Acetylcholinesterase was first studied by using the form found in electric fish, such as the torpedo ray. These fish have massive arrays of nerve-like structures in the organs that generate electricity, so acetylcholinesterase is particularly abundant. The form shown here, from PDB entry 1acj, forms a dimer in the crystal structure. It normally has lipids attached to the protein chains, which anchor the enzyme to the cell membrane. The lipids were removed in the crystal structure, however, to allow crystallization. The active site is found in a deep pocket, just big enough for the acetylcholine to slip down inside. At the base of the pocket is a triad of three amino acids--serine-histidine-glutamate--that is almost identical to the triad used in the serine proteases like trypsin and chymotrypsin.