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J Biol Chem, Vol. 274, Issue 13, 8491-8499, March 26, 1999

Organophosphorylation of Acetylcholinesterase in the Presence of Peripheral Site Ligands
DISTINCT EFFECTS OF PROPIDIUM AND FASCICULIN

From the Department of Pharmacology, Mayo Foundation for Medical Education and Research, and the Department of Research, Mayo Clinic Jacksonville, Jacksonville, Florida 32224

Structural analysis of acetylcholinesterase (AChE) has revealed two sites of ligand interaction in the active site gorge: an acylation site at the base of the gorge and a peripheral site at its mouth. A goal of our studies is to understand how ligand binding to the peripheral site alters the reactivity of substrates and organophosphates at the acylation site. Kinetic rate constants were determined for the phosphorylation of AChE by two fluorogenic organophosphates, 7-[(diethoxyphosphoryl)oxy]-1-methylquinolinium iodide (DEPQ) and 7-[(methylethoxyphosphonyl)oxy]-4-methylcoumarin (EMPC), by monitoring release of the fluorescent leaving group. Rate constants obtained with human erythrocyte AChE were in good agreement with those obtained for recombinant human AChE produced from a high level Drosophila S2 cell expression system. First-order rate constants k_OP were 1,600 ± 300 min¹ for DEPQ and 150 ± 11 min¹ for EMPC, and second-order rate constants k_OP/K_OP were 193 ± 13 µM¹ min¹ for DEPQ and 0.7-1.0 ± 0.1 µM¹ min¹ for EMPC. Binding of the small ligand propidium to the AChE peripheral site decreased k_OP/K_OP by factors of 2-20 for these organophosphates. Such modest inhibitory effects are consistent with our recently proposed steric blockade model (Szegletes, T., Mallender, W. D., and Rosenberry, T. L. (1998) Biochemistry 37, 4206-4216). Moreover, the binding of propidium resulted in a clear increase in k_OP for EMPC, suggesting that molecular or electronic strain caused by the proximity of propidium to EMPC in the ternary complex may promote phosphorylation. In contrast, the binding of the polypeptide neurotoxin fasciculin to the peripheral site of AChE dramatically decreased phosphorylation rate constants. Values of k_OP/K_OP were decreased by factors of 10³ to 10⁵, and k_OP was decreased by factors of 300-4,000. Such pronounced inhibition suggested a conformational change in the acylation site induced by fasciculin binding. As a note of caution to other investigators, measurements of phosphorylation of the fasciculin-AChE complex by AChE inactivation gave misleading rate constants because a small fraction of the AChE was resistant to inhibition by fasciculin.

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