| Abstract
| - The structure of a complex of Torpedo californicaacetylcholinesterase with the transition state analoginhibitorm-(N,N,N-trimethylammonio)-2,2,2-trifluoroacetophenonehas been solved by X-ray crystallographic methodsto 2.8 Å resolution. Since the inhibitor binds to the enzymeabout 1010-fold more tightly than thesubstrateacetylcholine, this complex provides a visual accounting of theenzyme−ligand interactions that provide the molecularbasis for the catalytic power of acetylcholinesterase. The enzymeowes about 8 kcal mol-1 of the 18 kcalmol-1 offree energy of stabilization of the acylation transition state tointeractions of the quaternary ammonium moiety withthree water molecules, with the carboxylate side chain of E199, andwith the aromatic side chains of W84 and F330.The carbonyl carbon of the trifluoroketone function interactscovalently with S200 of the S200−H440−E327 catalytictriad. The operation of this triad as a general acid−basecatalytic network probably provides 3−5 kcalmol-1 of thefree energy of stabilization of the transition state. Theremaining 5−7 kcal mol-1 of transitionstate stabilizationprobably arises from tripartite hydrogen bonding between the incipientoxyanion and the NH functions of G118,G119, and A201. The acetyl ester hydrolytic specificity of theenzyme is revealed by the interaction of theCF3function of the transition state analog with a concave binding sitecomprised of the residues G119, W233, F288,F290, and F331. The highly geometrically convergent array ofenzyme−ligand interactions visualized in the complexdescribed herein envelopes the acylation transition state andsequesters it from solvent, this being consistent with thelocation of the active site at the bottom of a deep and narrowgorge.
|
