Acetylcholinesterase: Effects of Ionic Strength and Dimerization on the Rate Constants

Brownian dynamics simulations are used to calculate diffusion‐controlled rate constants for the binding of a positively‐charged ligand to acetylcholinesterase (AChE) at 300 K, pH 7, and several ionic strengths. Models of the enzyme were constructed on the basis of the crystal structure of Torpedo californica AChE, and the ligand was modeled as a 5‐Å sphere. Assignment of the charge distribution of the enzyme is based on calculation of the fractional charges of its ionizable groups as a function of pH and ionic strength, by the finite difference Poisson‐Boltzmann method. We find that the mean charge of the enzyme increases significantly with increasing ionic strength, with most of the increase occurring between 0 and 200 mM ionic strength. The charge distribution results in a very high dipole moment for the monomeric subunit of the protein: 1500 D relative to the Center of Diffusion. The magnitude and orientation of the dipole moment are relatively insensitive to the ionic strength. At physiological ionic strength, electrostatic steering of the ligand increases the rate constant of the enzyme‐ligand encounter by more than one order of magnitude. The increase in protein charge with rising ionic strength weakens the ionic strength dependence of the rate somewhat. The calculations reproduce the experimentally observed decrease of the rate constants with increasing ionic strength. We observe no intrinsic rate difference for dimeric AChE as compared to the monomer: the rate constant of the dimer is twice that of the monomer.