Organophosphorylation of acetylcholinesterase in the presence of peripheral site ligands - Distinct effects of propidium and fasciculin

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 min(-1) for DEPQ and 150 +/- 11 min(-1) for EMPC, and second-order rate constants k(OP)/K-OP were 193 +/- 13 mu M-1 min(-1) for DEPQ and 0.7-1.0 +/- 0.1 mu M-1 min(-1) 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 ho, 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 10(3) to 10(5), 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.