Covalent inhibition of hAChE by organophosphates causes homodimer dissociation through long-range allosteric effects

Abstract

Acetylcholinesterase (EC 3.1.1.7), a key acetylcholine-hydrolyzing enzyme in cholinergic neurotransmission, is present in a variety of states in situ, including monomers, C-terminally disulfide-linked homodimers, homotetramers, and up to three tetramers covalently attached to structural subunits. Could oligomerization that ensures high local concentrations of catalytic sites necessary for efficient neurotransmission be affected by environmental factors? Using small-angle X-ray scattering (SAXS) and cryo-EM, we demonstrate that homodimerization of recombinant monomeric human acetylcholinesterase (hAChE) in solution occurs through a C-terminal four-helix bundle at micromolar concentrations. We show that diethylphosphorylation of the active serine in the catalytic gorge or isopropylmethylphosphonylation by the R_P enantiomer of sarin promotes a 10-fold increase in homodimer dissociation. We also demonstrate the dissociation of organophosphate (OP)-conjugated dimers is reversed by structurally diverse oximes 2PAM, HI6, or RS194B, as demonstrated by SAXS of diethylphosphoryl-hAChE. However, binding of oximes to the native ligand-free hAChE, binding of high-affinity reversible ligands, or formation of an S_P-sarin-hAChE conjugate had no effect on homodimerization. Dissociation monitored by time-resolved SAXS occurs in milliseconds, consistent with rates of hAChE covalent inhibition. OP-induced dissociation was not observed in the SAXS profiles of the double-mutant Y337A/F338A, where the active center gorge volume is larger than in wildtype hAChE. These observations suggest a key role of the tightly packed acyl pocket in allosterically triggered OP-induced dimer dissociation, with the potential for local reduction of acetylcholine-hydrolytic power in situ. Computational models predict allosteric correlated motions extending from the acyl pocket toward the four-helix bundle dimerization interface 25 Å away.

Keywords: 4-helix bundle; SAXS; acetylcholinesterase; allosteric behavior; enzyme structure; molecular dynamics; organophosphate intoxication; oxime reactivation; paraoxon; small-angle X-ray scattering; structure–function.