Abstract: The transient opening of a backdoor in the active-site wall of acetylcholinesterase,
one of nature’s most rapid enzymes, has been suggested to contribute to the efficient traffic of
substrates and products. A crystal structure of Torpedo californica acetylcholinesterase in
complex with the peripheral-site inhibitor aflatoxin is now presented, in which a tyrosine at
the bottom of the active-site gorge rotates to create a 3.4-A˚ wide exit channel. Molecular
dynamics simulations show that the opening can be further enlarged by movement of Trp84.
The crystallographic and molecular dynamics simulation data thus point to the interface
between Tyr442 and Trp84 as the key element of a backdoor, whose opening permits
rapid clearance of catalysis products from the active site. Furthermore, the
crystal structure presented provides a novel template for rational design of inhibitors and
reactivators, including anti-Alzheimer drugs and antidotes against organophosphate poisoning.