Acetylthiocholine Binds to Asp74 at the Peripheral Site of Human Acetylcholinesterase as the First Step in the Catalytic Pathway

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Abstract
Studies of ligand binding to acetylcholinesterase (AChE) have demonstrated two sites of interaction. An acyl−enzyme intermediate is formed at the acylation site, and catalytic activity can be inhibited by ligand binding to a peripheral site. The three-dimensional structures of AChE−ligand complexes reveal a narrow and deep active site gorge and indicate that ligands specific for the acylation site at the base of the gorge must first traverse the peripheral site near the gorge entrance. In recent studies attempting to clarify the role of the peripheral site in the catalytic pathway for AChE, we showed that ligands which bind specifically to the peripheral site can slow the rates at which other ligands enter and exit the acylation site, a feature we called steric blockade [Szegletes, T., Mallender, W. D., and Rosenberry, T. L. (1998) Biochemistry37, 4206−4216]. We also demonstrated that cationic substrates can form a low-affinity complex at the peripheral site that accelerates catalytic hydrolysis at low substrate concentrations but results in substrate inhibition at high concentrations because of steric blockade of product release [Szegletes, T., Mallender, W. D., Thomas, P. J., and Rosenberry, T. L. (1999) Biochemistry38, 122−133]. In this report, we demonstrate that a key residue in the human AChE peripheral site with which the substrate acetylthiocholine interacts is D74. We extend our kinetic model to evaluate the substrate affinity for the peripheral site, indicated by the equilibrium dissociation constant KS, from the dependence of the substrate hydrolysis rate on substrate concentration. For human AChE, a KS of 1.9 ± 0.7 mM obtained by fitting this substrate inhibition curve agreed with a KS of 1.3 ± 1.0 mM measured directly from acetylthiocholine inhibition of the binding of the neurotoxin fasciculin to the peripheral site. For Torpedo AChE, a KS of 0.5 ± 0.2 mM obtained from substrate inhibition agreed with a KS of 0.4 ± 0.2 mM measured with fasciculin. Introduction of the D72G mutation (corresponding to D74G in human AChE) increased the KS to 4−10 mM in the Torpedo enzyme and to about 33 mM in the human enzyme. While the turnover number kcat was unchanged in the human D74G mutant, the roughly 20-fold decrease in acetylthiocholine affinity for the peripheral site in D74G resulted in a corresponding decrease in kcat/Kapp, the second-order hydrolysis rate constant, in the mutant. In addition, we show that D74 is important in conveying to the acylation site an inhibitory conformational effect induced by the binding of fasciculin to the peripheral site. This inhibitory effect, measured by the relative decrease in the first-order phosphorylation rate constant kOP for the neutral organophosphate 7-[(methylethoxyphosphonyl)oxy]-4-methylcoumarin (EMPC) that resulted from fasciculin binding, decreased from 0.002 in wild-type human AChE to 0.24 in the D74G mutant.