ACETYLCHOLINESTERASE - DIFFUSIONAL ENCOUNTER RATE CONSTANTS FOR DUMBBELL MODELS OF LIGAND

For some enzymes, virtually every substrate molecule that encounters t he entrance to the active site proceeds to reaction, at low substrate concentrations. Such diffusion-limited enzymes display high apparent b imolecular rate constants ((k(cat)/K-M)), which depend strongly upon s olvent viscosity. Some experimental studies provide evidence that acet ylcholinesterase falls into this category. Interestingly, the asymmetr ic charge distribution of acetylcholinesterase, apparent from the crys tallographic structure, suggests that its electrostatic field accelera tes the encounter of its cationic substrate, acetylcholine, with the e ntrance to the active site. Here we report simulations of the diffusio n of substrate in the electrostatic field of acetylcholinesterase. We find that the field indeed guides the substrate to the mouth of the ac tive site. The computed encounter rate constants depend upon the parti cular relative geometries of substrate and enzyme that are considered to represent successful encounters. With loose reaction criteria, the computed rates exceed those measured experimentally, but the rate cons tants vary appropriately with ionic strength. Although more restrictiv e reaction criteria lower the computed rates, they also lead to unreal istic variation of the rate constants with ionic strength. That these simulations do not agree well with experiment suggests that the simple diffusion model is incomplete. Structural fluctuations in the enzyme or events after the encounter may well contribute to rate limitation.