SIMULATION OF CHARGE-MUTANT ACETYLCHOLINESTERASES

A recent experimental study of human acetylcholinesterase has shown th at the mutation of surface acidic residues has little effect on the ra te constant for hydrolysis of acetylthiocholine. It was concluded, on this basis, that the reaction is not diffusion controlled and that ele ctrostatic steering plays only a minor role in determining the rate. H ere we examine this issue through Brownian dynamics simulations on Tor pedo californica acetylcholinesterase in which the surface acidic resi dues homologous with those mutated in the human enzyme are artifically neutralized. The computed effects of the mutations on the rate consta nts reproduce quite well the modest effects of the mutations upon the measured encounter rates. Nonetheless, the electrostatic field of the enzyme is found to increase the rate constants by about an order of ma gnitude in both the wild type and the mutants. We therefore conclude t hat the mutation experiments do not disprove that electrostatic steeri ng substantially affects the catalytic rate of acetylcholinesterase.