THE X-RAY STRUCTURE OF A TRANSITION-STATE ANALOG COMPLEX REVEALS THE MOLECULAR-ORIGINS OF THE CATALYTIC POWER AND SUBSTRATE-SPECIFICITY OF ACETYLCHOLINESTERASE

The structure of a complex of Torpedo calfornica acetylcholinesterase with the transition state analog inhibitor ,N,N-trimethylammonio)-2,2, 2-trifluoroacetophenone has been solved by X-ray crystallographic meth ods to 2.8 Angstrom resolution. Since the inhibitor binds to the enzym e about 10(10)-fold more tightly than the substrate acetylcholine, thi s complex provides a visual accounting of the enzyme-ligand interactio ns that provide the molecular basis for the catalytic power of acetylc holinesterase. The enzyme owes about 8 kcal mol(-1) of the 18 kcal mol (-1) of free energy of stabilization of the acylation transition state to interactions of the quaternary ammonium moiety with three water mo lecules, with the carboxylate side chain of E199, and with the aromati c side chains of W84 and F330. The carbonyl carbon of the trifluoroket one function interacts covalently with S200 of the S200-H440-E327 cata lytic triad. The operation of this triad as a general acid-base cataly tic network probably provides 3-5 kcal mol(-1) of the free energy of s tabilization of the transition state. The remaining 5-7 kcal mol(-1) o f transition state stabilization probably arises from tripartite hydro gen bonding between the incipient oxyanion and the NH functions of G11 8, G119, and A201. The acetyl ester hydrolytic specificity of the enzy me is revealed by the interaction of the CF3 function of the transitio n state analog with a concave binding site comprised of the residues G 119, W233, F288, F290, and F331. The highly geometrically convergent a rray of enzyme-ligand interactions visualized in the complex described herein envelopes the acylation transition state and sequesters it fro m solvent, this being consistent with the location of the active site at the bottom of a deep and narrow gorge.