Chemico-Biological Interactions 119 – 120 (1999) 43 – 52 A preliminary comparison of structural models for catalytic intermediates of acetylcholinesterase Israel Silman a, *, Charles B. Millard a,b , Arie Ordentlich c , Harry M. Greenblatt d , Michal Harel d , Dov Barak c , Avigdor Shafferman c , Joel L. Sussman d,e a Department of Neurobiology, Weizmann Institute of Science, Rehoot 76100, Israel b US Army Medical Research Institute of Chemical Defense, Aberdeen Proing Ground, MD 21010, USA c Israel Institute for Biological Research, Ness Ziona 70450, Israel d Department of Structural Biology, Weizmann Institute of Science, Rehoot 76100, Israel e Biology Department, Brookhaen National Laboratory, Upton, NY 11973, USA Abstract Determination of the three dimensional structure of Torpedo Californica acetyl- cholinesterase (Tc AChE) provided an experimental tool for directly visualizing interaction of AChE with cholinesterase inhibitors of fundamental, pharmacological and toxicological interest. The structure revealed that the active site is located near the bottom of a deep and narrow gorge lined with 14 conserved aromatic amino acids. The structure of a complex of Tc AChE with the powerful ‘transition state analog’ inhibitor, TMTFA, sug- gested that its orientation in the experimentally determined structure was very similar to that proposed for the natural substrate, acetylcholine, by manual docking. The array of enzyme-ligand interactions visualized in the TMFTA complex also are expected to envel- ope the unstable TI that forms with acetylcholine during acylation, and to sequester it from solvent. In our most recent studies, the crystal structures of several ‘aged’ conjugates Abbreiations: ACh, acetylcholine; AChE, acetylcholinesterase (EC 3.1.1.7); BChE, butyryl- cholinesterase (EC 3.1.1.8); DFP, diiso propylphosphorofluoridate; MeP – AChE, methylphosphonylated AChE; MES, (2-[N-morpholino]ethanesulfonic acid); Mi PrP-, monoiso propylphosphoryl; OP, organophosphorus; PEG, polyethylene glycol; Tc, Torpedo californica ; TI, tetrahedral intermediate; TMTFA, m-(N,N,N-trimethylammonio)-2,2,2-trifluoroacetophenone. * Corresponding author. Tel.: +972-8-934-2128/3649; fax: +972-8-947-1849. E-mail address: bnsilm@weizmann.weizmann.ac.il (I. Silman) 0009-2797/99/$ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved. PII:S0009-2797(99)00012-5