Atomic Structure of Acetylcholinesterase from <i>Torpedo californica</i> : A Prototypic Acetylcholine-Binding Protein

TLDR

Modeling of acetylcholine binding suggests the quaternary ammonium ion is accommodated by aromatic residues lining the gorge rather than a negatively charged anionic site. The crystal structure of Torpedo californica acetylcholinesterase at 2.8 Å shows a glycolipid‑anchored homodimer whose 537‑residue monomer contains a 12‑stranded β sheet and 14 α helices, closely resembles other hydrolases, and features an unusual active site with Glu in the catalytic triad positioned near the bottom of a deep gorge.

Abstract

The three-dimensional structure of acetylcholinesterase from Torpedo californica electric organ has been determined by x-ray analysis to 2.8 angstrom resolution. The form crystallized is the glycolipid-anchored homodimer that was purified subsequent to solubilization with a bacterial phosphatidylinositol-specific phospholipase C. The enzyme monomer is an α/β protein that contains 537 amino acids. It consists of a 12-stranded mixed β sheet surrounded by 14 α helices and bears a striking resemblance to several hydrolase structures including dienelactone hydrolase, serine carboxypeptidase-II, three neutral lipases, and haloalkane dehalogenase. The active site is unusual because it contains Glu, not Asp, in the Ser-His-acid catalytic triad and because the relation of the triad to the rest of the protein approximates a mirror image of that seen in the serine proteases. Furthermore, the active site lies near the bottom of a deep and narrow gorge that reaches halfway into the protein. Modeling of acetylcholine binding to the enzyme suggests that the quaternary ammonium ion is bound not to a negatively charged "anionic" site, but rather to some of the 14 aromatic residues that line the gorge.