Electrostatic Properties of Human Acetylcholinesterase

Abstract

We recently proposed (Ripoll et al., 1993) an electrostatic mechanism of substrate attraction for the enzyme acetylcholinesterase (AChE). The proposed mechanism was based on the calculated electrostatic properties of Torpedo californica AChE (TAChE), which show that the active site is entirely embedded in a region that has the lowest values of the electrostatic potential. Recent experimental results on human recombinant AChE (HrAChE), designed to test this hypothesis (Shafferman et al., 1994), show that HrAChE mutants, in which as many as seven negative groups near the entrance to the active-site gorge have been neutralized, have catalytic activities comparable to those of the wild type enzyme. Moreover, these authors presented computational data showing that the isopotential surface, −1kT/e, for their seven-residue mutant (M7), was much smaller than the corresponding surface for the wild type enzyme. They concluded, therefore, that electrostatic effects do not contribute to the catalytic rate of this enzyme. In our earlier study on TAChE (Ripoll et al., 1993) we pointed out that it is important to take into consideration not only the isopotential surfaces, but also the electric field, before drawing any conclusion about the influence of electrostatic forces on AChE activity. In our present study, we calculate the electrostatic field vectors for wild type HrAChE (WT) and for the mutant in which seven negative acidic residues have been changed to neutral residues (M7).