and 9.6 kcal - mol-1, respectively. Possible conformational alteration of the inhibited enzyme arising from the binding of oximes is discussed.