Acetylcholinesterase is among the most efficient enzymes known. In order to provide an explanation for its catalytic and regulatory mechanisms, including the high turnover rate, the specific amino acid residues involved in substrate binding and hydrolysis need to be identified. In this article, Ferdinand Hucho, Jaak Järv and Christoph Weise describe the topography of the enzyme as deduced from protein chemistry studies. One result of this approach is the finding that the binding pocket for the substrate's cationic cholinium group appears to be hydrophobic rather than anionic.