Molecular docking study on the "back door" hypothesis for product clearance in acetylcholinesterase

Acetylcholinesterase (AChE) is one of the fastest enzymes known, even though the active site is buried inside the protein at the end of a 20-angstrom deep narrow gorge. Among the great variety of crystal structures of this enzyme, both in the absence and presence of various ligands and proteins, the structure of a complex of AChE with the pseudo-irreversible inhibitor Mf268 is of particular interest, as it assists in the proposal of a back door for product clearance from the active site. Binding of Mf268 to AChE results in the carbamoylation of Ser200 and liberation of an eseroline-fragment as the leaving group. The crystal structure of the AChE-Mf268 complex, however, proves that eseroline has escaped from the enzyme, despite the fact that the Ser-bound inhibitor fragment blocks the gorge entrance. The existence of alternative routes other than through the gorge for product clearance has been postulated but is still controversially discussed in the literature, as an experimental proof for such a back door is still missing. We have used Monte Carlo-based molecular docking methods in order to examine possible alternative pathways that could allow eseroline to be released from the protein after being cleaved from the substrate by Ser200. Based on our results, a short channel at the bottom of the gorge seems to be the most probable back-door site, which begins at amino acid Trp84 and ends at the enzyme surface in a cavity close to amino acid Glu445.